T L Chng

I S Orel

S M Starikovskaia

I V Adamovich

Electric Field Induced Second Harmonic (E-FISH) Generation for Characterization of Fast Ionization Wave Discharges at Moderate and Low Pressures

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Electric field induced second harmonic (E-FISH) generation for characterization of fast ionization wave discharges at moderate and low pressures

Tat Loon Chng, Inna Orel, Svetlana Starikovskaia, I.V. Adamovich

To cite this version:

Tat Loon Chng, Inna Orel, Svetlana Starikovskaia, I.V. Adamovich. Electric field induced second harmonic (E-FISH) generation for characterization of fast ionization wave discharges at moderate and low pressures. Plasma Sources Science and Technology, 2019, 28 (4), 045004 (8pp). 10.1088/1361-6595/ab0b22. hal-02406807

Introduction

Discharges generated by high peak voltage, ns duration pulses have been studied extensively over the last two decades, for applications including plasma-assisted combustion [START_REF] Yu | Plasma-assisted ignition and combustion[END_REF][START_REF] Starikovskaia | Plasma-assisted ignition and combustion: nanosecond discharges and development of kinetic mechanisms[END_REF][START_REF] Ju | Plasma assisted combustion: Dynamics and chemistry[END_REF], plasma chemical fuel reforming [START_REF] Petitpas | Electric field measurements in a near atmospheric pressure nanosecond pulse discharge with picosecond electric field induced second harmonic generation[END_REF] and CO2 dissociation [5], high-speed flow control [6], and plasma medicine [7]. Nanosecond pulse discharges generate nonequilibrium low-temperature plasmas over a wide range of pressures, from below a mbar [8,9] to several bar [10,11]. At moderate pressures, from several mbar to several tens of mbar, the discharge develops as a Fast Ionization Wave (FIW), kinetics of which are reviewed in great detail in [12,13]. A recent study of the FIW discharge in a capillary tube in air at 30 mbar [14] demonstrates that it produces nearly complete dissociation of molecular oxygen, suggesting that it can be used for the efficient plasma chemical generation of highly reactive radical species.

One of the most challenging aspects of characterization of kinetic processes in these plasmas, which occur on ns or even sub-ns time scales, is accurate diagnostics of electric field, electron density and temperature, and species number densities. A widely used method of electric field measurements in fast ionization wave discharges is by a calibrated capacitive probe [15,16], with a temporal resolution up to ~ 1 ns but a relatively low spatial resolution, ~ 1 cm. More recently, development and use of electro-optic probes has resulted in high temporal resolution (~ 1 ns), high spatial resolution (~ 1 mm) field measurements in atmospheric pressure plasma jets [17]. However, the presence of the probe perturbs the plasma to some extent, especially at the conditions when surface charge accumulation may occur on the probe housing. Development of non-intrusive, field vector sensitive, high spatio-temporal resolution diagnostics is essential for quantitative understanding of ionization kinetics in the wave, charge transport and species generation in the plasma behind the wave front, and wave propagation and attenuation.

The objective of the present work is to measure the electric field in a fast ionization wave, ns pulse discharge in nitrogen using the recently developed fs/ps Electric Field Induced Second Harmonic (E-FISH) diagnostic [18,19]. The principal advantage of this approach is sub-ns time resolution, limited primarily by the sampling rate of the detector and the oscilloscope. Previously, ps and fs E-FISH measurements have been done in atmospheric pressure ns pulse discharge plasmas in air, hydrogen, and argon [START_REF] Simeni Simeni | Electric Field Distribution in a Surface Plasma Flow Actuator Powered by Ns Discharge Pulse Trains[END_REF][START_REF] Simeni Simeni | Electric Field in Ns Pulse and AC Electric Discharges in a Hydrogen Diffusion Flame[END_REF][START_REF] Goldberg | 1-D Electric Field Measurements Using Field Induced Second Harmonic Generation[END_REF], as well in a ns pulse discharge in 100 Torr of air [19]. In the latter set of measurements, the onset of the ionization wave propagating between the discharge electrodes has been detected. However, it appears that in these experiments the wave did not fully develop, most likely due to a relatively short discharge gap, 1.3 cm. In the present work, a ns pulse discharge in nitrogen at 20-100 mbar is generated in a quartz tube between the electrodes set 8.0 cm apart, such that the development and propagation of the ionization wave could be studied in greater detail, over a wide pressure range.

Experimental

Figure 1 shows a schematic of the experimental apparatus. The discharge is sustained in a quartz tube with an internal diameter of 20 mm, between two hollow truncated cone brass electrodes, as illustrated in Fig. 1. The gap between the electrodes is 8.0 cm. Nitrogen is flown through the tube at the flow rate of 10 sccm and the pressure of 20-100 mbar. The discharge in the tube is initiated using positive polarity high-voltage pulses, with peak voltage of 9.3 kV, pulse rise/fall time of 4 ns, and pulse duration (FWHM) of 30 ns, at a repetition rate of 10 Hz, produced by an FPG 12-1NM (FID GmbH) high voltage pulse generator. The incident, reflected, and transmitted voltage pulse shapes are monitored by two calibrated, high bandwidth current shunts [START_REF] Anikin | Study of the oxidation of alkanes in their mixtures with oxygen and air under the action of a pulsed volume nanosecond discharge[END_REF] respectively installed in the coaxial transmission line 12.5 m before the high-voltage electrode and 12.5 m after the grounded electrode (see Fig. 1). Each current shunt represents an approximately 2 cm long insertion into the coaxial cable shielding comprising of several low induction resistors soldered in parallel. The voltage drop across the current shunt is a timeaccurate, amplitude-scaled waveform of the actual high-voltage pulse moving along the line as discussed in detail in our previous work [START_REF] Klochko | Investigation of capillary nanosecond discharges in air at moderate pressure: comparison of experiments and 2D numerical modelling[END_REF]. The distribution of the electric potential ) , ( t x  along the discharge gap is measured by a calibrated, high bandwidth, custom-made capacitive probe [15,[START_REF] Anikin | Breakdown development at high overvoltage: electric field, electronic level excitation and electron density[END_REF], a capacitive divider based on a low induction coaxial capacitor. The probe is moved along a slit machined in a cylindrical copper screen surrounding the discharge tube. The axial electric field

x t x t E x    / ) , ( ) (
 is calculated by measuring the electric potential at different points along the length of the tube with a step size of 1 mm. Broadband plasma emission images are taken using a Princeton Instruments PI-MAX 4 ICCD camera, and the ionization wave speed along the discharge tube is inferred from the emission images by tracking the time evolution of the wave front development. The camera gate width for these experiments is set at 1.17 ns, limited by the performance of the camera. Sequential emission images are captured every 0.2 ns for pressures 20, 40 and 70 mbar, and every 1 ns for the case of 100 mbar. The longer time delay chosen in the latter case is due to the slower ionization wave speed. The images are acquired from the moment of initiation of the ionization wave at the high-voltage electrode until its arrival at the grounded electrode, i.e. when the inter-electrode gap is closed. The position of the wave front, determined by locating the position of maximum emission intensity in each image, is plotted as a function of time, and the ionization wave speed is determined from the slope of this curve.

The schematic of the ps E-FISH diagnostic, also shown in Fig. 1, is similar to the one used in Ref. [19]. Briefly, the vertically polarized 1064 nm fundamental output beam of a ps laser (EKSPLA PL2241, pulse duration 30 ps, pulse energy 6 mJ) is rotated to horizontal using a halfwave plate, and is directed through the quartz discharge tube halfway between the high-voltage and grounded electrodes, near its horizontal plane of symmetry, as shown schematically in Fig. 1. The beam is focused in the center of the tube by a 20 cm focal distance lens. The confocal parameter of the lens, assuming a Gaussian laser beam, is estimated to be b≈1 mm. No optical access windows are used to pass the laser beam through the discharge tube. The laser pulse energy is kept sufficiently low, around 6 mJ, to prevent damage of the tube by the laser beam. A long pass filter, placed immediately after the first focusing lens, is used to block the stray second harmonic signal generated in the optical system components prior to the discharge tube. After exiting the discharge tube, the laser beam is collimated by a second 20 cm focal distance lens, and the second harmonic signal is separated from the fundamental beam using a dichroic mirror and a dispersive prism, as shown in Fig. 1. The intensity of the residual 1064 nm is monitored by a photodiode (Thorlabs DET10A, rise time 1 ns). The 532 nm second harmonic beam passes through a polarizer to isolate the horizontally polarized signal proportional to the square of the axial electric field in the discharge, and is focused with a 10 cm focal length lens into the IR H7422-50P Hamamatsu photomultiplier module with a rise time of 1 ns. An iris and 532 nm bandpass filter (FWHM 10 nm) are placed at the entrance of the PMT for the purpose of stray light rejection. The photodiode and PMT signals are monitored by a digital oscilloscope (LeCroy WaveRunner 64Xi-A) with a 5 GHz sampling rate and 600 MHz bandwidth. For the lowest pressure of 20 mbar, the time-resolved plasma emission detected by the PMT during the discharge pulse (emission background) is measured with the laser turned off, and subtracted from the PMT signal measured with the laser on (see Section 3) .

