ARTICLE pubs.acs.org/JPCA
Photoionization of Propargyl and Bromopropargyl Radicals: A Threshold Photoelectron Spectroscopic Study Patrick Hemberger,† Melanie Lang,† Bastian Noller,† Ingo Fischer,*,† Christian Alcaraz,‡ Barbara K. Cunha de Miranda,‡,§ Gustavo A. Garcia,|| and Heloïse Soldi-Lose|| †
Institute of Physical and Theoretical Chemistry, University of W€urzburg, Am Hubland, D-97074 W€urzburg, Germany Laboratoire de Chimie-Physique, UMR 8000 CNRS & Universite Paris-Sud 11, F-91405 Orsay Cedex, France § Laboratorio de Espectroscopia e Laser, Instituto de Física, Universidade Federal Fluminense, 24210- 340, Niteroi, RJ, Brazil Synchrotron SOLEIL, L’Orme des Merisiers, Saint Aubin - BP 48, F-91192 Gif-sur-Yvette Cedex, France
)
‡
bS Supporting Information ABSTRACT: In this Article, we present mass-selected threshold photoelectron spectra of propargyl as well as the 1- and 3-bromopropargyl radicals. The reactive intermediates were produced by flash pyrolysis of suitable precursors and ionized by VUV synchrotron radiation. The TPES of the propargyl radical was simulated using data from a recent high-level computational study. An ionization energy (IE) of 8.71 ( 0.02 eV was obtained, in excellent agreement with computations, but slightly above previous experimental IEs. The pyrolysis of 1,3-dibromopropyne delivers both 1- and 3-bromopropargyl radicals that can be distinguished by their different ionization energies (8.34 and 8.16 eV). To explain the vibrational structure, a Franck-Condon simulation was performed, based on DFT calculations, which can account for all major spectral features. Bromopropargyl photoionizes dissociatively beginning at around 10.1 eV. Cationic excited states of 1- and 3-bromopropargyl were tentatively identified. The dissociative photoionization of the precursor (1,3-dibromopropyne) was also examined, delivering an AE0K (C3H2Brþ/C3H2Br2) of 10.6 eV.
’ INTRODUCTION It is widely accepted that the resonantly stabilized propargyl radical (H2CCCH) plays a major role in the polycyclic aromatic hydrocarbon (PAH) growth mechanisms.1 Zhang et al. observed the radicals recently in a benzene/gasoline flame by VUV spectroscopy.2 Especially the formation of the first ring is initiated by a recombination of two propargyl radicals. Three possible components can be established: 1,2-hexadiene-5-yne, 3,4-dimethylenecyclobutene, and 1,5-hexadiyne, which are all able to isomerize to benzene over different pathways.3 The addition of a further propargyl radical and H-abstraction leads to phenylpropargyl radicals,1 which were recently studied in our group by threshold photoelectron photoion coincidence (TPEPICO) techniques.4 From a spectroscopic view, the propargyl radical (H2CCCH) has been thoroughly studied. Deyerl et al. investigated its photodissociation dynamics, which leads after UV excitation (265240 nm) to cyclopropenylidene (c-C3H2) þ H.5 After some debate on the symmetry of this state, assignment to the 3 2B2 state was recently confirmed.6-8 A time constant of 50 fs for the deactivation into a lower-lying excited state by internal conversion was determined.9 Also, infrared and microwave spectroscopy were carried out on the propargyl radical.10-13 Infrared photodissociation experiments of C3H3 cations and their clusters with N2 and Ar were performed by several groups, rendering vibrational frequencies of the cation. Both C3H3þ isomers, r 2011 American Chemical Society
HCCCH2þ and the cyclic isomer c-C3H3þ, were observed during the experiments.14,15 Both play an important role in plasma chemistry. 16,17 Numerous calculations were also carried out, yielding adiabatic ionization energies between 8.6 and 8.8 eV.6 Recently, Botschwina et al. presented a high-level study, employing explicitly correlated coupled cluster theory to calculate the photoelectron spectrum (PES) of linear C3H3.18 They calculated the adiabatic ionization energy to be 8.696 and 8.705 eV depending on the used basis set. On the other hand, experimental data on the photoionization are scarce. A conventional photoelectron spectrum yielded an ionization energy of 8.67 eV.19 A zero kinetic energy photoelectron spectrum was recorded several years ago,20 but it could not be reproduced.21 A value of 8.70 eV was obtained by threshold photoelectron spectroscopy (TPES) using synchrotron radiation, but only little vibrational activity was observed due to a low signal-to-noise ratio.22 This situation leads us to reinvestigate the TPES of propargyl. Concerning the brominated propargyl radicals, only little information is available in the literature. C3H2Br was observed Received: December 21, 2010 Revised: February 4, 2011 Published: March 02, 2011 2225
