O Proux
email: proux@esrf.fr

J Mimault

T Girardeau

CONTRIBUTION OF THE FLUORESCENCE IN THE CONVERSION ELECTRON YIELD XAFS MEASUREMENTS

Influence of an extra electron yield induced by the fluorescence process has been experimentally shown and analytically described. Analyse of the Conversion Electron Yield signal induced by pure bulk silver (Ag K-edge) and pure bulk nickel or copper recovered by a thin iron layer (Ni and Cu K-edges) has been performed. This fluorescence-electron yield leads to enhance the edge-height to modify the shape of the XANES and to decrease the EXAFS amplitude. Extrapolation of the model concerning unrecovered bulk samples was carried out in order to explain the EXAFS amplitude reduction often observed in CEY measurements referring to that obtained in the transmission mode. Such reduction / increases with the atomic number Z of the studied element. / can be considered as negligible for low-Z element (Ni and Cu). It becomes important for high-Z element. K-dependence of the amplitude reduction is rather limited and such reduction can be simply explained by a proportional factor. In order to correct this unwanted effect, theoretical calculation process can be performed or sample structure can be especially designed to minimise it. Thin films deposited on substrate composed of low-Z element do not generate significantly such extra electron yield.

§ 1. INTRODUCTION

Measurements of Extended X-ray Absorption Fine Structure (EXAFS) obtained via the total electron yield detection are now commonly used to carry out information about the local structure of specimens which cannot be probed by transmission or fluorescence mode. The resulting Conversion Electron Yield signal (CEY) is essentially characteristic of atoms confined at or near the probed surface. 1 This specificity is well-suited to deepen the structure obtained in thin layers deposited on absorbing substrates or to follow structural modifications induced by various surface treatments such as ion neutron or electron irradiations -which are depth-limited. Experiments have to be planned taking this CEY depthdependence into account. This parameter is now relatively well-known and several models can be used to determine its behaviour versus the chosen edge and its atomic nature as well as the composition and structure of probed samples. 2 , 3 , 4 However it remains numerous questions about an amplitude reduction frequently observed on EXAFS spectra deduced from CEY when it is compared to a reference one obtained on a transmission device 5 , 6 .

The decrease can be attributed to the detection technique itself which collects all kinds of electrons including those due to the self-absorption of fluorescence photons. The latter are specific to the investigated edge absorption and are emitted during the decay process. 7 Apart from some very particular configurations -as for instance very low atomic concentration -this signal fraction shows its main failing which consists in a non-linear absorption behaviour. EXAFS oscillations can then be modified differently versus the probed edge and their subsequent use leads to an erroneous structure modelisation. Edgeheights specific of the different atomic species which constitutes a sample have to be considered with care before any comparison and use. This last point has already been underlined in previous studies 2 about the alloy composition so obtained.

We have made a brief recall on the dependence of the sampling depth profiles versus the spectra energy.

The main purpose of this paper though is firstly to give experimental evidence of this unwanted fluorescence yield secondly to precise its extent on the height of the experimented edge and finally to give an order of its influence at once on the amplitudes of the EXAFS signal and XANES part.

Experimental and theoretical investigations have been here restricted to the K-edge absorption. First of all we outlined several experimental evidences of this additive electron yield obtained for pure metallic samples -some of them being recovered by a thin deposited film which nature and thickness were chosen to emphasise its part in the total yield. Second of all we splitted up the whole additive electron emission process and adapted the formalism developed by Schroeder et al. 5 to derive an analytical expression. This theoretical results is compared to the observed differences between signals issued from CEY and transmission modes.

1 Kordesch, M.E. and Hoffman, R.W., 1984, Phys. Rev. B, 29, 491 2 Erbil, A., Cargill III, G.S., Frahm, R. and Boehme, R.F., 1988, Phys. Rev. B, 37, 2450 3 Girardeau, T., Mimault, J., Jaouen, M., Chartier, P. and Tourillon, G., 1992, Phys. Rev. B, 46, 7144 4 Schroeder, S.L.M., 1996, Solid State Commun., 98, 405 5 Schroeder, S.L.M., 1996, PhD

Sampling depth

Pre-edge or edge-height behaviour versus the deposited thickness has often been probed in order to define a sampling depth. In a previous work 2 experimental evidences were found of a closely relation between the probed depth range and the energy E p of the "primary" electrons. These electrons result from the atomic excitation or desexcitation process and are at the basis of the measured electron yield. The term "primary" means a photoelectron (K or L) or an Auger electron (KLL or LMM) which either exits directly out of the surface or generates secondary electrons in cascades of inelastic collisions. Pre-edge and K-edge yields seem quasi exclusively related to the L photoelectron and KLL Auger electron energies respectively. It was experimentally shown that excited atoms located at a distance z away from the irradiated surface approximately provide an escaping current probability P 1 (z) which is exponentially decreasing versus z.8 A sampling depth D p were then naturally defined to fit such an experimental behaviour: exp(-z/D p ) (D p =D Ph for photoelectrons, D Au for Auger electrons).

Referring to the previously mentioned experimental results and versus the thickness  of the probed layer the total electron yield behaviour can be analytically plotted by the expression :

        P 0 1 1 D 1 S dz z P S          exp (1)
D p values always are much shorter than the 1/(E K ) absorbing X-ray length where E K is the K-edge energy and (E K ) the absorption cross-section at this energy. For layers where δ is of D p order the incoming X-ray flow is then considered to be rather constant through the whole specimen and the fluorescence phenomena intensity already rather negligible. S  means the electron yield due to a large (semi-infinite) sample and free of any fluorescence contribution.

Theoretical models were also recently developed to estimate an escaping probability function where E p appears through a distance parameter. Numerical computations were then done 4 using a Monte-Carlo simulation algorithm and taking into account elastic inelastic scattering and energy losses to evaluate an escaping probability function P 2 (z). They result in a set of calculated points which were well-plotted analytically by the following function:

P 2 (z)=0.76[1-2(z/R B )] exp[-2.7(z/R B )] (2)
The Bethe range (R B ) of the primary electrons is the total path length covered before all its kinetic energy is lost. 9 Starting with this analytic function a theoretical profile S 2 () of the total electron yield is immediately obtained as in equation 1. The result is generally more adapted to the experiment than the simplest and approximate S 1 (). For instance concerning the largest thickness , S 2 () becomes more rapidly constant. This saturation is generally effective for a distance D S  2 D p .