In spite of attempting to achieve sub-ns synchronization, there was an approximately 30 ns timing jitter between the laser and the high-voltage pulses. In order to address this issue, at each operating condition, a data acquisition strategy was employed whereby photodiode and PMT waveforms are saved for every laser shot (typically 40,000 shots per run). This procedure results in an ensemble of laser shots which are randomly distributed in time with respect to the transmitted high-voltage pulses. During the data post-processing, the E-FISH signal waveforms (with timing determined from the respective photodiode signals monitoring the laser pulses) are synchronized with respect to the discharge pulse by placing them into "time bins" 0.2 ns long, accumulating about 20 to 250 shots per time bin. Shots within each time bin are then integrated over time and averaged, as was done in our previous work [START_REF] Simeni Simeni | Electric Field Distribution in a Surface Plasma Flow Actuator Powered by Ns Discharge Pulse Trains[END_REF][START_REF] Goldberg | Electric Field Measurements in a Dielectric Barrier Nanosecond Pulse Discharge with Sub-Nanosecond Time Resolution[END_REF]. This approach greatly accelerates the data acquisition and yields the sub-ns time-resolution of the data, in spite of the significant timing jitter (see Fig. 2). The stability of the laser output pulse energy, which is critical for the accuracy of the electric field inference from the second harmonic signal, is monitored by the photodiode. Based on the photodiode data (see Fig. 2), the laser pulse energy variation does not exceed a few percent. Care was taken to ensure that the PMT signal was not saturated. This was verified by confirming that the PMT voltage, as well as the temporal profile and the peak value of the signal, were within their acceptable limits, determined based on previous experiments.

To verify that the second harmonic signal scales quadratically with the axial electric field, the setup is modified for calibration such that an axial Laplacian field is sustained between two circular, parallel plate electrodes placed in the discharge tube 4 mm apart, with the laser beam passing between the electrodes. The electric field calibration data have been taken for all four operating pressures, 20, 40, 70, and 100 mbar. The results of the electric field calibration indicate that over this pressure range, most of the second harmonic signal induced by the electric field in the discharge originates within the quartz wall of the discharge tube, as discussed in greater detail in Section 3. For this reason, two alternative calibration techniques have been tested and found to produce consistent results. Firstly, for all pressures, the absolute value of the electric field was obtained by comparing the second harmonic signal with the current shunt signals, after the ionization wave has reached the grounded electrode, i.e. at the conditions when the axial electric field distribution along the discharge gap is known to be uniform [START_REF] Klochko | Investigation of capillary nanosecond discharges in air at moderate pressure: comparison of experiments and 2D numerical modelling[END_REF]. In addition, the E-FISH signals obtained at 20 mbar were compared to the axial electric field profile calculated from the distribution of the electric potential along the discharge gap, obtained from capacitive probe measurements. From the calibration, the detection limit of the present E-FISH measurements is approximately 0.25 kV/cm.

Results and Discussion

Figure 3(a) shows a 200 shot averaged broadband plasma emission image measured in a ns pulse discharge in nitrogen at P=20 mbar, 3.2 ns after the beginning of the voltage rise. The camera gate is 1.17 ns, and the field of view is approximately 2.5 cm x 6.8 cm. In Fig. 3(a), the high-voltage electrode is on the left, the grounded electrode is on the right, and the flow is left to right. It can be seen that the emission intensity in nitrogen is significantly higher near the discharge tube walls, which is illustrated further by plotting the radial distributions of the emission intensity in Fig. 3(b), at the axial locations indicated by a dashed line in Fig. 3(a). This result, which is consistent with our previous experimental and kinetic modeling studies of fast ionization wave discharges in nitrogen [16,[START_REF] Lepikhin | Fast gas heating and radial distribution of active species in nanosecond capillary discharge in pure nitrogen and N2:O2 mixtures[END_REF], shows that the ionization wave is propagating predominantly near the walls of the discharge tube. Kinetic modeling predictions [16] reveal that this occurs due to the higher electric field near the wall, caused by the jump in permittivity, and due to the secondary photo-electron emission from the wall irradiated by the UV photons from the plasma. At higher pressures, when the electron transport becomes slower, this effect is more pronounced. It is interesting to note that such radial non-uniformity is also present in a capillary discharge sustained in a significantly smaller diameter quartz tube of 1.5 mm [START_REF] Lepikhin | Fast gas heating and radial distribution of active species in nanosecond capillary discharge in pure nitrogen and N2:O2 mixtures[END_REF].

Figure 4 plots the second harmonic signal calibration lines obtained by measuring the Laplacian field maintained between two circular, parallel plate electrodes D=20 mm in diameter and d=4 mm apart placed in the discharge tube, at four different pressures. The voltage pulse is applied to the electrodes by the same ns pulse generator as was used to sustain the discharge in the tube. The calibration data are taken only during the voltage rise, before breakdown occurs in the tube. The voltage rise is determined from the incident and reflected pulses measured by the current shunt connected to the high voltage electrode, while the occurrence of breakdown is inferred from the rise of the transmitted voltage pulse, measured by the current shunt connected to the grounded electrode. For all four pressures, the square root of the second harmonic signal is proportional to the applied electric field (see Fig. 4), as expected. For a plane wave approximation and for a uniform electric field, the line-of-sight integrated intensity of the horizontally polarized secondharmonic signal,

) 2 (  x I , is [28],   2 2 ) ( ) 3 ( ) 2 ( 2 / 2 / sin ] [ ~         k L k I E I x x xx x    . ( 1
)
In Eq. ( 1), x E is the horizontal (axial) component of the electric field, proportional to the number density), L is the length of the region where the electric field is applied, in the direction of the laser beam, and Δk is the difference between the fundamental and the second harmonic wave vectors. At the present conditions, the coherence length in nitrogen at P=20-100 mbar, Lc,N2 = π/(2kω -k2ω) ≈ 60-300 cm, and in ambient air, Lc,air ≈ 6 cm, is very long, such that the phase matching is not expected to affect the signal in the plasma or in ambient air, where it is therefore proportional to L 2 . On the other hand, the coherence length in quartz (fused silica) is much shorter, Lc,FS ≈ 10 μm, such that the wall thickness strongly affects the electric field induced second harmonic signal generated in the wall.

From Eq. ( 1), the square root of the second harmonic signal in a gas is proportional to the number density. However, from the data plotted in Fig. 4, it is readily apparent that varying the pressure in the tube by a factor of 5 has essentially no effect on the signal. This indicates that most of the signal is generated not in the nitrogen within the tube, but either in the quartz wall or in the atmospheric pressure air outside the tube, induced by the electric field extending beyond the space between the electrodes. The third order susceptibility of fused silica at 1064 nm is nearly 4 orders of magnitude larger compared to that of atmospheric pressure air, [START_REF] Guillet De Chatellus | Measurement of the third-order susceptibility of glasses by EFISH of femtosecond pulses[END_REF][START_REF] Gubler | Optical third-harmonic generation of fused silica in gas atmosphere: Absolute value of the third-order nonlinear optical susceptibility χ (3)[END_REF] [START_REF] Gubler | Optical third-harmonic generation of fused silica in gas atmosphere: Absolute value of the third-order nonlinear optical susceptibility χ (3)[END_REF][START_REF] Shelton | Measurements and Calculations of the Hyperpolarizabilities of Atoms and Small Molecules in the Gas Phase[END_REF]. The second harmonic signal from the ambient air would be comparable to that originating within the discharge tube wall only if it is produced over a distance of L ~ Lc,FS • ) 3 ( FS  / ) 3 ( air  ~ 7 cm. This is extremely unlikely, since the laser beam diameter over this distance is increasing from ≈ 0.5 mm at the wall to ≈ 4 mm, and since the electric field outside of the discharge tube is decreasing. We therefore conclude that at the present conditions, the signal is likely to be generated in the wall. This estimate is consistent with the results of Ref. [18], which show that for tight focusing configurations (i.e. for the lens focal distance of 15 cm and 30 cm), the second harmonic signal is also generated from the regions beyond the confocal beam parameter, such that the contribution of the electric-field induced signal from the tube wall could be dominant at the present conditions. Basically, the Laplacian field calibration indicates that a relatively slow reduction of the signal intensity with the distance from the beam focal point is offset by the much higher nonlinear susceptibility of fused silica. This suggests that in the present experiments the second harmonic signal is sensitive to the electric field generated within the wall of the discharge tube, such that the plane-to-plane Laplacian field calibration may well be inaccurate.