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in the pyrolysis of C3H2Br2 as an intermediate in the generation of propadienylidene (l-C3H2), and some theoretical calculations were also performed.23,24 We are interested in such brominated radicals because they permit one to study the influence of substituents on the structure, stability, and reactivity of radicals.25 Furthermore, brominated radicals are involved in ozone destruction and play an important role in atmospheric chemistry.26,27 In addition to propargyl, we therefore studied the C3H2Br products of the 1,3-dibromopropyne pyrolysis by threshold photoelectron photoion coincidence spectroscopy.
’ EXPERIMENTAL AND COMPUTATIONAL METHODS The experiments were carried out at the DESIRS beamline, which is located at the SOLEIL storage ring in St. Aubin (France).28 Only a brief description of the setup is given here.29 The undulator30 (OPHELIE 2) delivers the VUV radiation to a 6.65 m normal incidence monochromator, which is equipped with four gratings.30,31 For the present project, we chose the low dispersion, high flux, 200 gr/mm grating. An exit slit of 100 μm was used, providing a photon resolution of 5 meV at 9 eV. Higher harmonics were eliminated by a gas filter, which was operated with argon (p = 0.25 mbar).32 The differentially pumped vacuum chamber SAPHIRS contains a velocity map imaging spectrometer (VMI) and WileyMcLaren TOF spectrometer (DELICIOUS II).29,33 An extraction field of either 95 or 19 V cm-1 was used. Mass selected ion images were measured by merely reversing the polarity of the VMI. Radicals were produced in a continuous free jet of argon with the flash pyrolysis technique, using a 50 μm nozzle.34,35 The argon/precursor mixture was passed through a resistively heated (30-50 W) SiC tube, in which the radicals are generated. Propargylbromide (purchased from Sigma Aldrich) and 1,3-dibromopropyne, synthesized according to the literature, were used as precursors.36 Two parameters were adjusted before a measurement: the heating power of the tube, to get full conversion of the precursor and the distance of the tube to the skimmer, to minimize the amount of thermal background. For mass selected threshold photoelectron spectra, the photon energy was scanned in steps of 2.5-20 meV, and threshold electrons with 2-20 meV kinetic energy were chosen. All spectra were normalized by the photon flux, measured with a photodiode (AXUV-100 from IRD), and the background (false coincidences) was subtracted. Density functional theory (DFT) calculations were performed with the B3LYP functional and the 6-311þþG** basis set as implemented in the Gaussian 03 suite of programs.37,38 The radical and ionic (singlet/triplet) ground-state geometries were obtained applying tight convergence criteria and an ultrafine grid.39 To calculate the adiabatic ionization energy (IEad), the difference between the sums of electronic and zero point energies of the cation and the radical in their ground-state geometry was calculated. The spin contamination was negligible. To improve the accuracy of the IEad calculations, a CCSD (UCCSD) ccpVDZ geometry optimization was performed, and the IE was calculated on different levels of theory (cc-pVnZ/n = T, Q) utilizing the Gaussian 09 program.40-44 Dissociation energies of the cations were obtained by applying the B3LYP functional. Therefore, the difference between the sums of the electronic and zero-point energies of the fragments and the parent were calculated. Excited singlet states were computed by time-dependent DFT (TD-DFT) with the B3P86 functional and the 6-311þþG** basis set.45,46 Franck-Condon simulations were performed with the program FCFit 2.8.8, described by Spangenberg et al.47
Figure 1. TPES of the propargyl radical. The ν3 mode is active upon ionization and can be assigned. The red line shows a Franck-Condon simulation, obtained by convoluting the FC-factors from the literature with a Gaussian function.18
Table 1. Comparison of the Reported Ionization Energies of the Propargyl Radical IE/eV 19
method
Minsek and Chen
8.67
PES
Lau and Ng49 Botschwina and Ostwald18
8.679 8.696/8.705
ab initio ab initio
Sch€uβler et al22
8.70
TPES
Zhang et al2
8.674
VUV-PIE
this work
8.71
TPES
