N Bertrand

C Desgranges
email: clara.desgranges@cea.fr

M Nastar

G Girardin
email: gouenou.girardin@areva.com

D Poquillon

D Monceau

Chemical Evolution in the Substrate due to oxidation: A Numerical Model with Explicit Treatment of Vacancy Fluxes

Keywords: oxidation, modeling, defects, diffusion

To get a better understanding of oxidation behavior of Ni-base alloys in PWR primary water, a numerical model for oxide scale growth has been developed. The final aim of the model is to estimate the effects of possible changes of experimental conditions. Hence, our model has not been restricted by the classical hypothesis of quasi-steady state and can consider transient stages.

The model calculates the chemical species concentration profiles, but also the vacancy concentration profiles evolution in the oxide and in the metal as a function of time. It treats the elimination of the possible supersaturated vacancies formed at the metal/oxide interface by introducing a dislocation density at the interface and in the metal bulk. This latter density can be related to the cold-working state. Its effect on the vacancy profile evolution is studied in the case of a pure metal. Eventually an extension of the present model to the oxidation of Ni-base alloys is discussed regarding a recent vacancy diffusion model adjusted on Ni-base alloys.

Introduction

The understanding of the oxidation behavior of Ni-base alloys in PWR primary water is of major importance due to the cations released due to corrosion of the steam generators which is a source of the radioactivity of the primary circuit. Moreover, the oxidation process is the reason of the initiation of intergranular stress corrosion cracking (IGSCC) in alloys 600, 82 and 182.

The oxide layers formed on alloy 600, and alloy 690, in primary PWR water at 360°C are similar [START_REF] Combrade | 12th Int. Conf. on Environmental Degradation of Materials in Nuclear Power Systems-Water Reactors Proceedings[END_REF] in nature but two or three times thicker for the alloy 600. A typical oxide scale is composed of a thin Cr-rich inner layer at the metal/oxide interface, probably made of Cr 2 O 3 , under a Cr-rich oxide layer composed of a mixed Fe-Ni-Cr oxide of spinel structure such as NiCr 2 O 4 [START_REF] Marchetti | 6 th International Symposium on Contribution of Materials Investigations to Improve the Safety and Performance of LWRs Proceedings[END_REF]. Moreover, an outer Cr free oxide layer consists of crystallites spread over the surface containing Fe and Ni only. Extensive studies [START_REF] Marchetti | 6 th International Symposium on Contribution of Materials Investigations to Improve the Safety and Performance of LWRs Proceedings[END_REF][START_REF] Carrette | [END_REF][4][5][START_REF] Machet | EUROCORR 2004 Proceedings[END_REF][START_REF] Panter | [END_REF][START_REF] Panter | proceeding of Corrosion 2002[END_REF][START_REF] Soustelle | the Ninth International Symposium on Environmental Degradation of Materials in Nuclear Power Systems -Water Reactors Proceedings[END_REF][START_REF] Sennour | [END_REF] on the oxidation processes of Ni-base alloys in PWR primary coolant have demonstrated the importance of characterizing and modeling the initial stages of oxidation. Indeed, oxidation experiments on Ni-base alloys have demonstrated the occurrence of two different types of base metal damage: penetration of oxygen in the grain boundaries and the formation of a Crdepleted metal layer immediately below the Cr 2 O 3 oxide inner sub-layer as a consequence of the selective oxidation of Cr [START_REF] Combrade | 12th Int. Conf. on Environmental Degradation of Materials in Nuclear Power Systems-Water Reactors Proceedings[END_REF]. Nevertheless, the simple calculation of the mean free path of the Cr diffusion coefficient in the alloy from experimental profile [START_REF] Carrette | [END_REF] leads to a value a few orders of magnitude greater than the theoretically calculated one [START_REF] Barbe | [END_REF][START_REF] Barbe | also as Report CEA-R-6130[END_REF]. Similar Cr-depletion has been observed by Seo and Sato [START_REF] Seo | [END_REF][START_REF] Seo | the International Congress on Metallic Corrosion Proceedings[END_REF] in gaseous atmospheres at higher temperature (450-550°C). There is a continuing debate about the nature of the mechanisms involved in this accelerated diffusion. Several hypotheses have been proposed to explain this accelerated diffusion, such as the presence of a perturbed layer near the alloy surface that contains small grains and large density of defects [START_REF] Carrette | [END_REF] or a massive injection of vacancies into the metal caused by the growth of the oxide layer [START_REF] Seo | [END_REF][START_REF] Seo | the International Congress on Metallic Corrosion Proceedings[END_REF][START_REF] Pickering | [END_REF].

Therefore, modeling the oxide scale growth with an explicit treatment of the vacancy fluxes in the oxide and in the substrate should lead to a better understanding of the evolutions in the substrate and of the possible origin of the IGSCC in alloy 600. And then, it will allow to evaluate the safety margin of the alloy 690.

For a cationic transport mechanism, every models derived from Wagner's theory assume that all the vacancies are annihilated at the interface. Hence, the interface motion relative to the metal lattice is straightforwardly deduced from the quantity of metal recession due to oxide growth, and as a result, the movement of the interface follows parabolic kinetics. On the other hand, as revealed by voids formation which are sometimes evidenced, vacancies are not always all annihilated at the interface [16]. Indeed it should be more appropriate to consider that vacancies are neither all eliminated at interface nor all injected in the metal, but rather partially annihilated at the interface and hence partially injected. Pieraggi et al. [17] have linked the capacity of interfaces to annihilate incoming vacancies to the ability for interface dislocations to climb. In the case of pure cationic growing scales, they showed that cationic vacancies could only be annihilated by climbs of disorientation or misfit dislocations in the metal, these latter being energetically favored.

The aim of this work is then to build a numerical model to simulate an oxide scale growth taking into account vacancies as non-conservative species. This model should be able to calculate the evolutions of concentration profiles of the species and of their point defects in the oxide and in the substrate. Hence, this model called EKINOX (Estimation KINetics OXidation) should be an innovative tool for a better understanding of the oxidation behavior of Ni-base alloys in PWR primary water.

Presentation of the numerical model EKINOX for a pure metal M forming an oxide MOγ γ γ γ General description. The numerical model EKINOX is a one dimensional model that simulates the growth of an oxide layer whose composition is MO γ , using the explicit finite differences method as the integration algorithm. A description of the model EKINOX has already been given in [18]. It is divided into Ns layers of equal initial thickness. The substrate of metal M extends from the first to the Ni th layer and the oxide scale extends from the layer Ni+1 to the layer Ns γ . In the oxide, the two sub-lattices are considered for the cations and for the anions, whereas only one lattice is considered in the metal. Each sublattice is occupied either by the corresponding chemical species (metal M, oxygen O) or by the corresponding vacancies (V M , V O ). In the present version of the model, oxygen is supposed to be insoluble in the substrate. The model EKINOX does not make the classical steady-state hypothesis, and is thus able to study transient stages, evolutions in a finite size substrate due to oxidation and effects of microstructural changes, such as grain growth or dislocation density evolution.

Equations governing the evolution of the concentration profiles. Species transport is calculated from slab to slab with the explicit treatment of vacancy fluxes, following Fick's first law:

e e X X Ω D J 1 n n n V 1 n V n n V n V k k k k + + + - - = (1)
Equation 1 gives the flux

J k n V
of vacancies V k from slab n to n+1 in the reference frame corresponding to the nature of the considered slab n. e n is the thickness of the layer n, n V k X is the concentration in sites fraction of the vacancies V k in the layer n, n V k D is the diffusion coefficient of the vacancies V k in the layer n and n Ω is the molar volume of the layer n. Then, the variation of concentration X n V k

•

of vacancies V k in the layer n is given by the Eq. 2. A mirror condition is considered in the slab 1 which means that the model calculates the oxidation of a finite size sample.

n 1 n V n V n n V n V e J J Ω dt dX X k k k k - • - - = = (2)
Interface motion. For the layers beside the interfaces (Ni, Ni+1 and Ns) the conservation equation (Eq. 2) leads to a thickness variation of the slabs. Thus both interfaces are mobile, because of cationic and anionic fluxes. The corresponding variation with time of the slab thickness, Ns e

• and

1 i N e + &
, is given by the Eq. 3 and 4, where s eq M X and s M eq V X refer to the equilibrium concentration of ,respectively , cations and cationic vacancies at the surface of the oxide, i eq O X and i O eq V X refer to the equilibrium concentration of, respectively, anions and anionic vacancies at the metal/oxide interface. The anionic oxide growth leads to a recession of the metal layer thickness e Ni in the ratio of the molar volumes, that is to say the Pilling-Bedworth Ratio (

M MO Ω Ω γ ) : s M M s eq V 1 Ns V Ns eq M 1 Ns M Ns Ns X 1 J Ω X J Ω e - - = = - - • (3) i O i eq V 1 Ni VO 1 Ni eq O 1 Ni O 1 Ni 1 Ni X 1 J γ Ω X - J - γ Ω e - = = + + + + • + (4)
The implementation of the algorithm for moving boundaries is described in [19].