) 3 ( FS  =2•10 -22 m 2 /V 2
 =3•10 -26 m 2 /V 2
An alternative calibration method employed in the present work is based on a significant body of previous electric field measurements using calibrated back current shunts [START_REF] Anikin | Study of the oxidation of alkanes in their mixtures with oxygen and air under the action of a pulsed volume nanosecond discharge[END_REF] and capacitive probes [START_REF] Klochko | Investigation of capillary nanosecond discharges in air at moderate pressure: comparison of experiments and 2D numerical modelling[END_REF]15]. The results of these measurements show that after the ionization wave has reached the grounded electrode and the discharge gap is closed, the axial electric field distribution along the discharge tube becomes uniform and close to the difference between the potentials on the electrodes divided by the discharge gap. The potential difference between the electrodes, after the ionization wave "touchdown", is inferred from the two current shunt signals, one measuring the potential waveform in the incident and reflected voltage pulses (before the highvoltage electrode), and the other measuring the potential in the transmitted pulse (after the grounded electrode). For the absolute calibration, the square root of the second harmonic signal intensity is matched to the electric field obtained from the current shunt waveforms during the pulse voltage reduction, well after the wave reaches the grounded electrode. The confidence in the absolute calibration of the present measurements may well be increased by comparison with kinetic modeling, predicting electric field distributions both in the plasma and in the wall of the discharge tube, as has been done previously for electric field measurements using an electro-optic probe [START_REF] Bourdon | Numerical and experimental study of the dynamics of a μs helium plasma gun discharge with various amounts of N2 admixture[END_REF]. [START_REF] Anikin | Breakdown development at high overvoltage: electric field, electronic level excitation and electron density[END_REF][START_REF] Anikin | Study of the oxidation of alkanes in their mixtures with oxygen and air under the action of a pulsed volume nanosecond discharge[END_REF][START_REF] Klochko | Investigation of capillary nanosecond discharges in air at moderate pressure: comparison of experiments and 2D numerical modelling[END_REF], theoretical analysis, and kinetic modeling predictions [16]. The initial gradual rise of the signal corresponds to the Laplacian field generated ahead of the approaching ionization wave after it is initiated near the high-voltage electrode during the pulse-voltage rise. The peak occurs when the ionization front arrives at the measurement location, and the electric field begins to decrease rapidly, due to the charge separation in the plasma generated in the wave. The electric field behind the wave is reduced considerably, but remains above the detection limit at all pressures tested, such that the electrons created in the wave front are transported toward the high-voltage electrode, generating the conduction current. The secondary rise of the electric field occurs after the wave front arrives at the grounded electrode (cathode), the secondary emission is generated from the cathode, and the discharge transitions into a quasi-steady-state regime. Finally, the electric field decays to near detection limit as the applied voltage is reduced to zero.

Figure 6(a-d) plots time-resolved, absolute axial electric field distributions in a fast ionization wave discharge in nitrogen at P=20-100 mbar, shown together with the current shunt signal used for absolute calibration. It can be seen that the electric field waveform obtained by the E-FISH measurements matches the current shunt waveform after the moment when the wave reaches the grounded electrode (see Fig. 6(a-c), although at P=20 mbar the signal-to-noise ratio in the quasi-steady-state discharge becomes significantly lower (see Fig. 6(d)). At these low-pressure conditions, the emission from the quasi-steady-state discharge and the afterglow becomes much stronger, such that subtracting it from the PMT signal improves the signal-to-noise. The effect of the plasma emission in the afterglow is also visible but less pronounced at P=40 mbar (see Fig. 6(c)), where it has not been subtracted from the signal. At P=20 mbar, additional comparison has been made with the electric field measured by a calibrated capacitive probe, the technique described in detail in our previous work [15,[START_REF] Klochko | Investigation of capillary nanosecond discharges in air at moderate pressure: comparison of experiments and 2D numerical modelling[END_REF] (see Fig. 6(d)). Note that the capacitive probe measurements match very well the current shunt waveform in the quasi-steady-state discharge. However, it is readily apparent that the electric field peak in the ionization wave front, measured by the capacitive probe, is not fully resolved compared to the E-FISH data (the apparent peak is wider and the maximum value is lower). This behavior is expected, since the effective time resolution of the capacitive probe (~1.5-3.0 ns), limited by its spatial sensitivity, is up to an order of magnitude lower compared to that of the present E-FISH diagnostic (200 ps).

Comparison of the data taken at different pressures exhibits several dominant trends. First, in the entire pressure range, the peak electric field in the ionization wave exceeds the quasi-steadystate value established after the wave reaches the grounded electrode, by up to a factor of 4 to 5. This is much higher compared to a relatively modest electric field overshoot, ~10%, detected in a ns pulse discharge generated in 100 Torr of nitrogen across a relatively short gap of 13 mm, where the wave does not fully develop [19]. Second, the peak value of the axial electric field in the wave front exhibits a fairly weak pressure dependence, Epeak=8-11 kV/cm (compare Figs. 6(a-d)). The highest reduced electric field, E/N≈2000 Td, is produced at the lowest pressure of P=20 mbar, as expected (see Fig. 7). The gas density used in computing the reduced field assumes room temperature, which is anticipated given the low deposited energy density of this discharge. Third, reducing the pressure from 100 to 20 mbar, while keeping the discharge pulse voltage waveform the same, steepens the ionization wave front considerably, as illustrated in Fig. 7. At 100 mbar, the full width at half maximum of the transient electric field peak in the wave front is 3.0 ± 0.2 ns, while at 20 mbar it is reduced to 1.0 ± 0.2 ns, and remains fully resolved by the present diagnostic. This behavior is most likely due to the faster electron impact ionization at a significantly higher reduced electric field in the wave, as well as the more rapid electron transport behind the wave front, resulting in plasma self-shielding on a sub-ns time scale. Finally, the wave speed increases considerably as the pressure is reduced, from V = 0.4 cm/ns at 100 mbar to V = 1.6 cm/ns at 20 mbar. The wave speed at the present conditions was determined from the time-resolved plasma emission intensity distribution measurements, such as that shown in Fig. 3, with a relative uncertainty of 2%. The difference between the wave speed measured on the tube centerline and near the wall did not exceed 5%. The wave speed values are indicated in the legend of Fig. 7.

Summary

In this work, non-intrusive, time-accurate, absolute measurements of the electric field in a fast ionization wave discharge sustained over a wide range of pressures are obtained for the first time. Since the present results are based on the detection of the electric field within the discharge tube wall, they demonstrate that ps second harmonic generation using the present approach may be employed for electric field measurements at the conditions when the detection of the second harmonic signal generated directly in the plasma is challenging, i.e. (a) in low-pressure discharges, (b) in discharges sustained in small diameter capillary tubes, and (c) in discharges sustained in gases with very low nonlinear polarizability, such as helium. These results also show that the contribution of the discharge cell walls into the net electric-field induced second harmonic signal has to be identified and accounted for, e.g. using the electrostatic calibration over a wide range of pressures. Since the signal generated within the wall during the discharge is induced by the electric field, it cannot be easily subtracted from the net second harmonic signal, unlike the zero-field background.

In principle, the temporal resolution of the present measurements is limited only by the laser pulse duration, 30 ps, and may be improved further by using an oscilloscope with a higher sampling rate. In low and moderate pressure FIW discharges, this opens a possibility of detection and analysis of non-local electron kinetics effects in the presence of rapidly varying, high peak value reduced electric fields. Finally, with additional calibration, the same approach can be used to measure the radial component of the electric field in the ionization wave front. The present results can also be used for assessing the predictive capability of kinetic models of fast ionization waves generating breakdown on ns and sub-ns time scale.