’ RESULTS AND DISCUSSION a. Propargyl Radical. Mass spectra of propargyl bromide pyrolysis have already been shown in the literature.5,22 Although full conversion of the precursor can be achieved, dimerization of propargyl is known to occur as a side reaction under the conditions of a continuous expansion. To minimize this side reaction, the vapor pressure of the precursor has been reduced by cooling the sample, thus lowering the concentration. In Figure 1, the TPES of propargyl is depicted. It is dominated by a large peak. Its maximum at 8.71 eV is assigned to the adiabatic ionization energy. An error of 20 meV is estimated from the full width at half-maximum of this peak. The ionization energy of 8.71 eV, in agreement within error bars of our previous value, is higher than other experimental values, but is in excellent agreement with the high-level calculations of Botschwina and Oswald.18,19,22 All available ionization energies are assembled in Table 1. The appearance of only one intense peak indicates a small change in geometry upon ionization, because an electron is removed from a nonbonding orbital. A reproducible feature at 8.95 eV is also present in this spectrum. For comparison, we depict a simulation (red line) in Figure 1 that is based on the Franck-Condon factors reported by Botschwina and Oswald (red sticks).18 The stick spectrum was subsequently convoluted with a Gaussian function (fwhm = 25 meV) and is also given in Figure 1 as a red line. The computations show a small reduction of the C-CH2- (∼2.7 pm) and an elongation of the -CtCand the C-H-bond (both about 1 pm) upon ionization.18 2226
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Figure 2. Mass spectra at 10.2 eV showing the pyrolysis of the C3H2Br precursor to the C3H2Br radical. An almost complete conversion of the precursor to the radical could be ensured. Note the difference in scale of the two traces.
From the computations, one expects a short progression in the mode ν5 as well as the appearance of the ν3. Those bands are assigned to the pseudosymmetric (ν5) and pseudoantisymmetric (ν3) CC stretching vibration. The ν3 fundamental can be tentatively identified in the experimental spectrum. The CH2 scissoring mode (ν4) and the ν5 vibration, on the other hand, are probably below the noise level in our spectrum. For the vibrational excitation of the ν4, only small FC factors were predicted.18 Experimental values for IEad and the ν3 vibration are 8.71 ( 0.02 eV and 1950 cm-1, respectively, which compares with a calculated wavenumber of 2121 cm-1 and the value of 2077 cm-1 for ν3 from IRPD experiments.15,18 Overall, very good agreement between experiment and theory has been achieved. Hot and sequence bands contribute to the signal around the origin and complicate the determination of the exact ionization energy. In a recent photoionization study of the methyl radical, a vibrational temperature of about 550 K has been determined. The radicals are heated in the SiC after the expansion in a continuous molecular beam, where the initial adiabatic cooling is less pronounced than in a pulsed nozzle.48 b. Bromopropargyl Radicals. Mass spectra recorded at 10.2 eV show the precursor 1,3-dibromopropyne with the typical isotopic pattern 1:2:1, expected when two bromines are attached to a hydrocarbon molecule (Figure 2, lower trace). When the pyrolysis is turned on (Figure 2, upper trace), the precursor disappears, and the C3H2Br (m/z = 117/119) radical appears with a good yield. Several other masses can be identified: m/z = 38 is a C3H2 isomer, and m/z = 39 is the propargyl radical, which is produced by hydrogen addition to C3H2. Such bimolecular reactions were observed in former studies.50,51 Mass 74 might be assigned to a stable carbon chain, C6H2, which can be formed in the pyrolysis. However, it might also be due to diethyl ether, which was used in the synthesis as a solvent. The signal at m/z = 76 is the dimer of m/z = 38. When dealing with radicals produced by pyrolysis, one has to take into account that the cation can also be formed by dissociative photoionization of the precursor. Therefore, we investigated the photoionization and dissociative photoionization of the precursor to extract the ionization energy of the precursor and the appearance energy of the fragment ion (C3H2Brþ). Figure 3 shows the breakdown diagram (left-hand side) of the parent (C3H2Br2þ) and the daughter (C3H2Brþ) ions and the threshold photoelectron spectrum (right-hand side). Note that all threshold electrons have been collected, but contributions from masses other than C3H2Br2 are minor (cf., Figure 2). The latter shows a broad band, which indicates a change in geometry upon
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Figure 3. Breakdown diagram of 1,3-dibromopropyne (left) and threshold photoelectron spectrum of C3H2Br2 (right, all threshold electrons collected).