Different hypothesis at the interfaces. At the surface of the oxide scale, the thermodynamic equilibrium is supposed to be reached instantaneously. At metal/oxide interface, one can either consider an instantaneous thermodynamic equilibrium or a kinetics for the transfer of metallic species from the substrate to the oxide. The flux for such a transfer is then considered to be proportional to the transfer coefficient "α" and the difference between the effective and the equilibrium concentrations for cationic vacancies at metal/oxide interface (Eq. 5) [START_REF] Monceau | Poquillion in Oxydation des matériaux métalliques[END_REF] :

( ) i M M M eq V eff V 1 Ni trans V X X Ω α J - - = + (5)
Vacancy treatment. Metallic vacancies are treated as non-conservative species : the total amount of vacancies evolves during the simulation. They can be eliminated in the substrate by dislocations and at the metal/oxide interface by misfit dislocations between the oxide and the substrate. The velocity of the vacancy concentration variation is given by Eq. 6 where n ρ refers to the dislocation density in the slab n, and leads to the decrease of the corresponding layer thickness (Eq. 7) following [START_REF] Martin | [END_REF].

(

)

n M eq V n V V n n sink n V X X X D Ω ρ X M M M M - - =         • (6) ( ) n eq V n V V n n sink n e X X D Ω ρ e M M M - - =         • (7)
The dislocation density is not necessarily uniform in the substrate and can decrease from the surface into t the metal bulk which corresponds better to surface cold working

Results obtained with the model EKINOX for a pure metal and discussion

To explore the consequences of various hypothesis made on the treatment of vacancies, on the shape of concentration profiles and on the oxidation kinetics, three different illustrations of calculation results obtained with EKINOX are presented in the following part. All calculations have been done with an initial number of slabs N s =132 having a typical initial thickness of 0.25 µm.

Mixed diffusion mechanism controlling oxide scale growth. The parameters used for the calculation are given in Table 1. The cationic vacancies data are taken from the literature for the nickel oxide (NiO) growth at 1000°C [22]. The same values are arbitrarily chosen for the anionic vacancies in order to illustrate an anionic and cationic oxide scale growth with the same diffusivity for both species. The oxide scale growth kinetics is shown on Fig. 1. It is parabolic and the parabolic kinetics constant k p is equal to 2.64 10 -11 cm 2 .s -1 . The interface motion is shown on Fig. 2.

Both interfaces move and their displacement is symmetrical as we could expect from values given to input data.

Table .1 Input data for the simulation of an anionic and cationic diffusion controlled oxide scale growth.

Cationic vacancies

Anionic vacancies Partial control of the oxide scale growth by the interfacial reaction. In this case, only cations are considered as mobile species. The cationic vacancy diffusion data are the same as in the previous case, and the transfer coefficient at the metal/oxide is chosen so that the steady-state ratio k p /k l = 10 -3 cm. Used parameters are summarized in Table 2. For this calculation, the initial concentration profile for the cationic vacancies is chosen linear, with at both interfaces the equilibrium concentrations. Figure 3 shows the evolution of the cationic vacancy concentration profile for the 1000 first iterations of the simulation, corresponding to 1.25 s. A curvature appears in the concentration profile near the metal/oxide interface and the vacancies concentration at the interface increases. Indeed, the transfer coefficient is an obstacle to the exchange of cationic vacancies with metallic atoms in the substrate. Then, for longer durations, the cationic vacancies concentration profile keeps on being linear, and the concentration at metal/oxide interface decreases with time toward the equilibrium value (Fig. 4). It has been confirmed that after the transient stage, the cationic vacancy concentration at metal/oxide interface follows the stead-state evolution, given by the following equation [23]:

e α D X e α X D ) (X M i M s M M M V eq V eq V V state steady i V + + = - (8)
The shape of the simulated growth kinetics is a complete parabola (t= A + B.e + C.e²) (Fig. 5) as expected from calculation of the steady state case. The kinetics constant evaluated from EKINOX simulations using the analysis given in [24], are k p =1.30 10 -11 cm 2 .s -1 , and k l = 1.39 10 -8 cm.s -1 . The small variation between the steady-state and the simulated values comes from the transient stage. This small difference can be even less important if smaller values are chosen for the spatial division step and the time step for the numerical integration. Table .2 Input data for the simulation of an oxide scale growth controlled by the cationic vacancy diffusion and by the metal transfer at the metal/oxide interface. Vacancy concentration profile evolution in the substrate during oxide scale growth. This section illustrates the influence of the hypothesis made for the vacancy treatment on their concentration profile evolution in the substrate. In this case, the oxide scale growth is chosen to be controlled by cationic diffusion (Table 3) and thus the kinetics is parabolic. The data are taken from the literature for the nickel oxide (NiO) growth at 1200°C [22] and metallic vacancy diffusion in pure nickel [16].

Table .3 Input data for the simulation of oxide scale growth controlled by cationic diffusion .

Metallic vacancies Cationic vacancies

eq V Ni X Ni V D (cm 2 .s -1 ) i Ni eq V X s Ni eq V X Ni V D (cm 2 .s -1 )
3.6 10 -3 5 10 -8 2 10 -5 1.7 10 -4 7.57 10 -7

Two different hypothesis are made for the vacancy treatment, both considering dislocation density. The first one considers only misfit dislocations at the metal/oxide interface (ρ misfit =10 10 cm -2 ), and the second one considers also a non uniform dislocation density in the substrate, decreasing from the surface toward the depth of the substrate, simulating for example the effect of surface coldworking. The dislocation density profile chosen in this case is given on Fig. 6. in order to study the evolution of the vacancy concentration. In the case where only interfacial dislocations are considered, the vacancy concentration profile follows several stages. The whole evolution is given in the reference frame linked to the metal lattice in Fig. 7a. The vacancy concentration first increases in the substrate. Indeed the dislocations are not able to eliminate all the incoming vacancies. Then, when the oxide scale grows, incoming vacancy flux decreases due to the decrease of the concentration gradient in the growing oxide scale. Thus, the misfit dislocations become a sink efficient enough to eliminate all the incoming vacancies. Then, as there is no more vacancy injected in the substrate, the shape of the concentration profile evolves: vacancies that were accumulated in the core of the substrate diffuse backward toward the vacancy sink which is localized at the metal/oxide interface (Fig. 7b). Then the vacancies concentration profile in the substrate becomes flat and the concentration decreases uniformly.

Future developments: Simulation of the oxidation an alloy

From now, the EKINOX model will be adapted to simulate the oxidation of an alloy ABC that forms an oxide scale AO γ . To achieve this goal, the developments are made together with a segregation code already developed for multi-component alloys. This model uses a Self-Consistent Mean Field (SCMF) method to calculate the species and the vacancy transport in the substrate. The atomic diffusion model used in the SCMF model has been validated on different alloy compositions and is now suitable for Ni-base ternary alloys: Ni-Fe-Cr [START_REF] Barbe | [END_REF][START_REF] Barbe | also as Report CEA-R-6130[END_REF]. The diffusion fluxes of the species of the alloy are calculated from a microscopic model of atom-vacancy exchange, which depends on temperature and local composition of the alloy through thermodynamic and kinetic parameters and guarantees a coherent treatment of kinetics and thermodynamics. Thus diffusion coefficients are not extrapolated from high temperature but calculated from the mean exchange frequencies and differences between energy contributions at the saddle point and the initial point, both expressed in terms of nearest neighbor pair interaction energies [START_REF] Barbe | [END_REF]. The final model reaches a good agreement with the experimental data, as shown in Fig. 9 for binary alloys Ni-Cr, and it predicts that the mean exchange frequencies of Ni, Fe and Cr respectively should stay on a ratio approximately [START_REF] Carrette | [END_REF]4,[START_REF] Soustelle | the Ninth International Symposium on Environmental Degradation of Materials in Nuclear Power Systems -Water Reactors Proceedings[END_REF] for any composition, even at lower temperatures [START_REF] Barbe | [END_REF][START_REF] Barbe | also as Report CEA-R-6130[END_REF]

Fig. 1 .

 1 Fig. 1. Oxide scale growth kinetics calculated with the same diffusivity for the anions and the cations.

2 .

 2 Interfaces motions in the reference frame linked to the oxide lattice calculated with the same diffusivity for the anions and the cations.

Fig. 3 .Fig. 4 .

 34 Fig. 3. Evolution of the cationic vacancies concentration profile during the 1000 first iterations (1.25 s) when the oxide scale growth is controlled by diffusion and interfacial transfer.

Fig. 5 .Fig. 6 .