International Journal of Hydrogen



  is the third-order nonlinear susceptibility (in gases,

Figure 5

 5 Figure 5 plots a typical data set (square root of the second harmonic signal before the calibration is applied), taken in the ionization wave discharge in nitrogen at P=100 mbar. In this figure, black symbols indicate the raw single-shot data points taken during a 2-hour long run, and red symbols show the averaged data points in 200 ps time bins, obtained during the data postprocessing. The error bars indicate statistical uncertainty (standard deviation using the Student's tdistribution) of the data. From Fig. 5, different phases of the discharge development, labeled in the figure, are readily identified and well understood, based on the previous lower-fidelity electric field measurements by capacitive probes [15,[START_REF] Anikin | Breakdown development at high overvoltage: electric field, electronic level excitation and electron density[END_REF][START_REF] Anikin | Study of the oxidation of alkanes in their mixtures with oxygen and air under the action of a pulsed volume nanosecond discharge[END_REF][START_REF] Klochko | Investigation of capillary nanosecond discharges in air at moderate pressure: comparison of experiments and 2D numerical modelling[END_REF], theoretical analysis, and kinetic modeling predictions[16]. The initial gradual rise of the signal corresponds to the Laplacian field generated ahead of the approaching ionization wave after it is initiated near the high-voltage electrode during the pulse-voltage rise. The peak occurs when the ionization front arrives at the measurement location, and the electric field begins to decrease rapidly, due to the charge separation in the plasma generated in the wave. The electric field behind the wave is reduced considerably, but remains above the detection limit at all pressures tested, such that the electrons created in the wave front are transported toward the high-voltage electrode, generating the conduction current. The secondary rise of the electric field occurs after the wave front arrives at the grounded electrode

Figure 1 .

 1 Figure 1. Schematic of the experimental apparatus and ps second harmonic generation diagnostic. BT: beam trap; DM: dichroic mirror (532 nm reflecting, 1064 nm transmitting); DP: dispersive prism; PD: photodiode; FL: plano-convex lens; PB: 532 nm polarizer; ID: iris diaphragm; BF: 532 nm bandpass filter; PMT: photomultiplier tube; LP: long pass filter; HWP: 1064 nm half-wave plate; HVG: high voltage generator; BCS1, BCS2: back current shunts before the high-voltage electrode and after the grounded electrode, respectively. The inset shows the voltage waveform on the highvoltage electrode, top view of the discharge tube is shown.

Figure 2 .

 2 Figure 2. Histograms showing distributions of 40,000 laser shots (orange) and data points where the EM noise was sufficiently low (purple), plotted together with the laser pulse energy distribution over 0.2 ns time bins (green).

Figure 3 .Figure 4 .

 34 Figure 3. (a) ICCD camera image of a broadband emission from the plasma. The electrode polarity, the flow direction, and the axial location where the emission intensity distribution across the discharge tube is plotted are shown. (b) Plasma emission intensity distribution at the indicated axial location. Nitrogen, pressure 20 mbar, flow rate 10 sccm, discharge tube inside diameter 20 mm, camera gate 1.17 ns.

Figure 5 .Figure 6 .Figure 7 .

 567 Figure 5. Typical second harmonic signal data set, taken in a ns pulse discharge in nitrogen at P=100 mbar, before the calibration is applied. Black symbols indicate single-shot data points, red symbols are averaged data points in 200 ps time bins. Error bars indicate statistical uncertainty (standard deviation) of the data.

  mbar, V = 1.6 cm/ns P = 40 mbar, V = 1.2 cm/ns P = 70 mbar, V = 0.6 cm/ns P = 100 mbar, V = 0.4 cm/ns

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Acknowledgments

The work was partially supported by French National Research Agency, ANR (ASPEN Project), LabEx Plas@Par and the French-Russian international laboratory LIA KaPPA 'Kinetics and Physics of Pulsed Plasmas and their Afterglow'. The experiments were conducted at the Centre Laser de l'Université Paris Sud (CLUPS/LUMAT FR 2764). The authors are thankful to Ali Mahjoub, Bruno Dufour and Pascal Pariset for technical assistance, and to Dr. Michel Broquier and Dr. Gilles Grégoire for helping setting up these experiments. The support of Prof. Adamovich by the Ecole Polytechnique Gaspard Monge Visiting Professor (GMVP) Program is gratefully acknowledged.


Mireille Babineau
email: mireillebbineau@gmail.com

Rushen Shi

Anne Christophe

EXPECTATIONS ABOUT NOVEL WORDS 1 14-month-olds exploit verbs' syntactic contexts to build expectations about novel words

14-month-olds exploit verbs' syntactic contexts to build expectations about novel words

Introduction

When acquiring their native language, infants face the great challenge of assigning words to grammatical categories (e.g. noun, verb, adjective, etc.) and drawing inferences about their probable meanings. Since infant-directed speech is for the most part made up of sentences rather than isolated words [START_REF] Aslin | Segmentation of fluent speech into words : learning models and the role of maternal input[END_REF][START_REF] Brent | The role of exposure to isolated words in early vocabulary development[END_REF]van de Weijer, 1998), infants base their inferences about novel words mainly from the surrounding cues, such as the neighboring words (as well as the visual context). This situation is not problematic according to the well-known mechanism called syntactic bootstrapping (e.g., [START_REF] Arunachalam | Preschoolers' Acquisition of Novel Verbs in the Double Object Dative[END_REF][START_REF] Bernal | Syntax Constrains the Acquisition of Verb Meaning[END_REF][START_REF] Fisher | Syntactic bootstrapping[END_REF][START_REF] Gillette | Human simulation of vocabulary learning[END_REF][START_REF] Gleitman | The Structural Sources of Verb Meanings[END_REF], since the syntactic context of a word is highly informative with regard to its category, constraining its possible meaning and guiding children's visual attention towards specific parts of the world. For instance, a novel word such as "larp" appearing in a noun position (e.g., "This is a larp") can be interpreted as referring to a novel object, whereas if it appears in a verb position (e.g., "He is larping that"), it can be interpreted as referring to a novel event1. Accordingly, several studies have shown that infants from 18 months of age are able to use the other words of a sentence to infer the probable meaning of a novel word (e.g., [START_REF] Arunachalam | Grammatical Form and Semantic Context in Verb Learning[END_REF][START_REF] Bernal | Syntax Constrains the Acquisition of Verb Meaning[END_REF]de Carvalho, He, Lidz, & Christophe, 2018;[START_REF] He | Verb Learning in 14-and 18-Month-Old English-Learning Infants[END_REF][START_REF] Oshima-Takane | Early verb learning in 20-month-old Japanese-speaking children[END_REF][START_REF] Waxman | Twenty four-month-old infants' interpretations of novel verbs and nouns in dynamic scenes[END_REF]. These findings beg the question of how young children come to learn about syntactic contexts.

Syntactic contexts are mostly formed of function words (e.g., "the," "a," "she," "they"), which are so frequent that infants come to process, store, and recognize them during their first year of life (e.g., Hallé, Durand, & de Boysson-Bardies, 2008;[START_REF] Höhle | German-learning infants' ability to detect unstressed closed-class elements in continuous speech[END_REF]Shafer, Shucard, Shucard, & Gerken, 1998;[START_REF] Shi | Frequency and form as determinants of functor sensitivity in English-acquiring infants[END_REF][START_REF] Shi | The effect of functional morphemes on word segmentation in preverbal infants[END_REF]; for a review: [START_REF] Shi | Functional Morphemes and Early Language Acquisition[END_REF]. Soon after their first birthday, infants display a rudimentary understanding of grammatical categories based on their knowledge and use of frequent function words. Specifically, studies have found that infants can build expectations about the syntactic context in which novel nouns are likely to be next encountered, based on the contexts in which they have previously been heard [START_REF] Höhle | Functional elements in infants' speech processing: The role of determiners in the syntactic categorization of lexical elements[END_REF][START_REF] Shi | Syntactic Categorization in French-Learning Infants[END_REF]. For example, in [START_REF] Shi | Syntactic Categorization in French-Learning Infants[END_REF], French-1 In the sentence "This is a larp", the novel word "larp" appears in a noun position, because it follows a determiner (it is part of a DP), and is part of a complement of the copula "is". In the sentence "He is larping that", "larp" appears in a verb position because it follows the auxiliary verb "is" as the main verb (with aspectual morphology), and is in a transitive frame. learning 14-month-olds were familiarized with two new words (e.g. mige and crale), preceded by two function words, either two pronouns (je "I" and il "he") or two determiners (des "some" and ton "your"). Both groups were tested with trials presenting a different noun context (e.g., le mige "the mige") and trials presenting a different verb context (e.g., tu miges "you mige").