Numerous experimental values of D p can be found in literature for measurements done under He atmospheric pressure 2, 10 , 11 , 12 . They were determined by adjustment of the experimental edge-height behaviour. Results obviously show that the product D p is related to E p by a power law :

     p P E C D (3)
 being the specific density of the probed material. Fit gives 2.2 and C7.3 in the [4, 10 keV] energy range 2 D p being in nm and E p in keV.

Electron yield magnitudes

Considering a particular primary electron type it seems very reasonable to assume that the corresponding conversion electron yield is proportional to both its kinetic energy and sampling depth. Such an assumption has to be easily tested at the K-edge of each atomic element measuring the pre-edge to Kedge ratio value typical of the same spectra. To perform such calculus, both pre-edge and K-edge values are taken at the same energy and are characteristic of the same specimen. Much difficulty appears when the test concerns different atomic elements with different probed structures and energy ranges. Other parameters such as the incident X-ray measurement mode and surface density would be also considered 2 .

Following this assumption and for sufficiently deep sample -exp(-/D p )0 -the expression of the electron yield magnitude relative to the E p energy can be written as :

      P K P P K K 0 P , D E cos D E E 1 I S              (4)
 being the incidence of the X-ray beam on the sample surface, the (1-) factor the contribution of Auger electrons ( being the radiative decay probability), (E K ) and  K (E K ) the total and K-edge absorption cross-sections for the E K energy respectively. § 3. FLUORESCENCE ELECTRON YIELD

Composition of pre-edge and K-edge electron yields

Considering pure material and K-edge study, the fluorescent X-ray flow -which is incoming toward the front surface of the sample and consequently irradiates atoms near the surface -becomes only efficient when the energy of the incident X ray beam reaches or exceeds E K i.e. the probed K threshold energy. As 10 Chartier, P., Riviere, J.P., Mimault, J. and Girardeau, T., 1995, Nucl. Instrum a matter of fact the pre-edge electron yield is essentially built up from L photoelectrons. The subsequent atomic decay which follows an L absorption event involves energetically weak LMM Auger electrons which signal weight can be considered as relatively negligible. The complementary radiative decay fraction never goes beyond a few per cent for the present experimented elements. Moreover it induces very soft X-rays which are highly absorbed. So that even for thicker materials it can never induce an important X-ray fluorescent flow at its front surface. The S L pre-edge electron yield will then be practically considered as being always free of any fluorescent part.

Conversely just above the K-edge energy of K-photoelectrons is very low in comparison of the Auger KLL electrons issued from the subsequent decay. Besides, K  and K  fluorescence X-rays must greatly modify the Auger electronic yield. Such an assertion is qualitatively supported by several elements.

Firstly the decay radiative part is generally important apart from light atomic elements. Secondly, the K  and K  photons absorption coefficients become substantially lower relatively to the incident X-ray one. A cumulative increasing flow is consequently crossing towards the front surface. Their own atomic absorption gives rise to L photoelectrons which energy remains of the same order than the primary Auger ones. Therefore, an extra electron flow exits out of the surface. Its relative intensity is related to both thickness and atomic composition.

Selection of appropriate experiments

Relative effect on K-edge yields

The simpler way to probe quantitatively the "self absorption" weight in the collected electron yield is to compare the CEY pre-edge and K-edge values obtained on bulk samples and behaviours displayed by deposited layers versus the deposited thickness of thin films.

A bulk sample will allow the advisability of a complete attenuation of the X-ray incident beam. That makes further computations easier to evaluate the fluorescence electron yield ratio. This procedure becomes very relevant when the radiative decay probability  is large enough -that is effective for high atomic number elements. Absorption CEY spectra were then performed on silver material ( = 0.83) around the K-edge (E K =25.514 KeV).

The depth of deposited Ag layers was running from 125 to 1000nm. Such thickness range is large enough to obtain the saturation step of the CEY signal induced by Auger yield but also sufficiently far from the X-ray attenuation length -around 114 and 17m before and after the Ag K-edge respectively.13 Therefore this sample set is considered as free of any K  (Ag) and K  (Ag) fluorescence yield. Around 25 keV the substrate (polished quartz) displays very low absorption coefficients and its own electron yields counted in the pre-edge one is greatly weakened by the deposited silver layer. Moreover for incident photons at 25 keV energies of the Ag L photoelectrons and Si K ones are of the same order of magnitude.

Absolute effect on EXAFS or XANES spectrum

Compared to bulk material, layers deposited by sputtering process are generally constituted of very small crystallites which can induce a size-effect reduction on the amplitude of the correspondent EXAFS signal. Measurements have then to be free of such an artefact in order to probe the signal deformation exclusively due to the fluorescent K  and K  electron yield. Moreover copper or nickel materials are more well-suited than the silver one to obtain a good signal-to-noise ratio on EXAFS spectrum performed on the used beamline. A second set of samples was then prepared to especially give prominence to the fluorescence effect at Ni and Cu K-edges. Several pure bulk Ni (or Cu) samples were cut in the same piece of metal. Some of them being were recovered by a thin deposited iron film which thickness and nature were chosen in the following way.

Thickness is adjusted in order to try to annihilate the substrate Ni (Cu) CEY without any noticed modification though on the fluorescent Ni (Cu) K  and K  X-ray flow crossing the surface of the sample.

In Ni (Cu) materials, D S is expected to be largely lower than 200 nm for Auger "primary" electrons and X-ray attenuation depth is about 4 m just above the K-edges (McMaster et al. 1969). A 200 nm Fe sublayer is a good compromise between the two previous requirements. To appreciate any remaining substrate Ni (Cu) electron yield, the same iron deposition was done on a 200 nm thick Ni (Cu) underlayer. Moreover, this sublayer is sufficiently deep to give a quasi complete electron yield signal, at its own K-edge.

Nature of the sublayer is chosen in order to emphasise the resulting electron yield of the Ni (Cu) K  and K  X-ray absorption effect. Unlike into Ni (Cu) material, K  and K  Ni (Cu) X-rays absorption in Fe material derives mainly of a K process which is typical of a high absorption cross-section.