 56 Fig. 5. Calculated oxide scale growth kinetics when controlled by diffusion and interfacial transfer.

Fig. 9 .

 9 Fig. 9. Tracer diffusion coefficients of Cr and Ni in the Ni-Cr alloy at (a) 1373 K and (b) 1273 K as a function of the composition, obtained experimentally (squares) and by the SCMF model (solid lines). The dotted lines express the non-correlated part of the diffusion coefficients [12].

In the second case, vacancy sinks are also distributed in the substrate (Fig. 6), the evolution of the vacancy concentration profile is much simpler as shown in Fig. 8. Indeed, the concentration first increases near the metal/oxide interface because the rate of vacancy injection is initially high. But, those injected vacancies are eliminated in the entire volume of the metal by dislocations and so they do not accumulate in the substrate. Then, at longer durations, the rate of vacancy injection decreases and the concentration profile in the substrate becomes flat and decreases uniformly. Fig. 8. Evolution of vacancy concentration profile in the substrate (metal lattice as reference frame) when the oxide scale growth is controlled by cationic diffusion and vacancies are eliminated by dislocations (density profile given on Fig. 6).

Finally, these latter simulations results applied in the plain case of a pure metal show that the hypothesis for the treatment of vacancies in the substrate strongly affects the vacancy concentration profile. Vacancy injection could play a major role on the Cr depleted profile in the case of oxidation of Ni-base alloys. However to tackle the case of multi-element alloys, cross-diffusion coefficients in the substrate need to be handled. This can be done following previous work on a model developed to simulate the irradiation assisted segregation at grain boundaries of austenitic steels [25][26].


The first steps in the acquisition of syllables and geminates in Tashlhiyt Berber

Mohamed Lahrouchi

SFL, CNRS & Université Paris 8

A large body of literature on early language acquisition has shown continuity between phonetic forms in infant pre-linguistic vocalizations and earliest speech forms (cf. [START_REF] Locke | Phonological acquisition and change[END_REF][START_REF] Oller | The emergence of the sounds of speech in infancy[END_REF][START_REF] Stark | Stages of speech development in the first year of life[END_REF][START_REF] Stoel-Gammon | Patterns of early lexical and phonological development[END_REF], Vihman, Ferguson & Elbert 1986, among others). Strong similarities in sound types, sound combinations and syllable type preferences in different communities have been frequently documented across these periods, suggesting universal foundations for early production patterns. These similarities have been explained by articulatory constraints and structural factors as well (cf. [START_REF] Browman | Towards an articulatory phonology[END_REF][START_REF] Goldstein | The role of vocal tract gestural action units in understanding the evolution of phonology[END_REF], Vihman et al. 1985). It has also been proposed that input from the ambient language may influence the shaping of children's production preferences at some point in the late babbling and first word period (Werker & Lalonde 1988[START_REF] Konopczynski | Du Prélangage au Langage: Acquisition de la Structuration Prosodique[END_REF][START_REF] Snow | Intonation and final lengthening in early child language[END_REF].

In this paper, we present preliminary results of a longitudinal study that focuses on the developmental trajectory of speech production capacities in two Berber children acquiring Tashlhiyt from the babbling period to the emergence of early grammar. Very few studies, if any, are devoted to the Berber language acquisition, and Tashlhiyt, the variety spoken in South-west Morocco, presents very interesting phonetic and phonological characteristics to study in a developmental perspective. Probably the most salient characteristic is the use of complex consonants clusters, resulting in a highly marked syllable structure where any segment, even a voiceless obstruent, may occur in the nucleus position (cf. [START_REF] Dell | Syllabic consonants and syllabification in Imdlawn Tashlhiyt Berber[END_REF], 2002). Another feature, which deserves to be studied relates to consonant length. Tashlhiyt Berber contrasts singleton and geminated consonants in various contexts, including word-initial, medial and final positions. This paper is structured as follows. Section 1 provides a cross-linguistic overview of the preferred sound patterns in babbling and first words. Section 2 turns to Tashlhiyt Berber: data and methods are presented therein. The preliminary results of a longitudinal two-case study will be discussed in section 3. We will show that the two children followed during the babbling and first words periods exhibited production preferences that were often described as preferred productions in children learning different languages. In section 4, we discuss the acquisition of geminates, taking advantage of the case of Tashlhiyt Berber in order to shed light on the phonological representation of these segments. In particular, we will be testing the 'ambisyllabicity' hypothesis in relation with the assumption that any segment in Tashlhiyt Berber may function as a syllable nucleus. Section 5 concludes the paper.

Cross-linguistic preferences in babbling and first words

In this section, we briefly review the similarities in sound and syllable types, which have been documented across the babbling and first word periods.

Consonants and vowels

Several studies have shown that children from different ambient language backgrounds prefer producing consonants with a complete closure in the front part of the vocal tract, including labials and coronals, oral as well as nasal. The reader is referred to [START_REF] Kent | Vocalizations of one year olds[END_REF], Vihman et al. (1985), Robb & Bleihle (1994), Davis & MacNeilage (1995) on American English, [START_REF] Kern | Emergent complexity in early vocal acquisition: Cross-linguistic comparisons of canonical babbling[END_REF] on Dutch and French, and Cataño, Barlow & Moyna (2009) on Spanish. These preferences constitute a universal tendency for early consonant patterns, although not shared by all children, especially when acquiring languages with an important amount of dorsal and pharyngeal consonants. In this respect, [START_REF] Rosenhouse | The acquisition of Arabic as a mother tongue (mainly in Israel)[END_REF] reported an important use of velars, uvulars, pharyngeals and glottals in children acquiring Arabic spoken in Israel before the age of 24 months. The use of fricatives is also found in children first words in Egyptian Arabic [START_REF] Omar | The acquisition of Egyptian Arabic as a native language[END_REF] and Jordanian Arabic [START_REF] Amayreh | Phonetic inventories of young Arabicspeaking children[END_REF].

As to vowels, children begin producing the ones belonging to the lower left quadrant of the vowel space, in particular, mid [ε] / [e], low front [ae] / [a] and central [ə]. This has been reported in various languages, including American English [START_REF] Kent | Vocalizations of one year olds[END_REF][START_REF] De Boysson-Bardies | A cross-linguistic investigation of vowel formants in babbling[END_REF][START_REF] Davis | Acquisition of correct vowel production: A quantitative case study[END_REF], 1995, among others), Arabic (de Boysson-Bardies et al. 1989[START_REF] Kern | Emergent complexity in early vocal acquisition: Cross-linguistic comparisons of canonical babbling[END_REF]), French (de Boysson-Bardies et al. 1989[START_REF] Kern | Emergent complexity in early vocal acquisition: Cross-linguistic comparisons of canonical babbling[END_REF], Brazilian Portuguese [START_REF] Teixeira | Early sound patterns in the speech of two Brazilian Portuguese speakers[END_REF], and Fufulde [START_REF] Cissé | Développement phonético-phonologique en fufulde et Bambara d'enfants monolingues et bilingues[END_REF]. Depending on the input language, certain vowels may emerge relatively early in child productions. In Egyptian and Jordanian Arabic, whose vowel systems contain only three cardinal vowels contrasting in length, children produce the high vowels [i] and [u] early in their first words period [START_REF] Omar | The acquisition of Egyptian Arabic as a native language[END_REF][START_REF] Amayreh | Phonetic inventories of young Arabicspeaking children[END_REF]. According to [START_REF] Omar | The acquisition of Egyptian Arabic as a native language[END_REF], [u] is found even earlier in the pre-linguistic period (6 to 10 months) along with the vowels [ε, e, ӕ, a].

Syllable types

Open light syllables are the most predominant types, often reduplicated as CVCV or resulting from consonant cluster reduction and coda deletion, especially in first words [START_REF] Fikkert | On the acquisition of prosodic structure[END_REF][START_REF] Levelt | Syllable types in cross-linguistic and developmental grammars[END_REF][START_REF] De Boysson-Bardies | How language comes to children[END_REF][START_REF] Levelt | The acquisition of syllable types[END_REF]. This is actually not surprising as CV is the universally least marked syllable type. Closed syllables are very limited in children babbling and first words, regardless of the input language. However, in certain languages, children tend to produce a relatively high number of closed syllables. According to Fee et al. (1982) and [START_REF] Stoel-Gammon | Patterns of early lexical and phonological development[END_REF], some English-learning children had dominant closed syllables. In Dutch, one child followed by [START_REF] Elbers | Play Pen Monologues: The Interplay of Words and Babbles in the First Words Period[END_REF] is reported to have no preference for CV syllables in his first words. Another interesting case can be found in languages with phonemic geminates. In intervocalic position, these geminates are generally analysed as ambisyllabic, branching into coda and onset positions. This automatically increases the number of closed syllables as soon as the geminates emerge in children productions. As we shall see later in section 4, Tashlhiyt Berber is one such language where geminates are produced early in the babbling and first words periods.