ionization of 1,3-dibromopropyne. The onset of the electron signal, corresponding to the adiabatic ionization energy (IEad), is around 9.75 eV. For comparison, a value of 9.45 eV was computed for the IEad on a B3LYP level of theory. At higher photon energies, the ion starts to fragment and loses a bromine atom. This effect can be observed in the breakdown diagram (Figure 3 left) above 10.35 eV. The extraction of the 0 K appearance energy of the fragment ion requires modeling the whole breakdown diagram. The modeling was carried out using methods described in the literature, assuming a fast fragmentation, so that the effect of the unimolecular fragmentation rate constant is ignored.52-55 In brief, a thermal energy distribution (TED) was calculated by weighing, at a given temperature, the rovibrational density of states, obtained by a modified Beyer-Swinehart algorithm.56 This TED function was integrated and fitted, delivering an AE0K of 10.6 eV and an internal temperature of 300 K. This value is in agreement with the computed one of 10.25 eV (B3LYP), which suggests that the effect of the reverse barrier is negligible. Taking into account the IEad and the AE0K of 1,3 dibromopropyne, one can calculate the C-Br bond energy of the C3H2Br2 cation. Neglecting the reverse barrier of this reaction, a bond dissociation energy of 0.85 eV (82 kJ/mol) is obtained. However, two observations require further discussion: First, a fitted temperature of 300 K seems too high, and second, the signal of the parent does not disappear completely. One explanation could be the presence of dimers (C3H2Br)2 and clusters with argon (C3H2Br)Ar in the molecular beam expansion, which photoionize dissociatively to monomer ions. This would shift the AE0K to higher energies and results in a broader breakdown diagram with an artificially high temperature. This effect was observed in the dissociative photoionization of iodomethane57,58 and has also been discussed for CF3Br.59 A further possibility is an isomerization on the cationic surface, because some of the C3H2Brþ isomers lie very close in energy (Figure 5, vide infra). This has recently been observed for dichloroethylene ions and leads to a daughter signal that is a mixture of two species.60 In the next step, we studied the C3H2Br radical by TPE spectroscopy. The photon energy was scanned in 2.5 meV steps, and data were averaged for 40 s per point (pyrolysis turned on). The resulting TPES is given in Figure 4. Four maxima appear at 8.16, 8.22, 8.34, and 8.43 eV. The slow onset from 7.8 to 8.1 eV is most likely due to hot or sequence bands, because the pyrolysis source produces vibrationally and rotationally excited radicals in a continuous expansion (vide supra).48 For a better understanding of the spectrum, we performed a Franck-Condon (FC) simulation.47 The radical and ion equilibrium structure of the 3-bromopropargyl (H2CCCBr) was calculated, and an IEad of 8.09 eV was obtained on a B3LYP level of theory. Both molecules have C2v symmetry. The radical has a 2B1 2227
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Table 2. Comparison of Calculated and Experimental IEad of Both C3H2Br Isomers IEad/eV
Figure 4. TPES of C3H2Br: The spectrum contains contributions from two isomers as evident from the FC simulations of BrCCCH2 (red dashed line) and HBrCCCH (blue dashed line). The green line is the sum of both simulations.