Bulk materials are known to have large crystalline grains which minimise the previously mentioned sizeeffect amplitude reduction. Then, bulk recovered samples are expected to measure a signal quasi proportional to the only fluorescent electron yield. Specific data of bare bulk samples will be used for further theoretical tests. Cu and Ni 5 m thick foils have also been probed in a transmission mode to give XANES or EXAFS signal references. § 4. EXPERIMENT

Experimental details

Layer deposition was performed by using an argon-ion beam sputtering technique on a pure Fe, Cu, Ni or Ag target. The beam energy was fixed to 1.2 keV for a ionic current of 80 mA. Base pressure in the chamber was 10 -7 torr and did not exceed 10 -4 torr during the deposition step. Samples were all 12x18 mm 2 surface sized. Bulk substrates were polished till a mirror finish before their use. Absorption spectroscopy spectra were performed at LURE (Orsay, France) using synchrotron radiation from the DCI storage ring operating with an energy of 1.85 GeV. Data was recorded on the D42 station using a (331) Si channel-cut monochromator. For the Ag sample set -which requires several different incident angles -CEY measurements were carried out by using a room temperature detector. 14 Incidences can be fixed with an error bare of 2°. The other samples, which were probed for the details of their spectra, were introduced, at a constant incidence, into a device designed to be cooled at liquid nitrogen temperature. 15 An improved signal-to-noise ratio is then obtained on an enlarged energy range of the EXAFS spectra. All the data points were collected during 2s, under an atmospheric helium pressure, with an energy step of 0.5 eV or 2 eV in their XANES or EXAFS range, respectively. Each set of samples was probed with the same beamline and detection adjustments and all the samples were strictly positioned at the same place on the sample holder.

Results

Silver spectra

As a first evidence that an extra electron yield is introduced in the K edge signal for bulk samples, we report on Fig. 1 an absolute absorption CEY spectra relative to the bulk and the 1000 nm layer silver for the same beam incidence of 60° applied on both surfaces. One can get evidence that bulk materials exhibit an enhanced K-edge magnitude, until the pre-edge signal remains identical in both cases. As shown on this figure and to have further computations in mind to do, experimental magnitudes will be extracted just above the K-edge absorption threshold. Such procedure requires a very limited linear extrapolation of the pre-edge curve. Relative behaviour of the K-edge magnitude versus the silver layer thickness is reported on Fig. 2 for two incident angles (30° and 75°) in order to complete this first experimental view. The saturated signal is taken as a reference (S  =1). The variation is very similar for all the incident angles and the CEY saturation obviously appears around 500nm. Previously stated theoretical profiles of single-layer electron yield have been applied. The best fit being is obtained with D Au = 270nm for the simplest exponential expression S 1 () (eq. 1). The Monte-Carlo model agrees with a saturation process around 500nm. One can then consider that the fluorescence electron yield ratio, which is increasing with the incident angle, is

given by the relative difference between the bulk and the 1000nm layer results. It is an important amount since it varies relatively to the total signal approximately from 25% to 40% for 75° to 30° incidence conditions respectively (Tab. 1). Relative error bare on this percentages were estimated to be lower than 5%.



Effects of a Fe sublayer

We report on Fig. 3 Fe pre-edge and K-edge heights are very close to each other for both spectra. They can be considered as significant of quasi identical Fe deposited layers probed with the same X-ray beamline adjustments around this particular K-edge for both experiments. Nevertheless it is preferable to compare relative values, the mean slope of the whole spectrum being observed slightly different. The difference between the two Fe K-edge to pre-edge ratios is much lower than the error bare. In the same way Ni K-edge (Cu) to pre-edge ratios have been reported on table 2. Their difference is considered as weighting the pure fluorescence electron yield specific of the bulk material. Both normalised Ni K-edge spectra are superimposed on figure 4. On one hand the 400nm bilayer Fe-Ni case yields a spectra undoubtedly reliable to the well-known pure nickel one. On the other hand the Fe sublayer-Ni bulk case shows important modifications at once in its threshold shape and its oscillation amplitude with respect to the same standard one. Taking into account the relative weakness of the K  /K  emission ratio and to get further computations more easy, we will assume that the K-radiative decay is purely of K  type. The fluorescence phenomenon being also assumed to be spatially isotropic we will first consider X-rays which are crossing from the depth z around an incidence  towards the surface.

                     d F cos E cos E E dS K Ni i Ni i Ni , K (5)
As expected the final expression yields to a non linear behaviour part versus the Ni absorption crosssection  K,Ni (E i ) (appearing both at numerator and denominator) which depends of the orientation  of the surface sample relatively to the incident X-ray beam. The fluorescence-induced signal has then a reduced oscillation amplitude with respect to the reference. This reduction is moreover all the more important since the  angle is high -the  Ni (E i ) denominator term being weighted by the 1/cos factor.

Without any additive approximation the integration over  from 0 to /2 can only be performed by the way of numerical computation. This is especially true if one want precisely to test the fluorescence effect on the EXAFS oscillations versus the incident energy E i . Values reported on table 3 The computation process is somewhat different in the particular case of pure bulk sample. No sublayer has to be considered and the K  fluorescent X-ray beam essentially creates L photoelectrons (cf. §6.3).

We will use  Ag (E K )=143 cm -1 for E K =22.162 KeV in the particular case of bulk Ag sample studied at the Ag K-edge. Values of the absorption cross section were obtained from the Mac Master's table (McMaster et al. 1969) for each studied energies by using a Victoreen behaviour for extrapolation. § 6. DISCUSSION

Relative intensities

Silver K-edge

Relative intensity of the fluorescent electron yield can be extracted from spectra reported on figure 1 by subtracting the K-edge magnitude obtained for the deepest layers (S  ) from the bulk one (S bulk ). This quantity [S bulk -S  ]/S bulk -which is obviously representative of [S fluo /S K-edge ] or [S fluo /(S fluo +S Auger )] -were then measured for different  angular incidences. Results are gathered on the first line of table 1 and can be compared to values computed via the model described in §5 (2 nd line). Only the KLL Auger electrons (respectively the L photoelectrons) were considered for the S Auger (respectively the S fluo ) computation.

Computed and experimented values shows a similar behaviour versus the  angular incidence in spite of some surely crude approximations (as

1 P E S   
) which are certainly at the basis of the absolute differences. An experimental singularity revealed around =60° and confirmed by several measures must also be noticed.