Materials and methods

Tashlhiyt Berber

Berber belongs to the Afroasiatic family. It is spoken in large parts of North Africa, mainly in Morocco and Algeria, and to a lesser extent in Mali, Niger, Libya, Egypt, Tunisia and Burkina Faso. Berber communities also live in Diasporas, mainly in France, Spain, Holland, and Belgium. Tashlhiyt, whose native speakers are estimated at four millions based on the data of the official census of 2004,1 is spoken in South-west Morocco. The language presents interesting phonetic and phonological features, which can serve as a testing ground for important theoretical proposals. In particular, it allows complex consonant clusters, which may result in utterances without any vocalic segment (e.g. tsːlkmtːnt 'you made them (fem.) arrive'). Based on this characteristic, [START_REF] Dell | Syllabic consonants and syllabification in Imdlawn Tashlhiyt Berber[END_REF][START_REF] Dell | Syllabic consonants in Berber: Some new evidence[END_REF], 2002) argued that in Tashlhiyt any segment, even a voiceless obstruent, can be syllabic (see also [START_REF] Boukous | Phonotactique et domaines prosodiques en berbère[END_REF].

Regarding the phonemic system, Tashlhiyt Berber has only three vowels /i, a, u/. The so-called "transitional voicoids", which appear in certain consonant clusters (e.g. nkər 'wake up, stand up', lkəm 'arrive'), have no syllabic status according to [START_REF] Dell | Syllables in Tashlhiyt Berber and in Moroccan Arabic[END_REF], while [START_REF] Coleman | Declarative syllabification in Tashlhiyt Berber[END_REF][START_REF] Coleman | The phonetics and phonology of Tashlhiyt Berber syllabic consonants[END_REF] argues that they are epenthetic, filling syllabic nuclei that would otherwise remain empty. As to consonants, Tashlhiyt has 33 consonants and two glides /j, w/, which often occur in complementary distribution with the corresponding high vowels /i/ and /u/ (cf. [START_REF] Guerssel | Glides in Berber and syllabicity[END_REF][START_REF] Lahrouchi | A propos de I et de U en berbère : de la phonologie, de la morphologie et des phases[END_REF]. The consonantal inventory is given below in table (1). Each of the above consonants has a geminate counterpart. Table 1 also shows pharyngealized coronals and labialized dorsals, as opposed to their plain counterparts. All of these phonological properties will prove interesting in the study of language acquisition. In the remainder of this paper, we will be examining how these properties arise in babbling and first words.

Data collection and methods

Two children acquiring Tashlhiyt Berber participated in our study: a girl named Imane and a boy named Reda. They were recorded every two weeks in their home from 7 to 24 months (the whole data is downloadable from CHILDES 2 ). Parents were told to follow their normal types of activities with their child. No extra material was used in the recording sessions, so that samples reflected the children's typical vocalizations in familiar surroundings. We ended up with 31 sessions of one hour for Reda and 26 for Imane. The data were broadly transcribed using IPA. They were entered into the PHON software designed for describing phonetic patterns (Rose et al. 2006). Then, they were divided in two periods: period 1 from 7 to 12 months and period 2 from 13 to 24 months. Only babbling utterances were considered in the first period and word utterances in the second period. Frequency of segments and syllable types were calculated for both periods.

For the purpose of our study, consonants are grouped according to 1) place of articulation: labial (bilabial, labiodental, labio-palatal and labio-dorsal), coronal (2002), whose basic idea is that any segment, even a voiceless obstruent, can occur in the nucleus position. Some of the constraints proposed in the original work have been slightly modified here. The whole set of constraints is listed below in (1).

(
(1)

Two principles:

-The licit consonantal nuclei thesis, which states that any segment even a voiceless obstruent can act as a syllabic nucleus.

-The sonority-driven syllabification thesis, which holds that in the competition for the status of syllabic nucleus, more sonorous segments are favoured over less sonorous segments.

Segments are ranked along the following scale where they appear in order of decreasing sonority: a > high vocoids > liquids >nasals > fricatives > stops (cf. Dell & Elmedlaoui 2002:76).

Six constraints:

-Complex onsets are prohibited (Dell & Elmedlaoui 2002:114). This constraint has been slightly modified in this paper, suggesting that only domain-initial onsets can be complex.

-Any rime contains at most three consonants. If so, the last two consonantal slots host a geminate (Dell & Elmeldaoui 2002:98).

-The coda position cannot be more sonorous than the nucleus (Dell & Elmedlaoui 2002:102).

-Every syllable has an onset, except domain-initially where the onset may be empty (Dell & Elmedlaoui 2002:92).

-Any sequence with a sonority peak must contain a nucleus (Dell & Elmedlaoui 2002:100).

-A geminate cannot branch into an onset followed by a nucleus (Dell & Elmedlaoui 2002:102). However, in cases where none of the above constraints leads to the appropriate syllabification such as in words made of one geminate (e.g. ʃː 'eat') as well as in domain-initial position (e.g. kːtnt 'cross them (fem.)'), the geminate seems to constitute an onset-nucleus sequence, inevitably running counter Dell & Elmedlaoui's proposal.

Results

This section briefly presents our results about sound and syllable patterns in Imane and Reda's productions. The reader is referred to Lahrouchi & Kern (2015) for further details and analysis.

Ratio of consonants to vowels

Table 2 shows the frequency of segments and utterances. Both kids produced more consonants than vowels as illustrated by the C/V ratio, departing from the patterns observed in other languages such as French, Turkish, German and Tunisian Arabic (cf. [START_REF] Kern | Emergent complexity in early vocal acquisition: Cross-linguistic comparisons of canonical babbling[END_REF]. This is probably due to the ambient language effect, namely the fact that Tashlhiyt Berber has common complex consonant clusters.

Consonants and vowels

Both kids produced more oral stops than other manners of articulation in the babbling period, thus confirming the preferences that were often described in other languages. The liquids and glides are the least represented categories in their productions with less than 10% of occurrences in both categories. As to fricatives and nasals, Imane produced more nasals than fricatives (18% of fricatives in the first period vs. 26% of nasals), whereas Reda showed the reverse order (34% of fricatives vs. 18% of nasals). This is mainly due to the frequent use of the laryngeal In the first words period, we noted a much more varied vocalic repertoire, with an increasing use of the high vowels [i] and [u] over the months. We also noted an increasing use of the low vowel [a] as opposed to [ae]. For instance, Reda's preference for [ae] shifted down from 82% in the babbling period to 48% in the first words. In addition, both kids displayed a few instances of schwa in babbling and first words as well. These schwas deserve to be examined given their controversial status in Tashlhiyt Berber.

Syllable types

Open syllables and more precisely CV ones were predominant in both kids productions during the whole period of study. Out of 3000 syllables produced by as ambisyllabic typically branching into coda and onset positions. The following section is dedicated to this type of segments, focusing on their syllabic analysis.

Geminates

Geminates have been the source of much debate in phonological theory, with regard to their representation and their behaviour in phonological processes (cf. [START_REF] Hayes | Inalterability in CV phonology[END_REF][START_REF] Hayes | Compensatory Lengthening in moraic phonology[END_REF][START_REF] Barry | On geminates[END_REF][START_REF] Selkirk | A two-root theory of length[END_REF][START_REF] Kenstowicz | Phonology in generative grammar[END_REF][START_REF] Davis | Geminate consonants in moraic phonology[END_REF], 1999[START_REF] Davis | The controversy over geminates and syllable weight[END_REF], 2011). Berber is no exception to this debate, as the language contrasts singleton and geminates in various structural contexts, which inevitably raises the problem of their syllabic representation. This section discusses some aspects of this problem, in light of the facts used in the present study.

The phonology of geminates, an overview

Cross-linguistic study of geminates states interesting implicational relationships, whereby a language having word-initial and word-final geminates also has word-medial ones (cf. [START_REF] Taylor | Some patterns of geminate consonants[END_REF][START_REF] Thurgood | Geminates: A cross-linguistic examination[END_REF][START_REF] Kraehenmann | Initial geminates[END_REF]. The general finding is that word-medial, specifically intervocalic, is the preferred position for this kind of segments. Among the languages with phonemic geminates, very few contrast them with singleton consonants in word-initial position. Tashlhiyt Berber is one such language in which lexical geminates occur not only in medial position (e.g. [ratːftu] 'she will go').

Taking advantage of autosegmental representations, most phonologists (cf. [START_REF] Guerssel | Constraints on phonological rules[END_REF][START_REF] Dell | Les géminées en berbère[END_REF], 2002, 2011[START_REF] Lahrouchi | Aspects morpho-phonologiques de la dérivation verbale en berbère (parler chleuh d'Agadir)[END_REF][START_REF] Ridouane | Geminates at the junction of phonetics and phonology[END_REF] analyse geminates in Berber as a single melodic unit associated to two adjacent skeletal slots, as opposed to singleton consonants which attach to only one slot. This is illustrated below in (2).