The group familiarized with the noun phrases distinguished the two types of test trials, exhibiting longer looking times when the new content words were presented in the incongruent verb context. These results suggest that around 14 months of age, infants already know (some of) the syntactic contexts that support nouns, since they can generalize that a novel word heard in the context of two determiners is likely to also co-occur with a third one. In contrast, the 14month-olds in [START_REF] Shi | Syntactic Categorization in French-Learning Infants[END_REF], as well as in Höhle et al. (2004, in German), did not seem able to draw any inference from having heard a novel word in verb contexts, since they showed no listening preference, at test, for the novel words occurring in noun vs verb contexts.

Sensitivity to verb contexts for novel verb categorization seems to only emerge a few months later (e.g., [START_REF] Massicotte-Laforge | The role of prosody in infants' early syntactic analysis and grammatical categorization[END_REF].

Tracking the syntactic contexts of words may be an essential first step towards discovering the link between syntactic and semantic categories. From the literature discussed above, it is clear that shortly after their first year of life infants have managed to efficiently track noun contexts, i.e. nouns can follow different determiners [START_REF] Shi | Syntactic Categorization in French-Learning Infants[END_REF][START_REF] Höhle | Functional elements in infants' speech processing: The role of determiners in the syntactic categorization of lexical elements[END_REF]). However, it is less clear if they manage to do so with verb contexts involving subject pronouns, since previous work failed to show evidence of generalization to different subject pronouns at that age. The distributional properties of nouns and verbs may partly explain this discrepancy in infants' ability to build expectations about novel words in laboratory settings.

Based on a corpus in French (Cécyre & Shi, 2005), nouns appear in more stable contexts (i.e. consistently co-occurring with a preceding determiner in 71% of cases, or with a preceding adjective, in 26% of cases) than do verbs (i.e. preceded by a subject pronoun, 59%).

Nonetheless, the co-occurrence of verbs with subject pronouns might still be noticeable early on. Given that at 14 months of age infants have acquired a receptive vocabulary that includes both frequent nouns and verbs (e.g. [START_REF] Bergelson | The Acquisition of Abstract Words by[END_REF], 2015), it is plausible that they also track the contexts in which those known words occur. If that is the case, and infants have linked subject pronouns (e.g., il "he", tu "you") to known verbs (memorizing sequences such as il mange "he eats", tu manges "you eat"), then presenting known verbs along with tobe-learned novel words might help infants access what they already know about verb contexts.

When encountering a novel word such as dase in the same contexts as a known word (e.g., il dase "he dases", je dase "I dase", il mange "he eats", je mange "I eat"), infants could expect dase to occur in the syntactic context tu dases "you dase", based on its similarity with the known verb. This account is highly plausible based on a substantial body of work showing that preverbal infants are able to track patterns of co-occurrences in speech-only tasks (e.g., Gòmez & Gerken, 1999), enabling them to create form-based categories around 12 months of age (e.g., [START_REF] Gerken | Infants can use distributional cues to form syntactic categories[END_REF][START_REF] Gómez | A first step in form-based category abstraction by 12month-old infants[END_REF].

The idea that known words play a role in early syntactic development has recently been put forward by the semantic seed hypothesis [START_REF] Christophe | Bootstrapping the syntactic bootstrapper[END_REF]Gutman, Dautriche, Crabbé, & Christophe, 2014, see also [START_REF] Christodoulopoulos | An incremental model of syntactic bootstrapping[END_REF], which also reiterates the importance of distributional cues for dividing words into syntactic categories. Based on this hypothesis, infants could exploit their early lexicon as a seed for assigning novel words into categories. The meaning of initial lexical items may be acquired from visual cues and through cross-situational learning. Using their distributional learning skills, infants would then track the syntactic contexts of these known words (e.g., 'ball' and 'car' appear in "This is a ball", "This is a car"), and later infer that a novel word appearing in the same context is likely to share some properties with these known words (e.g., "This is a" + "dax" -> dax = object). This end-point follows the established proposal that young children expect words that share conceptual or semantic properties to occur in similar syntactic contexts [START_REF] Gleitman | The Structural Sources of Verb Meanings[END_REF]Pinker, 1984). In sum, before building semantic expectations about novel words based on syntactic cues (e.g. making deductions about the intended object referent via syntactic bootstrapping), infants would be capable of tracking cooccurrence patterns between function words and familiar content words.

The present research aimed at examining infants' ability to use words they already know to build expectations about the kind of syntactic contexts in which novel words would be likely to occur. Although our design was inspired by the distributionally-based assumptions of the semantic seed hypothesis, we do not provide a direct test of its underlying mechanism. In Experiment 1, we tested 14-month-olds' ability to build expectations about novel words based on function word knowledge only. We expected to replicate previous findings from Shi and Melançon's study, namely that 14-month-olds would show the ability to build expectations about novel words encountered in noun contexts, but not about novel words encountered in verb contexts. In subsequent experiments, we examined whether infants can exploit the presence of a few known content words during the familiarization phase, occurring in the same contexts as the novel words, to boost their performance: that is, to build expectations about the syntactic contexts in which the novel verbs would likely appear (i.e. following a pronoun in the test phase given that they followed pronouns in the familiarization phase). Note that the presence of a few known words within the familiarization phase makes the experiment more ecological, in that it is closer to what infants around one year of age hear in real life (i.e., novel words are likely to be introduced within conversations that also include known words).

Experiment 1: Building expectations based on co-occurring function words alone

In Experiment 1, two groups of 14-month-olds were habituated to two novel words presented either in noun contexts (following determiners, Novel Nouns condition) or in verb contexts (following pronouns, Novel Verbs condition). To test whether infants were able to build expectations about upcoming contexts for the novel words, both groups were tested with trials presenting these same novel words within a different noun context and trials presenting them within a different verb context. Hence, one type of test trials presented the novel words in a context that was congruent with the one used during familiarization, while the other test trials presented them in an incongruent context. As in [START_REF] Shi | Syntactic Categorization in French-Learning Infants[END_REF], differential looking times during congruent vs incongruent trials would indicate that infants successfully predicted the type of function words that could co-occur with the novel words.

Method

The study reported in this paper, including the entire method, analysis and criteria for exclusion of participants were pre-registered on the OSF (Open Science Framework) database before running the experiment. The preregistration2 can be accessed with the following link: https://osf.io/psqwk/?view_only=b41cae1698d24628aae24bc63271382b. The materials, collected data, and data analyses are also freely available to readers through this link.

Participants

A total of 56 French-learning monolingual 14-month-olds (28 in each group) with no known hearing problems participated in the experiments (Novel nouns group: mean age: 14 months, 0 day; age range: 13 months, 18 days -14 months, 14 days; 14 girls; Novel verbs group: mean age: 14 months, 0 day; age range: 13 months, 18 days -14 months, 13 days; 17 girls. An additional 12 infants were tested in the Noun condition, but excluded from the analyses due to extreme fussiness/crying (8), failure to habituate (1) or parental interference (3). An additional nine infants were tested in the Verb condition, but excluded from the analyses due to fussiness/crying (4), failure to habituate (2), technical failure (1), parental interference (1) or experimenter error (1).

The present study was conducted at the babylab of the École Normale Supérieure in Paris according to guidelines laid down in the Declaration of Helsinki, with written informed consent obtained from a parent or guardian for each child before any assessment or data collection. Since our study involves human subjects, it was carried out in accordance with the recommendations of our local ethics committee, the CERES (Comité d'éthique de la recherche en santé -Paris), which approved our protocol beforehand.

Stimuli

We used two pseudo-words, dase [daz] and nuve [nyv], which conform to the phonological structure of French. They appeared in short utterances, either in a noun position after a determiner (i.e., in a Det + N structure, where Det was: des "some", ton "your"; and un "a" for the test phase), or in a verb position after a subject pronoun (i.e. in a Pron + V structure where Pron was: il "he", je "I"; and tu "you" for the test phase).

A female native French speaker (the last author) recorded multiple repetitions of the twelve utterances in child-directed speech style (sampling frequency 44.1 kHz). All utterances were used in the habituation phase, except for utterances using the determiner un "a" and the pronoun tu "you" which were reserved for the test phase. The final stimuli consist of three exemplars of the eight habituation utterances (four noun contexts, four verb contexts), and six exemplars for each of the four test utterances.