Fe sublayer results

Applied to the Fe layer / Ni bulk (or Fe layer / Cu bulk) sample, the previous described model results in a fluorescent electron yield S fluo (E K ) at the Ni (or Cu) K-edge. This yield can be directly compared to the S Fe (E K ) signal obtained when the Fe layer is irradiated by the incident X-ray beam at the same K ,Ni or both cases in the Fe layer. The resulting S fluo (E K )/ S Fe (E K ) ratio is only sensitive to the intensity and directional efficiency parameters which characterise the backward fluorescence X-ray beam relatively to the incident one. Computed results referred to the same incident X-ray flow at E K and E K are reported on table 2 for both K edges. Extraction of experimental values from the data are less straightforward because of the efficiency of the incident X-ray detection which evolves in the [E K , E K ] range. Experimental Ni (and Cu) K-edge magnitudes must then be corrected to obtain the values which would be yielded if the incident beam X-ray detector was working at the E K,Ni or E K,Cu energy. We roughly assume a proportionality factor    air (E K )E K . Magnitudes have then been divided by this factor  which takes into account the variations of both the absorption cross section and the photoelectron output energy. The so-obtained results (table 2) show a good agreement with the computed ones. A logarithmic ratio study was performed on the filtered oscillations of the first coordination shell 16 in order to quantify the amplitude reduction of the EXAFS signal. Experimental k(k) EXAFS data were Fourier transformed on the [2.5 -12.5 Å -1 ] k-window using a Kaiser form (=2.5). The subsequent filtering was applied on the [1.7, 2.7 Å] range. By this way the amplitude reduction is expressed via two parameters:

XANES and EXAFS signals

-N fluo /N ref , ratio of the near-neighbouring atom number rather specific of a mean value, - 2 , difference of the Debye-Waller term which specifies its k-dependence. Moreover incertitude on the  2 value is usually around 10 -3 Å 2 . The difference  2 is rather negligible considering the error bare. The amplitude reduction can be considered in first approximation as quasi independent of k. Experiments leads to values of the same order. Nevertheless slight differences are observed on both parameters particularly on the  2 values. Such a limited misfit is not very surprising when one considers 1) the smallness of the used experimental electron yield signal and 2) the large energy range where the extrapolation of the pre-edge must be performed to operate the normalisation step. Besides, as shown on Fig. 3.b, the non negligible bulk contribution evidenced on Fig. 2.b certainly alters the result.

Amplitude reduction factor of the EXAFS signal for mono-element bulk samples

The here-proposed model correctly quantifies the amplitude reduction observed for the Fe sublayer on bulk Ni or Cu substrate. The extension of such a computation to the analysis of unrecovered bulk samples can be easily performed even if the formalism is a bit different. Absorption of the K  fluorescent photons leads to the creation of L photoelectrons ( § 5) and no sublayer has to be considered. The same kind of procedure as in the case of bulk Ni sample recovered by a Fe layer (schedule on figure 5 and detailed on §5) is followed. After integration over ζ in the [0, z] range and z in the [0, ∞[ one, the expression of the S fluo signal generated by a single bulk material can be expressed as:

                        2 0 K L L ph L ph i K 0 fluo d F E cos 2 D E E I S (6.a) with                                        K L ph L ph i K i E D cos sin D 1 cos E 1 cos E cos E 1 F (6.b)
E ph-L and D ph-L are the energy and the sampling depth of the emitted L photoelectrons respectively. E ph-L is in first approximation equal to the mean KLL Auger electron energy :

KLL L K L K L ph E E 2 E E E E         (7)
The D ph-L values are consequently very close to the D Au ones for a given element ( §2.1). Values used for the calculus are these found from equation 3. S fluo values can be estimated from numerical integration versus the probed element and edge and the incidence angle β.

As in the §6.2, experimental transmission reference signals (S ref. ) of pure materials are used to take into account the EXAFS oscillations of the absorption cross-section μ(E i ) in the S fluo calculation. CEY signal (S CEY ) having then two contributions -the fluorescence-induced (eq. 6) and the KLL Auger electrons (eq. 4 with KLL Auger electrons as primary electrons) signals -it can be easily numerically estimated:

S CEY = S fluo + S ∞,KLL (8)
Such calculation has been performed for 4 different bulk materials -pure Ni, Cu, Mo and Ag samplesstudied at the K-edge for various incidences running from 10° to 87.5°. In order to take into account the amplitude reduction of the EXAFS signal, we have estimated a k-independent parameter / defined by:

      0 . ref 0 . ref 0 CEY k k k /         (9)
where k 0 is as (k 0 ) is maximum.  CEY (k) and  ref.

(k) are the EXAFS signal deduced from S CEY and S ref.

respectively. Results of the calculations are plotted on Fig. 7. Two parts can be distinctly observed on the evolution of / versus , for 1)  > 80° and 2)  < 80°. 1) When the beam incidence becomes near the grazing incidence, / strongly increases. Such deviation is not due to the fluorescent-induced electron yield but to the electron yield itself (eq. 4). This nonlinearity effect has been previously observed 5,5 (Schroeder 1996b).

2) Considering non grazing incidence the reduction of EXAFS amplitude / is almost constant for a given element and edge. This evolution might appear contradictory compared with the relative S fluo signal intensity decreasing with the  increase (table 1 for the Ag K-edge). But for the resulting amplitude the relative S fluo intensity decrease is counterbalanced by the increase of the reduction of the linked oscillations amplitude (cf. eq. 5). / is all the more important since the atomic number of the considered element is high. / is around -6.5% and -7%, for Ni and Cu respectively (low Z-number elements). These reductions are around the error bare often considered for the N value. Experimental evidence of reduction amplitude around -5% / -10% 2 have been previously obtained for Ni K-edge. / values are higher for high Z-number elements. They are respectively found around -19.5% and -23.5% for Mo (Z=42) and Ag (Z=47). Such values agree well with those obtained for Pd (Z=46) by Schroeder 5 around -26±3% from experiment and -22% from calculation. (k) signals have to be corrected before any simulations of the data for high Z-number K-edge and for bulk samples. § 7. CONCLUSION

The experimental results of the present paper agree with an extra electron yield induced by the fluorescent process. This result have been put experimentally in evidence by analysing the CEY signal induced by bulk samples: pure bulk silver (Ag K-edge) and pure bulk nickel or copper recovered by a thin iron layer (Ni and Cu K-edges). Such upper deposited film enhances the effect of the fluorescence beam and strongly reduces the electron emission from the substrate. This fluorescence-electron yield leads to enhance the edge-height, to modify the shape of the XANES and to decrease the EXAFS amplitude.

Extrapolation of the model for mono-element bulk samples was carried out to quantify the EXAFS amplitude reduction due to this phenomenon. Such reduction increases with the atomic number Z of the studied element. The effects are rather limited for low-Z elements while they become noticeable for high-Z elements. The amplitude reduction can be well-described by a scale factor, its dependence versus the wave vector k being rather limited in the studied k range. This phenomenon is however negligible for experiment performed at the absorption edge of low atomic number element or on samples with a low concentration on the probed element. Samples can then be designed to experimentally avoid the consequences of the fluorescence-induced electron yield. Films deposited on substrate composed of low-Z element of thickness sufficiently inferior to the absorption length of the incident beam do not generate significantly such extra electron yield. Choice of the experimental references, used for the determination of the backscattering amplitude and phase, is crucial when these conditions can not be realised. In order to have a systematic amplitude reduction, the reference have to be as close as possible from the studied samples in a macroscopic point of view.