2) a. b.

x x x | \ / C C
This kind of representation inevitably raises the issue of syllabification, leading to cases where the two skeletal positions in (2b) are dominated either by one or by two syllabic constituents. The later case is found particularly in the intervocalic position, where geminates are ambisyllabic, attached to coda and onset positions. [START_REF] Dell | Syllabic consonants and syllabification in Imdlawn Tashlhiyt Berber[END_REF], 2002, 2011) hypothesis that any segment in Tashlhiyt Berber can be syllabic allows this configuration (see (3a)) as well as the one where geminates branch into nucleus + onset positions (see (3b)). However, it prohibits any branching into onset + nucleus positions, relying on Hayes (1989: 258) idea that onset segments are weightless and that branching into onset and nucleus positions entails a 'flopped structure'.

(3) O, R, N and C respectively stand for Onset, Rime, Nucleus and Coda.

a. idːa 'he went' b. islːa 'he heard'

σ σ σ σ σ R O R R O R O R N C N N N N x x x x x x x x x i d a i s l a
In the following section, we present the frequency of geminates in children productions. Then we discuss some of the issues they raise with regard to ambisyllabicity.

Geminates in Tashlhiyt language acquisition

Both children started producing geminates early in their babbling. Reda produced 1725 geminates during the whole period of study, 53% of which involve coronal consonants, followed by labials (26%), dorsals (13%) and gutturals (8%).

Imane produced almost the same amount of geminates (1677), with the same preferences: 54% of the geminates are coronals, followed by labials (29%), dorsals (11%) and gutturals (6%). Figures 5 and6 present the results for both kids. 9 , 2 4 -1 0 , 2 2 1 1 , 5 -1 2 , 1 6 1 3 , 0 1 -1 4 , 2 6 1 5 , 0 1 -1 6 , 0 7 1 8 , 1 3 -1 9 , 0 1 2 0 , 0 8 -2 2 , 1 9 2 3 , 0 3 -2 5 , 0 4 Nb of occurrences 9 ; 2 -1 0 ; 1 4 1 1 ; 4 -1 2 ; 9 1 2 ; 2 3 -1 3 ; 2 0 1 5 ; 2 4 -1 7 ; 9 1 7 ; 2 3 -1 9 ; 1 2 1 9 ; 2 7 -2 1 ; 1 6 2 2 ; 4 -2 3 ; 2 3 Nb of occurrences

Age

Imane geminates

Labials

Age

Reda geminates

Labials

Coronals

Dorsals

Gutturals

Cross-linguistic studies (cf. [START_REF] Taylor | Some patterns of geminate consonants[END_REF][START_REF] Thurgood | Geminates: A cross-linguistic examination[END_REF][START_REF] Kraehenmann | Initial geminates[END_REF] in adult language state that geminates most commonly occur in word-medial position, particularly in the intervocalic position. Word-final geminates are also often allowed, while word-initial ones are relatively rare. The low frequency of word-final geminates in children productions can be explained by the late acquisition of consonants in the coda position, as opposed to the onset (cf. [START_REF] Ingram | The role of the syllable in phonological development[END_REF][START_REF] Fikkert | On the acquisition of prosodic structure[END_REF], Demuth 1995[START_REF] Kirk | Accounting for variabilty in 2-year-olds' production of coda consonants[END_REF], Gnanadesikan 2004, among others) and the preference for unmarked structures (cf. [START_REF] Jakobson | Child language, aphasia, and phonological universals[END_REF], namely open syllables.

As mentioned earlier in section 2.2, Dell & Elemdlaoui's approach to syllable structure in Tashlhiyt Berber allows:

-complex coda but no complex onset -geminates in the coda position but not in the onset position. Any geminate that can potentially occur in the onset position is split into an ambisyllabic structure coda + onset or nucleus + onset -geminates as a nucleus + onset sequence but not the mirror-image onset + nucleus.

In this study, we assume that complex onsets may occur only domain-initially for it is difficult to show that forms like kti 'remember' and bdu 'start' involve an initial syllabic (4) consonant. This is even more difficult in forms containing sonority-equal segments such as bdg 'be wet' and fsx 'invalidate', since sonority hierarchy cannot determine which consonant is syllabic. In the same way, we assume that domain-initial geminates belong to the same syllabic constituent, namely the onset: forms in (4) like tːɛ, bːi and mːae are analysed as monosyllabic. However, in post-vocalic position geminates are syllabified either as tautosyllabic, associated to the coda position, or as ambisyllabic when followed by a syllabic segment. The latter case is well attested in both child and adult productions. It is also stated in typological studies as the preferred context for ambisyllabic consonants (see section 4.1 for relevant studies).

As to ambisyllabic geminates attached to nucleus and onset positions, they are found only in Tashlhiyt Berber. As far as this assumption holds, the geminates in forms like ħlli and ʕmːi in (4) are syllabified exactly as in (3b). For the sake of convenience, both forms are represented below in (5).

(5) a. ħlli 'milk' b. ʕmːi 'my uncle'

σ σ σ σ O R O R O R O R N N N N x x x x x x x x ħ l i ʕ m i
In the competition for the status of syllabic peak, the liquid /l/ and the nasal /m/ have priority over the less sonorous fricatives /ħ/ and /ʕ/, resulting in bisyllabic forms where the ambisyllabic character of the geminate is reflected in its association to nucleus and onset positions. In case the first consonant is more sonorous than the following geminate, the resulting structures involve more common ambisyllabic geminates, interposed between two nuclei. The forms xtːa and ftːae illustrate the situation in (6). Likewise, Imane produced 8924, 22,24% of which are coda consonants. The relatively important percentage of coda consonants is due the high frequency of ambisyllabic consonants in their productions.

Conclusion

In this paper, we presented preliminary results of a longitudinal study that focused on the developmental trajectory of speech production capacities in two Berber children acquiring Tashlhiyt. Our data provide evidence for universal tendencies in babbling and first words: The subjects produced more stops, nasals and glides than other types of sounds; more coronals and labials than dorsals and gutturals; the vowel a is produced earlier than the remaining i and u. Open syllables and more precisely the CV syllable type are predominant in both babbling and words. In addition, we noticed the early emergence of geminates in both children productions. They occurred preferably in word-medial position. Word-initial geminates were favoured over wordfinal ones, departing from the patterns observed in adult language. This was explained by the late acquisition of coda consonants, as opposed to onset consonants. We also examined geminates in relation to syllable structure. We discussed two types of ambisyllabic geminates: Those attaching to a coda + onset sequence and those associated to nucleus and onset positions. We showed that there is no phonological reason for rejecting the syllabification of initial geminates either as a complex onset (typically when followed by a vowel) or as an onset + nucleus sequence, especially in words made of one geminate.

  Imane and Reda in the babbling period, 85% were open. In the first words period, Imane produced 2370 syllables, 73% of which were open, whereas Reda had 82% of the syllables open (2082 syllables in total). The results are given in tables 3 and 4 (underlined consonants are syllabic).

Figure 5 .

 5 Figure 5. Number of occurrences and types of geminates in Imane's productions

  insofar as it involves a coda + onset or a nucleus + onset sequence. One can still ask how relevant is the assumption that a language may have geminates attaching to a nucleus + onset sequence but not their mirror-image forming an onset + nucleus sequence. The ban of the latter structure in syllable theory generally relies on the argument that onsets do not contribute to weight, and more particularly the avoidance of the so-called 'flopped structure' (cf.Hayes 1989: 258, and[START_REF] Topinzi | On the existence of moraic onset geminates[END_REF] for an alternative view). However, as far as Tashlhiyt Berber is concerned, there is no phonological evidence for syllable weight, apart from metrics and verse structure which has been argued to rely on fixed alternations of light and heavy syllables (cf.[START_REF] Dell | Singing in Tashlhiyt Berber: A language that allows vowel-less syllables[END_REF][START_REF] Dell | Syllables in Tashlhiyt Berber and in Moroccan Arabic[END_REF][START_REF] Dell | Singing in Tashlhiyt Berber: A language that allows vowel-less syllables[END_REF] the reader is referred to Hammane 2010 for an alternative analysis without any distinction between light and heavy syllables). Furthermore, we are left with forms such as ʃː 'eat' (see 4) and kː 'cross' which, according to Dell & Elmedloui's approach, should be analysed as closed syllables only because they eschew prohibited branching into an onset + nucleus structure. This is all the more questionable in child phonology that open syllables are generally favoured over closed ones. Tashlhiyt Berber is no exception to this trend: During the whole period of study, Reda produced 5537, of which only 909 occurred in the coda position.

Table 1 .