For each condition, we created three different habituation lists which contained 16 utterances in different orders (e.g. for the Novel Nouns condition, each list contained four repetitions of des dases [dedaz], des nuves [denyv], ton dase [tɔ daz], ton nuve [tɔ ñyv]; see Table 1 for a full list of utterances). We also created two different lists used as test trials (one for verb contexts, and one for noun contexts) which contained 16 utterances (e.g. for the test trials with noun contexts, there were eight repetitions of un nuve and eight of un dase). The maximal length of a habituation trial was 29.5 sec, whereas the maximal length of a test trial was 27.5 sec. Within a trial (habituation and test), the order of the utterances was pseudorandomized with two restrictions: A particular utterance could not appear more than twice in a row, and the same novel pseudo-word could not occur more than twice in a row. In French, the final "s" in plural nouns is silent, making it possible to create ambiguous new content words (e.g., dase is pronounced as [daz] in both types of utterances, noun context: des dases [dedaz] "some dases", verb context: il dase [ildaz] "he dases"). In addition, the acoustic properties of the novel pseudo-words in noun versus verb contexts were carefully balanced (see Table 2 for the mean acoustic measures for all stimuli). An animation of a bird moving its mouth in synchrony with the audio stimuli was presented during each trial. As an attention-getter to attract infants' attention towards the screen between trials, we used a silent video of a butterfly perched on a leaf.

Design

The experiment contained two phases, a habituation phase and a test phase. Infants were randomly assigned to one of two habituation conditions (i.e. Novel Nouns condition vs Novel Verbs condition). Infants in the Novel Nouns condition were familiarized with trials presenting the two pseudo-words preceded by two determiners: ton "your" and des "some". Infants in the Novel Verbs condition were familiarized with trials presenting the pseudo-words preceded by two pronouns: je "I" and il "he". Once infants reached the habituation criterion (see Procedure), the habituation phase ended and the test phase started automatically.

In the test phase, both groups were presented with the same stimuli containing the pseudo-words. There were two types of test trials: verb contexts involving the pronoun tu "you", and noun contexts involving the article un "a". Both trial types contained a context that was not used in the habituation (i.e. a different pronoun and a different article); the only difference between the two trial types was that one is a verb context, and the other a noun context. Hence, one type of test trials was congruent with the syntactic contexts in which the pseudo-words were presented during the habituation and the other one was incongruent. For instance, for infants in the Novel Verb condition, the test trials presenting the pseudo-words following the pronoun tu (i.e. tu dases, tu nuves) were congruent with the habituation phase, whereas the test trials presenting the pseudo-words following the article un (i.e. un dase, un nuve) were incongruent.

Procedure

Infants were tested individually using a habituation paradigm implemented in a central fixation procedure. In a sound-attenuated double-walled booth, each infant sat on its caregiver's lap about 70 cm in front of a wall-mounted 27-inch monitor. Caregivers were instructed not to talk or point towards the screen. Sound was presented to the infant through two loudspeakers, positioned on each side of the monitor. Caregivers listened to music through noise-canceling headphones to prevent them from hearing the stimuli. A camera above the monitor was connected to a monitor placed outside the booth. This set-up enabled the experimenter to observe infants' reaction from an adjacent room without being aware of the stimuli presentation.

We presented the stimuli using Habit (Cohen, Atkinson, & Chaput, 2004). From the adjacent room, the experimenter pressed a computer key to initiate a trial when the infant looked toward the monitor, and she pressed another key whenever a look toward the monitor occurred during a trial. Trials ended either after the infant looked away from the screen for more than two consecutive seconds or after the maximum trial length was reached. A new trial started when the infant looked back towards the screen.

The habituation criterion was reached when an infant's average looking time during any block of three consecutive trials dropped to less than 65% of the average looking time for the most-attended block (i.e., the 3-trial block that has the longest total looking time). The total number of habituation trials each infant received could be different (between a minimum of four and a maximum of 12 trials). The presentation order of the habituation trials was counterbalanced across infants, and the three trials formed a block which could be repeated up to four times (i.e., each trial could either be presented as the first, second or third trial of the block). The test phase began either with a trial presenting the pseudo-words in verb contexts or a trial presenting the pseudo-words in noun contexts. The order of presentation was counterbalanced across infants (N V N V or V N V N). The two types of trials alternated for a total of four trials (two of each type).

Our experiment was inspired by [START_REF] Shi | Syntactic Categorization in French-Learning Infants[END_REF], with three main differences. First, Shi and Melançon used a familiarization paradigm with a predetermined 30 sec exposure, whereas we chose to use a habituation paradigm with a length of exposure determined by the infant. A habituation paradigm has the advantage of facilitating the prediction of the results, since a novelty effect (i.e. here, longer looking times during incongruent trials) is expected. Second, we chose different pseudo-words (i.e. nuve and dase vs mige and crale in Shi & Melançon). Third, we chose the indefinite determiner un "a" for the test phase instead of the definite le "the", since le can act as an object clitic when preceding a verb (e.g., je le mange "I eat it") as well as a determiner (e.g., le ballon, "the ball").

Results and discussion

Each infant's average looking times for the test trials presenting noun contexts and for those presenting verb contexts were calculated. The data for all infants were analyzed in a 2 x 2 analysis of variance (ANOVA) with Condition (Novel Nouns vs Novel Verbs habituation phase) as a between-participant factor, and Test Trial Type (noun vs verb context) as a withinparticipant factor. The results showed no effect of Condition (F (1,54)=.036, p=.85, η²=.001), nor of Test Trial Type (F(1, 54)=.824, p=.368, η²=.014). However, the interaction between Condition and Test Trial Type was significant (F(1,54)=5.64, p=.021, η²=.093). Follow-up paired t-tests assessed infants' performance in each condition. For the Novel Nouns condition, looking time was significantly longer for the trials presenting verb contexts (i.e. incongruent trials), (M=13.71, SE=1.364), than for the trials presenting noun contexts (i.e. congruent trials), (M=10.51, SE=1.248), t (27) = 2.135, p =.042, d=.404. However, infants in the Novel Verbs condition showed no significant difference between the congruent test trials presenting verb contexts (M=11.12, SE=1.269) and the incongruent test trials presenting noun contexts (M=12.54, SE=1.048), t (27) = -1.147, p=.262, d=-.217.

As shown in Figure 1, infants in the Novel Nouns condition looked significantly longer when the context was incongruent with their expectations, that is, when the novel words appeared in a verb context (red bars), than when it was congruent with their expectations, that is, when the novel words appeared in a noun context (blue bars). A total of 20 infants out of 28 showed that pattern of looking times, which is consistent with the interpretation that infants from the Novel Nouns condition were able to create expectations as to the syntactic context in which the novel words were likely to be next encountered: based on the habituation phase, in which the novel words occurred in noun contexts (following determiners), they were able to expect the novel nouns to occur after another determiner, but not after a subject pronoun. In contrast, infants in the Novel Verbs condition did not look significantly longer to the incongruent noun context relative to the congruent verb context, with only 15 infants out of 28 showing the expected pattern of listening times (although numerically, there was a trend in the right direction). Crucially, the significant interaction between Condition and Test Trial Type shows that the two groups of infants behaved significantly differently, therefore the longer looking times for incongruent contexts for infants from the Novel Nouns condition is not due to an intrinsic preference for verb contexts. These results confirmed previous findings, since [START_REF] Shi | Syntactic Categorization in French-Learning Infants[END_REF] also observed that 14-month-olds exposed to noun contexts successfully generalized to another noun context, while infants familiarized with verb contexts did not show the ability to generalize to another verb context. and Novel Verbs, Experiment 2: Novel Verbs + Known Verbs, Experiment 3: Novel Verbs + Mixed Known Words). Error bars represent the standard error of the mean. On the right side: Boxplot of the difference between looking times during the noun contexts and the verb contexts test trials for the four conditions. The black line represents the median. Each dot represents one participant. Successful categorization is indicated by longer looking times during the incongruent context (verb contexts for the Novel Nouns condition; noun contexts for the three Novel Verbs conditions).

Experiment 2: Can familiar lexical words boost infants' ability build expectations for verbs?

In Experiment 2, 14-month-olds were habituated to two novel words in verb contexts (following pronouns), just as in Experiment 1. To test whether presenting known verbs during the habituation phase would increase infants' performance, the habituation trials also contained familiar verbs in the same contexts as the novel words (e.g., il dase "he dases", il mange "he eats"). The same test trials as in Experiment 1 (i.e. a noun context vs another verb context) were used. If infants learn co-occurrence patterns by relying on a handful of words they know, then reminding them which known words occur in verb contexts might help them build expectations about what other contexts would be valid for the novel words (i.e. following the pronoun tu "you", but not following the determiner un "a").