Thickness and composition are then two crucial parameters. In the other cases, this unwanted effect can be corrected performing theoretical calculations adapted to the studied bulk materials (density, energy of the fluorescent K  photons, absorption cross-section at E K …) in order to determine with the better accuracy the coordination numbers around the central atom.

Figure 1 .Figure 2 .

 12 Figure 1. Comparison of the as-obtained Ag-K edge XANES spectra for two pure silver samples, bulk and deposited layer (1000 nm thickness).

Figure 3 .

 3 Figure 3. As-obtained Conversion Electron EXAFS spectra measured for the Fe/Ni system at Ni-K edge. Substrate of the 200 nm thick iron layer is a bulk nickel substrate (a) or a 200 nm thick nickel layer deposited on quartz (b).

Figure 4 .Figure 5 .

 45 Figure 4. Comparison of the normalised XANES spectra obtained at the Ni-K edge for the 200 nm thick iron layer deposited on a bulk nickel substrate (thick full line) or on a 200 nm thick nickel layer deposited on quartz (thin full line with black circles).

e)

  Atoms of the Fe sublayer located at the depth  are irradiated by an attenuated X-ray beam : factors exp[- Ni (E K ) (z- Fe )/cos] inside the Ni substrate and exp[- Fe (E K ) ( Fe -)/cos] inside the Fe sublayer. f) Gathering all the backcoming directions defined by an equivalent incidence  and included in a width d yields a new probability geometrical factor: sin d/2. Considering finally that the K ,Ni (or K ,Ni ) X-ray absorption by Fe atoms is mainly of K type an expression of non radiative decay probability specific to the infinitesimal slice (, + d) can be stated as (1- Fe ) Fe (E K )d/cos. The resulting probability has to be multiplied 1) by exp(-/D Au (Fe)) in relation to the depth of the interaction and 2) by E Au (Fe) the Auger electron energy to obtain a final expression of the electron yield Integration over z in the [d, [ range and  in the [0, d] one are straightforward and we get:

Figure 6 .

 6 Figure 6. Comparisons of the experimental (points) and calculated (full line) XANES spectra obtained for the 200 nm thick iron layer deposited on a bulk nickel substrate at the Ni-K edge (a) or on a bulk copper substrate at the Cu-K edge (b).

Figure 7 .

 7 Figure 7. Calculated EXAFS amplitude reduction / versus the incidence angle  for various mono-element bulk samples Kedge studies: Ni (■), Cu (◊), Mo (○), Ag (▼).

2. ANALYTICAL BEHAVIOUR IN CEY MODE

Thesis, University of Cambridge 6 Schroeder, S.L.M., Moggridge, G.D., Lambert, R.M. and Rayment, T., 1997, J. Phys. IV, 7, 91 | Schroeder, S.L.M., Moggridge, G.D., Lambert, R.M. and Rayment, T., 1998, Advances in Spectroscopy, Vol. 26, edited by Clark, R.J.H. and Hester, R.E., chapter 1, pp 1-70 §

35, 1429

  . Methods B, 95, 327 11 Bouldin, C.E., Forman, R.A. and Bell, M.I., 1987, Phys. Rev. B,

Table 1 .

 1 Comparison of the relative values of the fluorescence signal with respect to the total one, S fluo /S total , obtained for bulk silver by experiment and computation.

	°)	30	45	60	65	70	75
	experiment	0.41±0.01	0.37±0.01	0.31±0.01	0.34±0.01	0.31±0.02	0.26±0.02
	model	0.47	0.44	0.39	0.37	0.33	0.29

Table 2 .

 2 Experimental and computed results for the fluorescence yield signal versus the pre-edge one at the Ni and Cu K-edges for the two experimented systems.

	S Fluo (E K )/ S Fe (E K )	200 nm Fe / bulk Ni	200 nm Fe / bulk Cu
	experiment : Fe(200nm) / bulk	0.109	0.128
	experiment : Fe(200nm) / layer(200nm)	0.032	0.031
	difference : S fluo /S pre-edge	0.077	0.097
	model	0.075	0.089

Table 3 :

 3 were used to perform such a computation for Fe on Ni or Cu bulk cases. Numerical values using for computation.  is the Auger decay probability, E K and E K the K-edge and fluorescence photon K  energies,  Fe (E K )and  K,Fe (E K) the total and K-edge iron absorption coefficient for the particular energy E K and  el. (E K ) the nickel or copper total absorption coefficient for this same energy.

	element	 (%)	E K (keV)	E K (keV)	 Fe (E K )	 K,Fe (E K)	 el. (E K )
	Ni	58.6	8.333 keV	7.477 keV	2881 cm-1	2525 cm-1	532 cm-1
	Cu	55.5	8.979 keV	8.047 keV	2393 cm-1	2103 cm-1	459 cm-1

Table 4 .

 4 First shell relative correlation number N fluo /N ref. and mean square relative displacement  2 determined from logarithmic ratio analysis of the backscattering amplitudes for the iron recovering bulk sample.Results are gathered on table 4. Computed spectra are typical of a reduction parameter of 70% order.

	sample	200 nm Fe / bulk Ni	200 nm Fe / bulk Cu
	parameter	N fluo. /N ref.	 2 (10 -4 Å 2 )	N fluo /N ref.	 2 (10 -4 Å 2 )
	experiment	0.24	-24	0.31	-14
	computation	0.29	-1.3	0.31	-4

Tröger, L., Arvanitis, D., Baberschke, K., Michaelis, H., Grimm, U. and Zschech, E., 1992, Phys Rev B, 46, 3283

Jones, R.G. and Woodruff, 1982, Surf. Sci., 114, 38

Joy, D.C. and Luo, S., 1989, Scanning, 11, 176

Yanase, E., Dake, Y., Sakiyama, M., Tohkai, M., Watanabe, I.,Harada, M. and Takahashi, M., 1996, Oyo Buturi, 65, 1267

McMaster, W.H., Del Grande, N.K., Mallet, J.H. and Hubell, J.H., 1969, Lawrence Livermore Laboratory Report, Sec. II, Rev. 1

Lee, P.A., Citrin, P.H., Eisenberger, P. and Kincaid, B.M., 1981, Rev. Mod. Phys., 53, 769

ACKNOWLEDGEMENTS

The authors wish to acknowledge Agnès Traverse for her great assistance and fruitful discussion.