 1 The consonantal inventory of Tashlhiyt Berber.

		Labial		Dental	Alveolar	Palatal	Velar	Uvular	Pharyngeal	Laryngeal
	Stop	b	t	d		k	g	q q w
			tˤ	dˤ		k w	g w

Table 2 .

 2 Frequency of occurrence of segments and utterances.

		Utterances	Consonants	Vowels	C/V ratio
	Imane	3339	8924	7261	1,23
	Reda	2986	5537	5823	0,95
	Total	6325	14461	13084	1,09

  fricative[h] which represented over 50% of the fricatives in Reda's babbling.Regarding place of articulation, over 67% of the consonants produced by Reda and Imane in the babbling period were labials and coronals, while gutturals represented approximately 19% and dorsals around 13% for Imane and 17% for Reda. Similar patterns arose in first words, although we noticed an increasing use of

	gutturals, especially in Imane's productions where they reached 19% of the
	consonants compared to 9% in the babbling period. The use of laryngeal [h] explains
	the relatively high frequency of gutturals in both kids productions.
	As to vocalic patterns, the open vowel represented 80% of the vowels found in
	both children productions during the babbling period. Of particular interest is the use
	of the front [ae], specifically in Reda's productions. This is consistent with what has
	been found in other languages such as Tunisian Arabic, whose vocalic system is
	similar to that of Tashlhiyt Berber. Kern & davis (2009) noticed that children acquiring
	Similar patterns arose in period 2 (first words). Both children produced more
	oral stops than any other manner of articulation: 43.8% of the consonants found in
	Imane's productions are stops, as opposed to 33.5% in Reda's productions. Glides

and liguids are the least produced categories in first words, just as in the babbling period.

this language use the front [ae] much more than the central variant

[a]

. Another interesting point is the use of the mid vowel [e], especially in Imane's productions (23% of the vowels in the babbling period).

Table 3 .

 3 Syllable types in Reda's productions

				Open	Closed
		CV	CC V	C	CCV CCC VC CC CVC CCC VCC
	Babbling period 59,78 0,05 21,85 2,62 0,92	0 4,20 0,81 8,85 0,32 0,54
	Words period	57,10 0,28 22,04 2,01 1,15 0,04 5,42 0,09 11,43	0 0,38
	Table 4. Syllable types in Imane's productions
				Open	Closed
		CV	CC V	C	CCV CCC VC CC CVC CCC VCC
	Babbling period 75,77 0,11 8,27 0,23 0,11 0	1,55 0,23 13,54 0,11 0
	Words period	62,78 1,09 8,52 0,12 0,88 0	4,30 3,37 17,80 0,80 0,29

Among open syllables, CV ones are much more frequent in both children productions. CC syllables are far fewer though phonologically speaking they are equivalent to CV syllables in that they contain an onset plus a nucleus, both of which are simplex. The relatively high percentage of closed syllables (in particular CVC) in both children productions is due to the frequency of geminates, which are syllabified

[START_REF] Chaker | Bibliographie critique: langue, littérature, identité[END_REF] refers to three million natives at the beginning of the 90s. According toBoukous (2011:28),

28%of the population speaks Berber, of which 52% are native Tashlhiyt.


Won June Kim

Sarah Xing

Geoffroy Kremer

Muriel Sicot

Bertrand Kierren

Daniel Malterre

Giorgio Contini

Julien Rault

Patrick Le Fèvre

François Bertran

Dario Rocca

Yannick Fagot-Revurat
email: yannick.fagot@univ-lorraine.fr

Sébastien Lebègue
email: sebastien.lebegue@univ-lorraine.fr

Electronic Structure of Heavy Halogen Atoms Adsorbed on the Cu(111) Surface: a Combined ARPES and First Principles Calculations Study

By means of angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations, we investigate the electronic structure of a Br or I atom overlayer on the Cu(111) surface produced by the well known Ullmann coupling reaction. We found that the iodine adsorbate induces two spin-orbit splitted highly dispersive bands, which are well-separated from the bulk Cu-3d bands, whereas the bromine-induced bands are flat and largely hybridized with the copper states. Also, our measured constant energy maps show that the I-induced bands have a parabolic shape in the whole surface Brillouin zone, which is confirmed by our calculations. Overall, the agreement between theory and experiments is excellent, giving new insights concerning the electronic structure of halogen atoms on noble metals and its possible influence on the molecular electronic structure.

Introduction

The adsorption of halogen atoms on noble metal surfaces has been the subject of an intense research work in the field of surface science and electrochemistry in the last two decades. This is due to the fact that the adsorption of halogen atoms modifies the electronic structure and changes the surface work function, which is directly related to the electrochemical potential of the metal electrode. [START_REF] Magnussen | Ordered Anion Adlayers on Metal Electrode Surfaces[END_REF][START_REF] Andryushechkin | Adsorption of halogens on metal surfaces[END_REF] Therefore, there have been several theoretical [START_REF] Pettersson | Adsorbate ionicity and surface-dipole-moment changes: Cluster-model studies of Cl/Cu(100) and F/Cu(100)[END_REF][START_REF] Koper | Interaction of halogens with Hg, Ag and Pt surfaces: a density functional study[END_REF][START_REF] Migani | Chemisorption of atomic chlorine on metal surfaces and the interpretation of the induced work function changes[END_REF][START_REF] Migani | A Systematic Study of the Structure and Bonding of Halogens on Low-Index Transition Metal Surfaces[END_REF][7][START_REF] Bagus | Dependence of surface properties on adsorbatesubstrate distance: Work function changes and binding energy shifts for I/Pt(111)[END_REF][START_REF] Peljhan | Adsorption of Chlorine on Cu(111): A Density-Functional Theory Study[END_REF][START_REF] Pašti | Halogen adsorption on crystallographic (111) planes of Pt, Pd, Cu and Au, and on Pd-monolayer catalyst surfaces: First-principles study[END_REF][START_REF] Roman | Periodic Density-Functional Calculations on Work-Function Change Induced by Adsorption of Halogens on Cu(111)[END_REF][START_REF] Gossenberger | Change of the work function of platinum electrodes induced by halide adsorption[END_REF][START_REF] Roman | Halide adsorption on close-packed metal electrodes[END_REF][START_REF] Gossenberger | Equilibrium coverage of halides on metal electrodes[END_REF][START_REF] Zhu | Trends and Regularities for Halogen Adsorption on Various Metal Surfaces[END_REF][16] and experimental [START_REF] Citrin | Adsorption Sites and Bond Lengths of Iodine on Cu{111} and Cu{100} from Surface Extended X-Ray-Absorption Fine Structure[END_REF][START_REF] Jones | Bromine adsorption on Cu(111)[END_REF][START_REF] Inukai | Adlayer Structures of Chlorine, Bromine, and Iodine on Cu(111) Electrode in Solution: In-Situ STM and ex-Situ LEED Studies[END_REF][START_REF] Andryushechkin | Commensurate-incommensurate phase transition in chlorine monolayer chemisorbed on Ag(111): Direct observation of crowdion condensation into a domainwall fluid[END_REF][START_REF] Andryushechkin | Atomic structure of Ag(111) saturated with chlorine: Formation of Ag 3 Cl 7 clusters[END_REF][START_REF] Zheltov | Structural paradox in submonolayer chlorine coverage on Au(111)[END_REF][START_REF] Cherkez | Self-ordered nanoporous lattice formed by chlorine atoms on Au(111)[END_REF] studies to investigate the overlayer structures, to measure the changes in the work function, and to study their dependence on the coverage of the halogen adsorbates. Especially, some recent works have focused on the relationship between the direction of the work function shift and the character of the metal-halogen bonding, [START_REF] Roman | Periodic Density-Functional Calculations on Work-Function Change Induced by Adsorption of Halogens on Cu(111)[END_REF][START_REF] Roman | Halide adsorption on close-packed metal electrodes[END_REF] as well as the physical reasons for the formation of the particular patterns at low coverages. [START_REF] Andryushechkin | Commensurate-incommensurate phase transition in chlorine monolayer chemisorbed on Ag(111): Direct observation of crowdion condensation into a domainwall fluid[END_REF][START_REF] Andryushechkin | Atomic structure of Ag(111) saturated with chlorine: Formation of Ag 3 Cl 7 clusters[END_REF][START_REF] Zheltov | Structural paradox in submonolayer chlorine coverage on Au(111)[END_REF][START_REF] Cherkez | Self-ordered nanoporous lattice formed by chlorine atoms on Au(111)[END_REF] Also, halogen atoms, especially bromine and iodine, are the key ingredients of the wellknown Ullmann coupling reaction, which is one of the chemical reaction that synthesizes organic or organometallic polymers on a metal surface. [24][START_REF] Di Giovannantonio | Reversibility and intermediate steps as key tools for the growth of extended ordered polymers via on-surface synthesis[END_REF][START_REF] Clair | Controlling a Chemical Coupling Reaction on a Surface: Tools and Strategies for On-Surface Synthesis[END_REF] In the Ullmann coupling reaction, the deposition of halogenated molecules on noble metals leads to the formation of polymeric architectures, i.e., a molecular layer in a one or two-step reaction involving organometallic intermediates depending on the molecule/substrate affinity. [START_REF] Björk | Mechanisms of Halogen-Based Covalent Self-Assembly on Metal Surfaces[END_REF][START_REF] Di Giovannantonio | Insight into Organometallic Intermediate and Its Evolution to Covalent Bonding in Surface-Confined Ullmann Polymerization[END_REF][START_REF] Di Giovannantonio | Comment on 'Insight into Organometallic Intermediate and Its Evolution to Covalent Bonding in Surface-Confined Ullmann Polymerization[END_REF][START_REF] Basagni | Molecules-Oligomers-Nanowires-Graphene Nanoribbons: A Bottom-Up Stepwise On-Surface Covalent Synthesis Preserving Long-Range Order[END_REF][START_REF] Björk | Reaction mechanisms for on-surface synthesis of covalent nanostructures[END_REF][32] Changing the type of halogen in the precursor is known to lead to strong modification of the Ullmann reaction parameters, mainly due to its reactivity with the surface and its ability to participate to the final molecular architecture. [START_REF] Galeotti | The role of halogens in on-surface Ullmann polymerization[END_REF] Indeed, depending on the size of the precursive halogenated molecules, halogen atoms remaining at the surface as a by-product of the reaction play a role both in the nanopatterning of the polymeric layer and possibly on the corresponding electronic structure. [START_REF] Vasseur | Quasi one-dimensional band dispersion and surface metallization in long-range ordered polymeric wires[END_REF] Therefore, understanding the electronic structure of halogen-adsorbed metal surface is fundamentally important for deep studies of the above phenomena. Regardless of this importance, the direct measurement of the band structure of halogen atoms on the metal surfaces have not been conducted up to now. In this letter, we focus on the modification of the electronic structure of the Cu(111) surface when two halogen atoms, I and Br, are adsorbed, by using combined experimental and theoretical methods. Firstly, we have synthesized molecular layer structures from the deposition of 1,4-diiodobenzene (dIB) and 1,4-dibromobenzene (dBB) on the Cu(111) surface and we confirmed the existence of patches of I/Br overlayer structures in them. Then the electronic structure of the I/Br-adsorbed Cu(111) overlayers are analyzed by both angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) simulations.