Participants

A total of 28 French-learning monolingual 14-month-olds with no known hearing problems participated in the experiment (mean age: 14 months, 0 day; age range: 13 months, 17 days -14 months, 13 days; 14 girls). An additional 14 infants were tested, but excluded from the analyses due to extreme fussiness/crying (6), failure to habituate (3), parental interference (3), technical failure (1) or experimenter error (1).

Stimuli, design and procedure

The design and the procedure were the same as in Experiment 1, except that only one group of infants was tested, and this group was familiarized with verb contexts. In addition to the two pseudo-words, we used four frequent known verbs: donne "give", dort "sleep", mange "eat", regarde "look". These familiar verbs appeared in the same contexts as the pseudo-words, following the pronouns il "he" and je "I". These eight utterances were recorded by the same speaker as in Experiment 1, and three exemplars of each utterance were kept for the final stimuli. We created three new habituation lists including both the utterances with novel words (those used in Experiment 1) and the ones with known verbs. The utterances were pseudorandomized with two restrictions: a particular utterance could not appear twice in a row, and one of the novel pseudo-word occurred at least once every two utterances (e.g., il nuve, il regarde, je nuve, je dors). Each habituation list contained 16 utterances (half with familiar verbs and half with pseudo-verbs) in different orders, and lasted 29.5 sec. The stimuli for the test phase were identical to those in Experiment 1.

Results and discussion

As in Experiment 1, each infant's average looking time for the test trials presenting noun contexts and for those presenting verb contexts were calculated. The data from the Novel Nouns condition of Experiment 1 were analyzed along with the Novel Verbs + Known Verbs condition from Experiment 2 in a 2 x 2 analysis of variance (ANOVA) with Condition (Novel nouns vs Novel Verbs + Known Verbs) as a between-participant factor, and Test Trial Type (noun vs verb syntactic context) as a within-participant factor. The results showed no main effect of Condition (F (1,54)=.196, p =.66, η²=.004), nor of Test Trial Type (F(1, 54)=.167, p=.685, η²=.003). As in Experiment 1, the interaction between Condition and Test Trial Type was significant (F (1, 54)=8.411, p=.005, η²=.134), i.e. the two groups behaved differently. A follow-up paired t-test assessed infants' discrimination in Experiment 2. This time, with a habituation to both pseudo-words and familiar verbs following pronouns, the difference between the congruent test trials presenting verb contexts (M=11.60, SE=1.210) and the incongruent test trials presenting noun contexts (M=14.01, SE=1.393) approached significance, t (27) = -1.971, p=.059, d=-.372. A total of 18 infants out of 28 showed the expected pattern of listening times3.

The presence of familiar verbs in the habituation phase slightly increased 14-montholds' looking times towards syntactic contexts that were incongruent with what they heard 3 Note that infants' performance in Experiment 2 cannot be explained by a difference in the amount of exposure to the novel verbs. While the average habituation time in Experiment 1 (Novel Verbs) and Experiment 2 is comparable (Experiment 1: M=127.5 sec; range=24.7-271.9 sec vs Experiment 2: 126.08; range=37-263 sec), the average exposure to the novel words was higher in Experiment 1 (M=69.2) than in Experiment 2 (M=34.19), since familiar verbs were added in the latter condition. during familiarization. To test whether infants' behavior differed between Experiment 1 and Experiment 2, we conducted a direct comparison between the two verb groups (Novel Verbs in Exp. 1 vs Novel Verbs + Known Verbs in Exp. 2): this analysis did not reveal a significant interaction between Condition and Trial Type (F(1,54)=.315, p=.58, η²=.005), suggesting that the two groups of infants behaved similarly. However, it did reveal a significant main effect of Test Trial Type (F(1,54)=4.834, p=.032, η²=.082), reflecting the fact that infants looked significantly longer overall for the incongruent syntactic contexts. This result suggests that 14month-olds were able to build expectations about the syntactic contexts in which the novel verbs were likely to appear (i.e. following a subject pronoun). This effect appears to not be very strong, since it is observed only with the added power of the two groups of infants pooled together (56 infants total).

Although the analysis pooling Experiments 1 and 2 together suggests that infants are able to build expectations about the contexts in which novel verbs are likely to occur, 14month-olds might have shown the observed behavior simply because they noticed that the habituation phase contained only one kind of function words (only personal pronouns), and they were surprised at test to hear a function word from a different category (and this holds for both Experiment 1 and 2). Under that interpretation, infants might have ignored the novel words, and reacted to an unexpected function word during the test phase; as a result, they might not have made any predictions linked to the novel words themselves. To test whether infants paid attention to the novel words (not only to the function words), and used the familiarized verb contexts to build expectations about the novel verbs, the familiarization phase in Experiment 3 included both known verbs and known nouns (in addition to the novel verbs). This follow-up experiment alleviates the above concern, in that it allows us to test the hypothesis that infants use the specific contexts in which novel words have been encountered to build syntactic expectations about these novel words. It also provides a welcome replication.

Experiment 3: Can infants build expectations about novel verbs based on the specific syntactic contexts in which they appear?

In this experiment, 14-month-olds were familiarized with a mixed list containing novel words in verb contexts (preceded by subject pronouns), known verbs (also preceded by subject pronouns), and known nouns (preceded by determiners). If infants group the novel words with the familiar verbs (and not with the familiar nouns) based on the fact that the novel words appeared in verb contexts, then they should look longer during the test trials presenting the novel verbs in incongruent noun contexts. Such a result would suggest that infants do pay attention to co-occurring function words: they would not group words together simply because they both appear at the end of utterances (e.g. group "eats" and "balls" after hearing "he eats" and "the balls"), but rather group words which appear in the same syntactic contexts (e.g., group "eats" and "sleeps" together after hearing "he eats" and "he sleeps").

Method

Participants

A total of 28 French-learning monolingual 14-month-olds with no known hearing problems participated in the experiments (mean age: 14 months, 0 day; age range: 13 months, 16 days -14 months, 9 days; 18 girls). An additional 12 infants were tested in the Novel Verbs + Mixed Known Words condition, but excluded from the analyses due to extreme fussiness/crying (6), failure to habituate (1), technical failure (2) or parental interference (3).

Stimuli, and procedure

As in the previous experiments, three habituation lists were prepared. These lists were twice as long as in Exp. 1 and 2 (i.e. 58 sec instead of 29.5 sec) since we aimed at providing as much variety while ensuring that infants would get enough repetitions of the pseudo-words and enough support from the crucial type of semantic seed (i.e. familiar verbs). The familiar nouns were the following: ballon "ball", chat "cat", doudou "blankie", livre "book". They appeared in noun contexts, i.e. following the determiners des "some" and ton "your". Each trial presented 16 utterances with the pseudo-words, and 16 utterances with familiar words (half with known verbs, half with known nouns). As in the previous experiments, the utterances were pseudorandomized with two restrictions: a particular utterance cannot appear twice in a row, and one of the novel pseudo-word occurs at least once every three utterances. Furthermore, in order to facilitate the recognition of familiar words, the two contexts involving a particular word (e.g. mange "eat") were only separated by three other utterances (e.g., je mange "I eat", des doudous "some blankets", je nuve "I nuve", il dase "he dases", il mange "he eats"). That is, since the number of familiar words presented in the habituation lists doubled, we aimed at facilitating recognition and activation by presenting two cases of a particular familiar word in a short time window. Once again, the test trials were the same as those used in Experiment 1. The procedure is the same as in the previous conditions.