Faustine Cantalloube

Kjetil Dohlen

Julien Milli

Wolfgang Brandner

Arthur Vigan

Various structures are visible within Spectro-Polarimetric High-contrast Exoplanet REsearch instrument (SPHERE) images that are not always straightforward to interpret. In this article we present a review of these features and demonstrate their origin using simulations. We also identify which expected or unexpected features are limiting the contrast reached by the instrument and how they may be tackled. This vision paves the way to designing a future upgrade of the SPHERE instrument and the next generation of high-contrast instruments such as those planned for the Extremely Large Telescope (ELT).

Imaging exoplanet and circumstellar discs with SPHERE

Direct imaging provides key information for understanding the nature of exoplanets, their formation and evolution processes. It is complementary to other detection techniques since (i) it is biased towards distant giant gaseous planets hosted by young stars, (ii) it enables direct extraction of the planet's thermal emission and its spectrum and (iii) it reveals the planet in its birth environment and its connection to the circumstellar disc's properties. Dedicated instruments have been built worldwide to scrutinise very short separations (below 500 milliarcseconds), and reach very high contrasts (more than 10 -6 ) between the host star and its planetary companions in the near infrared (NIR).

In 2014, the SPHERE instrument was installed on Unit Telescope 3 (Melipal) of the Very Large Telescope (VLT) at the

Telescopes and Instrumentation

Peering through SPHERE Images: A Glance at Contrast Limitations ESO Paranal Observatory [START_REF] Beuzit | [END_REF]. The common path infrastructure of SPHERE is equipped with an extreme adaptive optics (AO) system known as SAXO (Fusco et al., 2006) and coronagraphs (such as the apodised Lyot Coronagraph, APLC; see Carbillet et al., 2011 and[START_REF] Guerri | [END_REF], allowing it to recover the diffraction-limited angular resolution of the 8-metre telescope and to reach a contrast of 10 -4 at a few hundred milliarcseconds in the raw images. This common path infrastructure feeds three scientific instruments 1 : the InfraRed Dual-band Imager and Spectrograph (IRDIS; Dohlen et al., 2008), the Integral Field Spectrograph (IFS; Claudi et al., 2008) and the Zurich Imaging POLarimeter (ZIMPOL; Schmid et al., 2018).

Since 2014, the SPHERE instrument has delivered a wide variety of astrophysical results and impressive images (including nine ESO press releases 2 ). So far, SPHERE has contributed to the discovery of two confirmed exoplanets (HIP65426b, Chauvin et al., 2017;and PDS70b, Keppler et al., 2018) and additional candidates are being followed up with the SpHere INfrared survey for Exoplanets (SHINE; Chauvin et al., 2017) within the Guaranteed Time Observations (GTO). From the current results, a clear paucity of giant planets appears beyond typically 10 astronomical units. Can we improve the current SPHERE limitations to access closer separations and fainter objects?

The unique combination of extreme AO correction and the coronagraph reveals very faint structures in the images that were not always expected. In order to better understand the images delivered by SPHERE and to provide clues about its current limitations, and thus to shed light onto future high-contrast instru mentation pathways, we present and explain the various features visible in the SPHERE images (see Figure 1) by comparing on-sky images to simulations.

To simulate SPHERE images, the following are included: (i) the VLT pupil showing central obstruction and spiders holding the secondary mirror; (ii) von Karman atmospheric turbulence (at seeing ~ 0.85 arcseconds) and its correction by the SAXO AO; (iii) the Apodized Pupil Lyot Coronograph (APLC); (iv) the remaining high-order optical aberrations within SPHERE; and (v) photon and detector noise. The overall light path is schematically presented in Figure 2 (from left to right) where the light is propagated from pupil planes to focal planes by a Fourier transform, under the Fraunhofer farfield approximation. At the entrance of this setup, the SAXO residual phase is simulated using a Fourier-based analytical code (Jolissaint et al., 2006) from which an instantaneous coronagraphic exposure is produced. "Pseudo-long exposures", are obtained by summing 500 temporally uncorrelated exposures.

The final simulated images have the same spectral response as the SPHERE filters 3 , by summing images at different wavelengths over every nanometre. Most of the figures presented here show two simulated images: one using an ideal coronagraph, which perfectly cancels the light diffracted by a fully circular telescope aperture (Sauvage et al., 2010), and which is affected only by residual phase errors from the AO system in addition to the phase term from which the feature originates; the other one using an APLC coronagraph, affected by all of the main phase error terms visible in SPHERE images. The figures illustrating the pupil plane images are shown in blue and the focal plane images are shown in red. A yellow circle in the top right-hand corner indicates real images from SPHERE. All DOI: 10.18727/0722-6691/5138 (Soummer et al., 2005). As a result, under very good conditions SPHERE can reach a contrast of up to 10 -4 at 250 milliarcseconds in the raw coronagraphic image with a 50% transmission at 100 milliarcseconds in the H-band 4 .

Effect of the VLT pupil on the coronagraph signature (Figure 4) The coronagraph design, its resulting performance, and the central image pattern are all driven by the shape of the telescope pupil. From a circular pupil (Figure 4a) to a centrally obscured pupil (Figure 4b), a brighter ring appears close to the star. When the spiders are added by placing an opaque mask in the focal plane before re-imaging the star (Figure 3b). The very central part of the image shows a bright spot where one might naïvely expect a dark spot; this is the so-called Poisson or Arago spot, which is due to diffraction by the coronagraphic focal plane mask.

To remove the diffraction pattern due to the telescope aperture, a Lyot stop is placed in the following pupil plane, consisting of a wider central obstruction, wider spiders and smaller outer diameter (Figure 3c). In order to smooth the sharp edges of the VLT pupil and thus avoid strong diffraction effects (ripples in the focal plane, as in the Gibbs effect), a pupil apodiser is placed upstream of the coronagraph focal plane mask (Figure 3d). Its transmission function has been optimised to avoid these ripples while keeping a high throughput and resolution images are cropped to 200 × 200 pixels (about 2.5 arcseconds).

Dissection of a SPHERE image

The coronagraph is essential to reaching high contrast at close separation from the star. Its role is to suppress as much diffracted light from the star as possible while preserving any other astrophysical signal present in the field of view. When the target is faint, the main error comes from the measurement noise. In the following, the target star is considered bright enough (less than 8 magnitudes in the V-band) to ignore this error.