Figure 5. Computed energy diagram of the stable isomers of C3H2Br. Given are the relative energies of four isomers, ionization energies, and energies for dissociative photoionization.
ground state, whereas the cationic ground state is of 1A1 symmetry. A strong change in geometry is observed in the C-Br bond, which is reduced by about 5.5 pm. Also, the CtC- and the CC-bond lengths decrease by 3.2 and 1.3 pm, respectively. For more details on the geometry, the reader is referred to the Supporting Information. With the calculated structures and force constants, a FC simulation was performed, depicted in red (convolution fwhm = 50 meV) in Figure 4. The second band at 8.22 eV is due to a C-Br stretching vibration (522 cm-1/ν5/a1) with some contribution from a combination band with the C-C-CH2 out-of-plane bending mode (170 cm-1/ν8/b2). Besides the ν5 progression, also some activity in the CtC stretching vibration (2171 cm-1/ν2/a1) is expected at around 8.43 eV from the simulations, but only with small intensity. When an electron is removed, the C-Br, C-C, and CtC bond lengths shorten, because the HOMO has antibonding character and the positive charge is stabilized by the free electron pairs of the bromine. Thus, upon ionization, the bond order increases. However, the high energy part of the spectrum including the third strong peak at 8.34 eV is not well described by the simulation. Because the electron signal is mass selected, the contribution of a different isomer constitutes a likely explanation.
exp.
B3LYP
CCSD
HBrCCCH
8.34
8.27
8.324 (cc-pVQZ)
H2CCCBr
8.16
8.09
8.133 (cc-pVTZ)
Therefore, we performed further calculations on the various C3H2Br isomers, which are summarized in the energy diagram in Figure 5. The H2CCCBr (3-bromopropargyl) isomer has an IEad = 8.09 eV, whereas the cyclic isomer and the H2BrCCC isomer show values of 6.52 and 10.91 eV (all values obtained from DFT). On the other hand, the HBrCCCH (1-bromopropargyl) isomer also has an IEad in the studied energy range (IEad,calc = 8.27 eV). A progression in the C-Br- stretching mode should be expected because the molecule reduces its C-Br bond length by about 8.5 pm upon ionization and the C-C-C angle is reduced by about 1.5°. The detailed geometries of all species are again given in the Supporting Information. In the next step, we simulated the photoelectron spectrum of 1-bromopropargyl (HBrCCCH). The expected spectrum of 1-bromopropargyl is given as a blue line/blue sticks in Figure 4. It is weighted relative to H2CCCBr by a factor of 0.8. As can be seen, it fits the high energy part of the spectrum well. The peak at 8.34 eV is assigned to the ionization threshold of this isomer. The broad band at 8.43 eV is then due to a combination of the C-Br stretching (770 cm-1/ν6/a0 ) and the C-C-C in-plane bending vibration (447 cm-1/ν8/a0 ) of the HBrCCCH cation. The broad band at around 8.50-8.55 eV originates from the ν6ν8 combination band and the second overtone of the ν6 mode. Assuming contributions from two isomers, H2CCCBr and HBrCCCH, leads to a good fit of the experimental TPE spectrum and can explain all major features (see green line in Figure 4). Comparing the experimental IEad of 8.16 eV for 3-bromopropargyl and 8.34 eV for 1-bromopropargyl with the B3LYP computational results, a difference of 60-70 meV was found. An even better agreement within 30 meV is obtained with CCSD calculations. Both calculations and experimental results are summarized in Table 2. Considering that a clean precursor was used to produce the H2CCCBr radical, one has to ask, where does the HBrCCCH radical come from? In the NMR spectrum of 1,3-dibromopropyne, only one peak is visible at 3.096 ppm in CDCl3, verifying the CH2 protons. One possibility is a rearrangement during the pyrolysis of the precursor. It is known in the literature that there is an equilibrium between propargylic and allenic structures (Scheme 1).36 Computations place the propargylic educt 29 kJ mol-1 higher in energy (BMK/6311þþG**). Turning the pyrolysis on, there is sufficient thermal energy deposited in the precursor to cross the barrier of 344 kJ mol-1 (BMK/6-311þþG**, QST3) to isomerization and partially form the allenic structure. In a true equilibrium, the allenic structure would dominate (97:3 at 1000 K). Although under the continuous flow conditions applied here, the contact time in the pyrolysis tube is significantly longer than under pulsed conditions, it might not be long enough to ensure a true thermal equilibrium. Note also that the ionization signal reflects not only the number of radicals, but also their ionization cross sections. The possible reaction is depicted in Scheme 1. The other two isomers shown in Figure 5 are too high in energy to be present in a sufficiently high concentration. 2228
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The Journal of Physical Chemistry A Scheme 1. Products of the Pyrolysis of 1,3-Dibromopropyne
Figure 6. TPE spectrum of C3H2Br. Both isomers contribute to the TPE signal with singlet and triplet states. Calculated vertical excitation energies of excited electronic states are given as arrows.