Results

In a series of three pre-registered experiments, we showed that 14-month-olds who are habituated to novel words preceded by common function words can build expectations about the syntactic contexts in which they are likely to be next encountered (co-occurring with a determiner or a pronoun). We first confirmed earlier findings indicating that 14-month-olds expect novel words heard in noun contexts (preceded by determiners) to later occur in other noun contexts (in French: [START_REF] Shi | Syntactic Categorization in French-Learning Infants[END_REF]in German: Höhle et al., 2004). Crucially, we went on to demonstrate that infants' sensitivity to the co-occurrence of verbs with pronouns begins much earlier than previously understood. At 14 months, French-learning infants can infer that a novel word heard in the context of subject pronouns is more likely to be heard following a different subject pronoun, than following a determiner, as shown in the overall analysis pooling together infants from all three verb groups (total of 84 infants). This effect was present even when familiar verbs and familiar nouns were both presented along with the novel words during the habituation phase (Experiment 3). Hence, when infants were familiarized with pronouns followed by novel words, they were not simply expecting more pronouns to occur later on. Instead, they paid attention to the specific co-occurrences between pronouns and novel words to build expectations about the other contexts that these novel words are likely to occur in. We started out this set of experiments under the hypothesis that 14-month-olds were in all likelihood able to track co-occurrence patterns between function words/morphemes and verbs (in languages such as French or English), since only a few months later, at the age of 18 months, they can be shown to exploit such co-occurrences to infer the probable meaning of a novel word (e.g. 'he's larping', 'larp' likely refers to an action; [START_REF] He | Verb Learning in 14-and 18-Month-Old English-Learning Infants[END_REF][START_REF] De Carvalho | Prosody and Function Words Cue the Acquisition of Word Meanings in 18-Month-Old Infants[END_REF]. Because previous experiments had failed to reveal an ability to track verb distributional contexts [START_REF] Shi | Syntactic Categorization in French-Learning Infants[END_REF][START_REF] Höhle | Functional elements in infants' speech processing: The role of determiners in the syntactic categorization of lexical elements[END_REF], and because we think that children have to rely on known content words in order to learn which contexts correspond to which categories (e.g. objects/actions), we speculated that adding familiar content words to the habituation lists might help infants to retrieve their knowledge about verb contexts. We did not find clear evidence that this was so, since there were no significant differences in infants' performance between the three conditions presenting novel verbs (with or without familiar words during the habituation phase). At any rate, even if adding familiar words to the habituation phase had significantly improved infants' performance in the task, this finding could not have been taken as direct evidence of the process through which they had learned distributional regularities. The improvement might simply be due to the fact that the task was made less boring, and/or more ecological (i.e. familiar words are present in typical childdirected input, and these familiar words might act as helpful contrasts/anchors for novel words).

Since our experiments did not test the meaning associated with the novel words, we can say nothing about whether infants went all the way up to evaluating their likely semantic properties, or whether they remained at the stage where only distributionally-based predictions (likelihood of co-occurring function words) are generated. It is unknown whether, at 14 months of age, infants can expect the novel words not only to appear in the same contexts as the familiar verbs, but also to share some semantic properties with them based on their conceptual category (e.g., refer to an action that can be performed by an agent). Note that teaching novel verb-event associations has been challenging in laboratory settings at this age. For instance, while 18month-olds have been shown to categorize a novel verb and infer its meaning based on its syntactic context in recent studies using a habituation-switch paradigm (in French: de Carvalho, [START_REF] De Carvalho | Prosody and Function Words Cue the Acquisition of Word Meanings in 18-Month-Old Infants[END_REF]in English: He & Lidz, 2017), younger infants (14-month-olds) failed to do so with the same design [START_REF] He | Verb Learning in 14-and 18-Month-Old English-Learning Infants[END_REF]. Even older children have difficulty learning the meaning of novel verbs in 'sparse' contexts (when the referent of the subject pronoun is uncertain, as in our study; see [START_REF] Arunachalam | Grammatical Form and Semantic Context in Verb Learning[END_REF][START_REF] Syrett | Slowly but Surely: Adverbs Support Verb Learning in 2-Year-Olds[END_REF]. Hence, given young infants' limited processing and mapping skills, novel paradigms need to be developed in order to discover the full scope of infants' early representations of newly-learned words during the initial stages of syntactic and lexical acquisition.

In sum, we have shown that by 14 months, infants notice the syntactic contexts in which known nouns and verbs occur. Although verbs occur in less consistent contexts than nouns in French, French-learning infants nonetheless pay attention to their co-occurrence patterns with specific function words. This knowledge is a powerful tool that can be deployed when new words are encountered, enabling infants to build expectations about their properties. Most importantly, infants around this age have already acquired rudimentary levels of syntactic and lexical knowledge that are highly connected, and greatly influence one another.

Figure 1 .

 1 Figure 1. On the left side: Mean looking time in seconds during the two trial types, i.e. noun contexts (in blue) and verb contexts (in red), for the conditions (Experiment 1: Novel Nouns

Table 1 .

 1 Stimuli of Experiment 1

	Habituation
	Novel Nouns condition:	Novel Verbs condition:
	Des dases "Some dases"	Je dase "I dase"
	Des nuves "Some nuves"	Je nuve "I nuve"
	Ton dase "Your dase"	Il dase "He dases"
	Ton nuve "Your nuve"	Il nuve "He nuves"
	Test
	Noun context: un dase "a dase", un nuve "a nuve"
	Vs
	Verb context: tu dases "you dase", tu nuves "you nuve"

Table 2 .

 2 Average acoustic values (and SD) across multiple exemplars of the novel words (NW) and function word (FW) stimuli.

	Acoustic measure	Mean for noun uses Mean for verb uses	2-tailed t-tests*
	Total utterance duration (sec)	1.097 (.172)	1.116 (.21)	t (46) <1
	FW duration (msec)	179 (44)	179 (40)	t (46) <1
	FW mean pitch (Hz)	268.6 (65.15)	271.6 (67.92)	t (46) <1
	FW mean intensity (dB)	78.42 (3.145)	78.16 (3.182)	t (46) <1
	NW duration (msec)	918 (147)	937 (196)	t (46) <1
	NW vowel duration (msec)	577 (13)	578 (154)	t (46) <1
	NW vowel mean pitch (Hz)	260.4 (28.32)	253.7 (27.95)	t (46) <1
	NW vowel mean intensity (dB)	77.37 (2.605)	76.7 (2.167)	t (46) <1
	*Note that t values varied from -1 to 1, with p >.05 for all analyses.

Note that an amendment to the original pre-registration containing information about Experiment

was added later on.

Acknowledgments

This research was supported by postdoctoral fellowships to Mireille Babineau, one from the Fyssen Foundation and another from the H2020 European Research Council [Marie Skłodowska-Curie grant No 799380) to Mireille Babineau. It was also supported by grants from the Fondation de France, and the French ANR (ANR-13-APPR-0012 LangLearn, ANR-17-CE28-0007-01 LangAge, and ANR-17-EURE-0017 FrontCog).

The authors thank Anne-Caroline Fiévet for her help in recruiting the children. We also thank all the parents and the children who participated in this study.

The data from the Novel Nouns condition of Experiment 1 were analyzed along with the one from Experiment 3 in a 2 x 2 analysis of variance (ANOVA) with Condition (Novel Nouns vs Novel Verbs + Mixed Known Words) as a between-participant factor, and Test Trial Type (noun vs verb context) as a within-participant factor. The results showed no effect of Condition, nor of Test Trial Type (both F(1, 54)<1). As in Experiment 1 and 2, the interaction between Condition and Test Trial Type was significant (F(1, 54)=9. 770, p=.003, η²=.153), i.e. the two groups behaved differently during the test phase. A follow-up paired t-test assessed the performance in Experiment 3. With a habituation phase that mixed novel verbs and familiar nouns and verbs, we obtained a significant difference between congruent test trials presenting verb contexts (M=10.17, SE=.832) and the incongruent test trials presenting noun contexts (M=13.10, SE=1.265), t (27) = -2.316, p=.028, d=-.438. A total of 18 infants out of 28 showed the expected pattern of listening times.

Note that infants from Experiment 3 were habituated to the novel words to the same extent as those from Experiment 2 (Experiment 2: M=126.09 sec, range=37-263 sec;

Experiment 3: M=120.3 sec, range=17.6-292.2 sec), yielding an average exposure to the novel words of about 34 times (Exp. 2: 34.2 times; Exp.3: 33.2 times). Nevertheless, the design of Experiment 3 yielded a different exposure to the known words in terms of quantity and quality.

First, familiar verbs were heard on average 34.2 times in Experiment 2 (Novel Verbs + Known Verbs) and only 16.6 times in Experiment 3 (Novel Verbs + Mixed Known Words), since familiar nouns were also presented about 16.6 times. In addition, two phrases involving a particular familiar verb (e.g. je mange "I eat", il mange "He eats") were separated on average by 7 utterances (range: 2-12) in the habituation trials of Experiment 2, whereas they were separated by only 3 utterances in Experiment 3, which might have promoted recognition of the known verbs.

To compare the behavior of the infants from the verb groups, we ran an ANOVA with all three verb groups (Exp. 1, Exp 2, and Exp 3): this analysis revealed a significant and strong effect of Trial Type (F(1,81)=9. 857, p=0.002, η²=.108), showing that overall, 14-month-olds expected novel verbs that had been presented in verb contexts to occur in other verb contexts (i.e. preceded by a subject pronoun). In addition, there was no interaction between Condition and Trial Type (F(1,81)=.38, p=.69, η²=.008), suggesting that the three groups of infants did not behave differently.

Discussion

Declaration of Conflicting Interests

The authors declare no conflicts of interest with regard to the funding source for this study.