Methods

Experimental Setup

All the experiments have been performed under ultra-high vacuum (UHV) conditions (base pressures below 2×10 -10 mbar). The Cu(111) surface was prepared before dosing by multiple cycles of Ar + sputtering at 700 eV followed by annealing at 500 precursor molecules (Sigma-Aldrich, 98 % purity) were deposited from their crystallized forms in a quartz crucible by sublimation at room temperature onto the Cu(111) substrate (SPL Laboratory) through a leak valve. A partial pressure of 10 -8 mbar was maintained in the chamber during few minutes while the substrate was kept at room temperature. The surface coverage was evaluated by recording STM images as function of sublimation time in our experimental set-up and by monitoring the XPS C-1s/Cu-2p peak intensity ratio on the CASSIOPEE beamline. Scanning tunneling microscope (STM) was performed with an Omicron LT-STM operating at liquid nitrogen or liquid helium temperatures, using a constant tunneling current, I, and a constant bias voltage, V BIAS , measured from the tip to the sample (the tip is grounded). STM images were analyzed using WSxM software and treated for line-by-line flattening, plane subtraction and contrast enhancement. ARPES measurements were performed at the CASSIOPEE beamline of the synchrotron SOLEIL at a photon energy of 35 eV with an energy and momentum resolution of 10 meV and 0.01 Å -1 respectively, using a VG-SCIENTA R4000 analyzer. Light polarization in a linear horizontal configuration, i.e., electric field perpendicular to the sample surface, was taken. All the photoemission data were recorded at 30 K. LEED patterns were recorded at an electron energy of 145 eV and at a temperature of 80 K.

Computational Details

Our density functional theory (DFT) calculations were performed with the projector augmented wave (PAW) method as implemented in the Vienna ab initio Simulation Package (VASP). [START_REF] Kresse | Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set[END_REF][START_REF] Kresse | From ultrasoft pseudopotentials to the projector augmentedwave method[END_REF] The generalized-gradient approximation (GGA), as designed by Perdew, Burke, and Ernzerhof (PBE), [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF] was used for the exchange-correlation functional and an energy cutoff of 600 eV was chosen for the plane wave basis set. Electronic iterations were continued until the total energy difference between steps became smaller than 10 -7 eV, and the geometry optimization was performed until the force of each atom became smaller than 5.0 × 10 -3 eV/Å. To build a model for our calculations based on these previous experimental studies [START_REF] Citrin | Adsorption Sites and Bond Lengths of Iodine on Cu{111} and Cu{100} from Surface Extended X-Ray-Absorption Fine Structure[END_REF][START_REF] Jones | Bromine adsorption on Cu(111)[END_REF][START_REF] Inukai | Adlayer Structures of Chlorine, Bromine, and Iodine on Cu(111) Electrode in Solution: In-Situ STM and ex-Situ LEED Studies[END_REF] and our STM/LEED measurements, we started by obtaining the equilibrium lattice parameter of bulk fcc-Cu. Our calculated lattice parameter a = 3.63 Å, obtained with a 12 × 12 × 12 Γ-centered k -point grid, is close to the experimental value of 3.615 Å. [START_REF] Villars | Atomic-environment classification of the chemical elements[END_REF] The slab to model the Cu surfaces were constructed using the optimized lattice parameter of bulk Cu: we con-

structed a √ 3 × √ 3 R30
• supercell for the Cu(111) surface, with a thickness of eight atomic planes and we added a vacuum of 15 Å to avoid significant interactions between periodically repeated slabs. To have a similar grid spacing in the reciprocal space to that of the bulk calculation, we used a Γ-centered k -grid of 10 × 10 × 1. The geometry optimization of the surfaces (with and without adsorbates) were performed by constraining the four atomic layers at the bottom of the slab to reproduce the bulk geometry of Cu while the other atoms were allowed to relax freely. The adsorption energies of the Br and I adsorbate atoms on the different Cu surfaces were calculated by subtracting the sum of total energies of the clean surface and the isolated atom from the total energy of (atom+surface) structure. Relativistic effects have been taken into account in our band structure calculations, since spin-orbit coupling is expected to be significant. The constant energy maps were calculated by using a dense k -point mesh of 30 × 30 × 1. mesurements. The PPP and OM chain periodicities of 4.4 and 6.8 Å are in agreement with previous works. [START_REF] Di Giovannantonio | Insight into Organometallic Intermediate and Its Evolution to Covalent Bonding in Surface-Confined Ullmann Polymerization[END_REF][START_REF] Vasseur | Quasi one-dimensional band dispersion and surface metallization in long-range ordered polymeric wires[END_REF] The iodine layer in zone 1 shows a periodicity of 4. Previous DFT studies have shown that for low coverages, both I and Br atoms occupy the 3-fold fcc site on the Cu(111) surface. [START_REF] Migani | A Systematic Study of the Structure and Bonding of Halogens on Low-Index Transition Metal Surfaces[END_REF][START_REF] Roman | Halide adsorption on close-packed metal electrodes[END_REF][START_REF] Zhu | Trends and Regularities for Halogen Adsorption on Various Metal Surfaces[END_REF] In our case, the measured LEED patterns of the √ 3 × √ 3 R30 • overlayer correspond to a coverage of 0.33 monolayer (ML) with a superperiodicity of 4.4 Å. To investigate the most stable sites and their consequences on the electronic structure, we calculated the adsorption energies (see table 1) of I and Br atoms for different sites using DFT. We have considered four different sites: top, 2-fold bridge, 3-fold fcc and hcp sites, as detailed in figure 1(e). Among them, 3-fold fcc is the most favorable site for both I and Br atoms, which is the same tendency as in previous DFT studies with lower coverages. [START_REF] Migani | A Systematic Study of the Structure and Bonding of Halogens on Low-Index Transition Metal Surfaces[END_REF][START_REF] Roman | Halide adsorption on close-packed metal electrodes[END_REF][START_REF] Zhu | Trends and Regularities for Halogen Adsorption on Various Metal Surfaces[END_REF] The 3-fold hcp site is less stable than 3-fold fcc, but the difference in the adsorption energies between the two sites is only 0.01 eV. Note that there is no value for the adsorption energy of the bridge site in table 1 since the I/Br atoms moved to the 3-fold fcc site during the geometry relaxation, indicating that this site is highly unfavorable. Also, it is noteworthy to mention that both the Cu-3d bands and the I-induced bands measured by ARPES are deeper in energy than in the DFT band structures. This is due to the well known problem of DFT when used with a semi-local approximation for the exchangecorrelation potential: it suffers from a self-interaction error, which make the eigenstates to be shifted in energy. This issue could be solved by using a more elaborate level of theory such as hybrid functionals or the GW approximation. However, these methods are computationally heavy, and their use would not change the qualitative comparison bewteen theory and experiments in the present study. We have also investigated the shape of I-induced band in the surface Brillouin zone (SBZ) by calculating and measuring constant energy maps. Figures 3(a

Results and Discussion

Conclusion

In summary, we have studied the geometry and the electronic structure of I/Br atoms adsorbed on a Cu(111) surface by STM, LEED, ARPES measurements combined with DFT simulations. Even if the two halogen elements, I and Br, adsorb at the 3-fold fcc site on the Cu(111) surface with a √ 3 × √ 3 R30 • super-periodicity, we found that they display different electronic structure. When Br atoms are adsorbed, the corresponding band presents a flat dispersion and is close to the Cu-3d bands, and, in this particular experimental case, superimposed with molecular states so that ARPES can hardly resolve it. However, the induced spin-orbit splitted bands by the I adsorbate are well separated from the Cu-3d bands.