The fitting error (Figure 5) As the number of HODM actuators is finite, only the low frequencies up to the cut-off frequency, defined by the HODM inter-actuator pitch, can be corrected. The simulated AO residual phase including only the fitting error (Figure 5a) shows residuals with a typical size equal to the inter-actuator spacing and smaller. In the focal plane image it creates a central circular dark zone, called the corrected area, delimited by the correction ring this bright secondary ring is broken into four petals (Figures 4c andd). These patterns are strongly dependent upon the observing wavelength as the size of the focal plane mask is fixed -Figure 4 shows the specific case of Y-band images at 1.02 µm to highlight this effect. typical cross shape along this preferential direction. Moreover the aliasing effect involves spatial frequencies close to the HODM cut-off frequency and therefore the aliasing effect is more intense close to the corrected radius. To bypass this aliasing effect, a field stop (a square hole of variable size) is placed upstream of the SH-WFS to filter out the high frequencies that can neither be analysed nor corrected (Poyneer et al., 2004;[START_REF] Fusco | SPIE[END_REF]. Depending on the seeing conditions, different filter sizes can be used to minimise aliasing; the smallest filter size can be used under very good observing conditions as this effect increases with the seeing.

The satellite spots (Figure 7) Sometimes two perpendicular sine waves are applied to the HODM (the so-called "waffle mode", Figure 7a). This pattern creates four satellite spots in the focal plane image. Each spot is a pure copy of the star image and hence shows the same aberrations (Figures 7b andc). The intensity of the satellite spots is given by the sine wave amplitude, their position by the sine wave frequency; their direction is perpendicular to the sine wave direction. Secondary orders create multiple satellite ZELDA image taken on internal source is shown in Figure 5c [START_REF] Vigan | O Links 1 The SPHERE subsystems[END_REF].

When propagating this phase, the resulting image shows these patterns (Figure 5d). In order to avoid the light diffracted by defective actuators of the HODM reaching the image, the coronagraph Lyot stop was remanufactured with 6 patches to hide dead actuators (Figure 2).

The aliasing error (Figure 6)

The WFS has limited spatial sampling of the incoming phase and, as a result, the uncorrected high spatial frequencies of the atmospheric turbulence may be seen by the WFS as low spatial frequencies (Figure 6a). The HODM then corrects these frequencies, but since they are not real some light is instead scattered into the corrected area (Figures 6b-d). This aliasing effect is amplified along the WFS sub-aperture directions, giving rise to a

Telescopes and Instrumentation Cantalloube F. et al., Peering through SPHERE Images spots in the image, which are usually too faint to be observed in SPHERE images.

In addition, owing to the finite spectral bandwidth of SPHERE, the satellite spots are always slightly radially elongated (Figure 7b).

This waffle mode is commonly applied at the beginning of the observing sequence to estimate the location of the centre of the star behind the coronagraph signature in the final image, which is precisely located at the intersection of the four satellite spots. Note that as a result of its manufacturing process, the grid of the HODM creates a similar pattern, provoking the presence of bright spots along the HODM grid direction (horizontally and vertically) located at 40 l/D in the SPHERE images (l being the observation wavelength and D the effective telescope diameter). speeds move the upper level atmospheric turbulence across the pupil considerably faster than the AO loop can correct for. The AO residual phase shows strong atmospheric residuals with a clear directional pattern along the wind direction (Figure 9a). When propagating this phase, it produces a typical butterfly-shaped structure in the focal plane image, along the wind direction (Figures 9b-d). This temporal error significantly affects the contrast reached by the instrument (Mouillet et al., 2018).

Recent studies have shown that the fast, high-altitude jet stream atmospheric layer (typically located at about 12 km above Cerro Paranal), whose wind speed can reach 50 m s -1 , is the main cause of the wind-driven halo (for example, Madurowicz et al., 2018). Moreover, this halo shows an unexpected asymmetry caused by interference between this temporal lag error and scintillation errors for in the AO arm, but that are not present in the light path of the scientific subsystems and vice-versa. Like the AO residuals, they distort the wavefront so that each incoming light ray interferes with the others in the focal plane to form the "speckle field" (Figure 8b). The size of each speckle is typically that of one resolution element (1 l/D), as for planetary signals, and their typical contrast can go up to 10 -4 , whereas that of the sought planetary signals is less than 10 -6 .

Advanced post-processing techniques are then necessary to detect exoplanet signals. NCPA that are located upstream of the coronagraph focal plane mask have been recently measured thanks to the ZELDA mask on the SPHERE internal source (Figure 8a, [START_REF] Vigan | O Links 1 The SPHERE subsystems[END_REF]. When comparing the image simulated using this NCPA measurement (Figure 8b) to the internal source image (Figure 8c), a similar speckle field is observed. Under good observing conditions, such a speckle field is indeed limiting the contrast reached in the AO-corrected zone (Figure 8d).

The wind-driven halo (Figure 9) This halo appears when high wind

The contrast killers

In the context of high-contrast imaging with instruments such as SPHERE, two major aspects greatly affect the final contrast performance: (i) the errors that provoke starlight leakage out of the coronagraph; and (ii) the errors that are not temporally stable, or more generally not deterministic, and hence cannot be removed by any current post-processing techniques. In the following we focus on the errors affecting the corrected area in the images, that is to say low-order residual aberrations.

The non-common path aberrations (Figure 8) Under very good conditions, current high-contrast images are limited by speckles originating from non-common path aberrations (NCPA). These are aberrations that are sensed and corrected before the coronagraph focal plane mask, to estimate the position of the PSF core every second; that is then centred by the tip-tilt mirror of SPHERE. When the target star is faint (around 8 magnitudes in H-band) the integration time on the differential tip-tilt sensor is longer, which potentially causes stronger low-order residuals. Also, as the size of the focal plane mask is fixed, the effect of the low-order residuals in the final image is stronger when the observing wavelength increases (Figure 10d).

The low wind effect (Figure 11) During the night, the M2 spiders can cool below the ambient air temperature by radiative losses as their emissivity is significantly higher than that of air. As a consequence, under low wind conditions, a layer of colder air -which therefore has higher refractive index -forms around the spider (Sauvage et al., 2015). When the windspeed is high, the dense air is blown away, but when the wind is slow, an abrupt change of air index is seen from one side of the spider to the other. As a result, and since the SH-WFS is insensitive to such a phase step, each quarter (or fragment) of the pupil shows a different piston, and sometimes tip-tilt whose effect is also stronger with higheraltitude turbulence (Cantalloube et al., 2018).

Low-order residuals (Figure 10) Tip-tilt errors (Figure 10a) create image jitter. Consequently, the PSF core is not correctly centred behind the coronagraph focal plane mask. In addition, the diffraction patterns from the pupil and the spiders are not entirely hidden by the Lyot stop (Figures 10b and10c). Fast loworder residuals may arise from residual atmospheric turbulence and telescope vibrations, while atmospheric dispersion residuals and differential thermomechanical effects cause slow low-order residuals.

In SPHERE, these slow residuals are minimised by a differential tip-tilt sensor (Baudoz et al., 2010). This differential tiptilt sensor uses 2% of the infrared light at the observing wavelength, picked-off just phase error (Figure 11a). The corresponding PSF shape is modified, often appearing with two bright side lobes surrounding the central PSF core, and hence this is unofficially referred to as the "Mickey Mouse effect". Since the starlight is no longer concentrated in the central core (Figures 11b and11c), this results in strong starlight leakage off the coronagraph focal plane mask (Figure 11d). This effect is always present to some degree and becomes dominant when the wind speed is too slow to reduce the temperature difference between the spiders and the ambient air (Figure 11e).

To mitigate the low wind effect at the VLT, the M2 spiders were covered with a lowemissivity coating, thus preventing strong radiative cooling. This solution has proven effective, reducing the occurrence of this effect from 18% to 3% (Milli et al., 2018). Towards the future generation of High Contrast Imaging (HCI) instruments

Going from the first generation of exoplanet imagers, such as the Nasmyth Adaptive Optics System -Near-Infrared Imager and Spectrograph (NaCo) at ESO, to the latest generation of instruments, such as SPHERE, the contrast reached increased by an order of magnitude. This gain also revealed all the instrumental structures that are presented in this article. Analysing the origin, behaviour and effects of these structures on the current contrast performance of SPHERE offers a better understanding of high-contrast instruments. It also adds clear constraints to lead the design of future high-contrast imagers and specifically ELT instruments equipped with a high-contrast mode.

Based on these considerations, SPHERE has demonstrated that scintillation, pupil fragmentation, AO temporal errors, and NCPA must be specifically tackled in the future.

In order to gain sensitivity, especially at closer separations to the star, the next generation of high-contrast instruments can build on these different aspects.

For instance, on the AO side, by using a WFS less sensitive to aliasing and noise measurement (such as a Pyramid WFS; Ragazzoni et al., 1999), a faster AO loop (for example, more efficient real time computer architecture and predictive control) and going with a faster, hence more sensitive, detector and a fast deformable mirror. On the coronagraph side, by designing a coronagraph that is achromatic and less sensitive to low-order residuals (for example, pupil plane coronagraph) and offers a smaller inner working angle. On the instrument itself, correcting for the NCPA by estimating them during the observing run and by applying offsets to the HODM or by applying advanced post-processing techniques.

For ELT instruments, these different aspects will be greatly affected by the design of the telescope itself. The AO system and coronagraph will have to deal with a larger central obstruction, thicker spiders and a segmented primary mirror potentially having co-phasing errors, differential transmission and missing segments. Stay tuned for the ESO Messenger 2025 edition.

This SPHERE image of the protoplanetary disc around the young star PDS 70 reveals a planet in the act of formation.

Figure 1 .

 1 Figure 1. Typical image obtained with the SPHERE-IRDIS instrument (H2 narrow-band filter at 1.6 µm). The various structures are highlighted with numbers referring to the figure(s) in which they are explained. The yellow dot in the top right-hand corner indicates that this is a real on-sky image taken with SPHERE.

Figure 2 .

 2 Figure 2. SPHERE-IRDIS APLC coronagraphic setup used to produce the simulations. Phase and amplitude are shown in the pupil planes (blue) and intensity in the detector plane (red).

Figure 3 .(

 3 Figure 3. Illustration of the APLC coronagraph effect (H2-band): (a) non-coronagraphic image, (b) coronagraphic image with only the focal plane mask, (c) adding the Lyot stop downstream of the focal plane mask, (d) adding the SPHERE pupil apodiser upstream of the focal plane mask.

AFigure 4 .

 4 Figure 4. Illustration of the VLT pupil effect on the APLC coronagraphic images (Y2-band): (a) full circular aperture, (b) pupil with central obstruction, (c) VLT pupil including spiders and central obstruction and (d) corresponding on-sky SPHERE-IRDIS image.

Figure 5 .

 5 Figure 5. Illustration of the correction radius due to the fitting error (H2-band): (a) AO residual phase with only the fitting error, (b) corresponding ideal coronagraphic image showing a perfect dark hole, (c) the real HODM physical shape is not homogeneous and (d) this results in additional patterns in the image.

Figure 6 .

 6 Figure 6. Illustration of the folded light in the corrected zone due to the aliasing error (H2-band): (a) AO residual phase showing lower spatial frequencies, (b) simulated ideal coronagraphic image, (c) simulated APLC coronagraphic image and (d) on-sky image where aliasing dominates.

Figure 7 .

 7 Figure 7. Illustration of the satellite spots (H2-band): (a) waffle pattern applied on the HODM with a frequency of 14 cycles per pupil diameter, (b) simulated ideal coronagraphic image obtained with the waffle pattern added to the AO residual phase resulting in four satellite spots located at 14 l/D, (c) simulated APLC coronagraphic image and (d) on-sky image taken with the waffle mode.

Figure 8 .

 8 Figure 8. Illustration of the non-common path aberrations (H2-band): (a) non-common path aberrations phase map upstream of the coronagraph focal plane mask estimated using the ZELDA mask, (b) simulated APLC coronagraphic image using this estimated ZELDA phase map, (c) internal source image and (d) on-sky image where the non-common path aberrations are dominant.

Figure 9 .

 9 Figure 9. Illustration of the wind-driven halo due to the Jetstream layer (IFS, Y-band): (a) AO residual phase map showing large atmospheric residuals as ripples perpendicular to the wind direction, (b) simulated ideal coronagraphic image using only this phase map, (c) simulated APLC coronagraphic image and (d) on-sky image where the wind-driven halo dominates.

Figure 10 .

 10 Figure 10. Illustration of the low order residuals (K1-band): (a) Tilt phase map added to the AO residual phase, (b) simulated ideal coronagraphic image with only tilt error added to the AO residuals, (c) simulated APLC coronagraphic image and (d) on-sky image where LORs dominate.

Figure 11 .

 11 Figure 11. Illustration of the low wind effect (H2-band): (a) Differential tip-tilt phase map due to low wind effect, (b) simulated non-coronagraphic PSF, (c) corresponding on-sky image of the non-coronagraphic PSF, (d) simulated APLC coronagraphic image including the differential tip-tilt phase map and (e) on-sky image where the low wind effect dominates.

Acknowledgements

Faustine Cantalloube acknowledges David Mouillet and Gaël Chauvin (IPAG) for their useful comments. Arthur Vigan acknowledges support from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 757561).