From the analysis of the band structures and of the constant energy maps, we found that the I-induced bands have a quadratic nature in k -space, a feature which is shown in both our calculations and our experiments. Overall, the work presented here solves the long standing issue of the electronic structure of heavy halogen atoms on noble metal surfaces. In this respect, it represents a fundamental step towards a better understanding of the complex interplay between reactants and products of the surface mediated Ullmann reaction.

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Figures 1 ( 5 Page 5 of 22 ACS

 1522 Figures 1(a) and 1(c) show the STM images of the Cu(111) surface on which dIB and dBB precursors were used for the Ullmann coupling reaction, respectively. Even if I/Br patches and poly(p-phenylene) (PPP) or organometallic (OM) chains co-exist, the two precursors lead to different behaviors on the surface superstructure after the dehalogenation. When the dIB molecules are in contact with the Cu(111) surface and after a moderate annealing at 150 • C for 10 mn (see figure 1(a)), large areas of iodine domains (zone 1) including few PPP chains (zone 2) as well as ordered OM chains (zone 3) are evidenced by our STM

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 622 4 Å, corresponding to that of √ 3 × √ 3 R30 • overlayer structure as expected from the periodicity of Cu(111) surface. The low-energy electron diffraction (LEED) pattern of the dIB-deposited Cu(111) surface presented in figure 1(b) exhibits a well-defined √ 3 × √ 3 R30 • surface superstructure compared to that of the bare Cu(111) surface in figure 1(f). We interpret it as the signature of Paragon Plus Environment The Journal of Physical Chemistry the overlayer structure formed by iodine patches. On the other hand, the STM image of the dBB-deposited Cu(111) surface (see figure 1(c)) shows that Br domains are small compared to the iodine domains in the dIB-deposited Cu(111) surface. As a result, the domains of self-assembled OM chains also contribute to the LEED pattern leading to a more complex one as presented in figure 1(d). A more detailed analysis of this molecular self-assembling is beyond the scope of this paper and will be presented elsewhere for clarity. For the rest of the discussion, we use I/Cu(111) and Br/Cu(111) to either refer to the actual annealed surface of Cu(111) after adsorption of dIB and dBB, respectively or to the theoretical model of the corresponding halogen atoms on the Cu surface.

From

  the ARPES measurements carried out on the I/Cu(111) surface shown in fig-

Figure 2 : 8 Page 8 of 22 ACS 9 Page 9 of 22 ACS

 2822922 Figure 2: Left panels: ARPES intensity map measured at 30 K around the Γ point on (a) I/Cu(111), (b) Br/Cu(111), and (c) the bare Cu(111) substrate; Right panels: DFT band structures for (d) the I/Cu(111), (e) Br/Cu(111), and (f) clean Cu(111) surfaces. The green fatbands in figure 2(d) and 2(e) depicts the projection of the band structure onto the halogen atomic states. Fermi levels are set to zero for all figures.

Figure 3 :

 3 Figure 3: Constant energy maps in the 2-dimensional k -space measured by ARPES for (a) the bare Cu(111) surface and (b)-(d) the I/Cu(111) surface taken for three different values of the energy. The black dashed and magenta dotted lines represent the surface Brillouin zone (SBZ) of the clean surface and of the surface with a √ 3 × √ 3 R30 • overlayer, respectively. Figure 3(e) shows the DFT constant energy maps of the band which contains the I-induced state (depicted as a black solid line in the bandstructure at the bottom of figure). For both upper and lower energy cuts, the smallest circle corresponds to the I-induced state. The black lines represent the SBZ. For details about the two energy values that were chosen, see the main text.

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 10221122 )-(d) are the ARPES constant energy maps for the Cu(111) and I/Cu(111) surfaces, respectively, with the binding energy cut between -1.5 to -1.7 eV. Whereas only a large hexagon-like structure is shown Paragon Plus Environment The Journal of Physical Chemistry in the map for the Cu(111) surface as expected for a photon energy of 35 eV (see figure 3(a)), the I/Cu(111) surface has additional circular structures around the Γ-point (see figure 3(b)-(d)), which comes from the parabolic I-induced band as shown in figure 2(a). As expected, the center of this parabolic state is repeated with a 1.63 Å -1 periodicity evidencing that these states are the clear signature of the I-induced states associated with the √ 3 × √ 3 R30 • -I surface superstructure. Furthermore, as the magnitude of the binding energy increases, the size of the circle structure increases, indicating that the I-induced bands have a 2-dimensional quadratic nature. The corresponding constant energy maps obtained from DFT are shown in figure 3(e). To calculate the constant energy maps, and we chose two energy values in our bandstructure, as depicted in the bottom of figure 3(e): one is -0.96 eV (upper cut), which is just below the maximum in energy of the I-induced bands at the Γ-point, and the other is -0.99 eV (lower cut). The smallest circle around the Γ-point corresponds to the circle-like structure of the I-induced band in the ARPES results, of which the size increases as the magnitude of the binding energy increases. A larger circle and hexagon-shape structures, which are depicted as blue lines, are also shown in each DFT constant energy map. These structures correspond to Cu surface bands in our DFT simulations and are not clearly identified by ARPES due to the attenuation of this signal by the presence of the iodine layer at surface. By going to higher binding energy, we also observed the evolution of the second I-induced band. Figure S2 (b)-(d) are the ARPES constant energy maps taken at the binding energies in a range between -2 to -1.8 eV, showing that the increase of the radius of the circular structure is repeated again from the Γ-point. The ARPES intensity curves taken at the vicinity of k x = -0.1 Å -1 and k y = 0.0 Å -1 (see figure S2(a)) proves that the second evolution of the circular structure corresponds to the second I-induced band (peak 3). The origin of the different shapes of the I-and Br-induced bands can be associated with the different size of the Br and I atoms. Since both I and Br have the same √ 3 × √ 3 R30 • structure on the Cu (111) surface, the nearest Br-Br and I-I distances are the same. However, Paragon Plus Environment The Journal of Physical Chemistry the atomic radius of I is larger than the one of Br, resulting in a larger overlap between the nearest I atoms than the nearest Br atoms. This idea is supported by the charge density plots of the I/Cu(111) and Br/Cu(111) overlayers shown in figure S3: our calculated charge densities show that the electrons of I are distributed in a wider region than the electrons of Br. Consequently, the I adsorbates form a band which is more dispersive than the one coming from Br atoms.

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Table 1 :

 1 Adsorption energies (in eV) and halogen-surface distances (in Å) of the I/Cu(111) and Br/Cu(111) overlayer structures. The most stable site for each composition is presented in bold.

	Site	∆E I ads (eV) r I-surf (Å) ∆E Br ads (eV) r Br-surf (Å)
	Top	-2.38	2.39	-2.65	2.27
	3-fold hcp	-2.69	2.22	-3.01	2.03
	3-fold fcc	-2.70	2.22	-3.02	2.03

Acknowledgement

The authors acknowledge financial support from the Agence Nationale de la Recherche (ANR) under Grant No. ANR-15-CE29-0003-01, the Italy-France International Program of 12 Page 12 of 22 ACS Paragon Plus Environment The Journal of Physical Chemistry Scientific Cooperation (PICS No. 6823, CNRS/CNR), and the French PIA project "Lorraine Université d'Excellence" (LUE). Calculations were conducted using GENCI-CCRT/CINES computational resources (Grant No. x2019-085106).

Supporting Information Available

• supp_info.pdf: supplementary figures and detailed analysis of surface bands This material is available free of charge via the Internet at http://pubs.acs.org/.