When the photon energy is tuned further to the blue (Figure 6), two bands at around 10.0 and 10.7 eV are found for m/z = 117/ 119. To elucidate which cationic states of both isomers contribute to the TPE signal, calculations of the excited states were performed. For both isomers, singlet and triplet cationic states were found in the relevant energy range. Detailed information is given in the Supporting Information. The lowest triplet state has a minimum with Cs symmetry (3A00 ) where the Br-C-C- angle is 152.6° (out of plane), with an vertical excitation energy of 9.62 eV. This strong change in geometry is in agreement with the TPE spectrum, where a broad band is observed around 10 eV. Also, a singlet state was found at around 9.93 eV (1A2/vertical value). We think that both states contribute to the signal and cannot be distinguished. Upon excitation of the HBrCCCH radical into the aþ 3A00 state, also some bond lengths change and especially the CtCstretching vibration should be active. A vertical ionization energy of 10.80 eV was calculated for this state. As in the case of the H2CCCBr isomer, a singlet state (1A00 ) lies also in the energy range of the scan at about 10.78 eV (vertical). Both states are indicated with arrows in Figure 6. Because dissociative photoionization of the small amount of unconverted precursors (C3H2Br2) might also contribute to the C3H2Brþ signal, a Franck-Condon simulation would not be meaningful. Above 10.1 eV, dissociative photoionization of the C3H2Br to C3H2þ is evident in the m/z = 38 mass channel. Looking at the threshold electrons of the C3H2þ channel, given in the
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Supporting Information, only a linear signal increase is visible. The onset of the signal does not correspond to the IE of any of the three C3H2 isomers.24,61,62 An ion image taken at 10.5 eV, depicted also in the Supporting Information, shows that at 10.5 eV a significant kinetic energy is present in the fragment m/z = 38, confirming that it originates from dissociative photoionization. Inspection of the energy diagram in Figure 5 shows that several close-lying fragmentation channels could be involved. Furthermore, isomerization on the ionic surface is possible, and some internal energy might be present in the radicals due to the pyrolysis. Therefore, no further conclusions on the structure of the fragment or the mechanism can be drawn from our data.
’ CONCLUSION The propargyl radical was studied by TPE spectroscopy, and an IEad of 8.71 ( 0.02 eV was obtained. A reproducible feature was observed in the spectrum and is assigned to the ν3 vibration of the cation. The experimental results are in good agreement with a recent theoretical study of Botschwina and Oswald.18 In addition, 1- and 3-bromopropargyl radicals were studied. The two bromopropargyl radicals are distinguishable due to their different ionization energies and can be identified in a single spectrum. The ionization energies were determined to be 8.34 and 8.16 eV, respectively, in good agreement with DFT calculations. A progression in the C-Br stretching vibration is observed in the spectra. Comparing the IEs of propargyl and bromopropargyl, a red shift is found, which is probably due to the electrondonating character of the bromine substituent. This effect stabilizes the cation and leads to a lower ionization energy of C3H2Br. Several cationic electronic states were calculated with TD-DFT/DFT methods and tentatively assigned in the spectrum of C3H2Br. The precursor 1,3-dibromopropyne was also studied, and a AE0K (C3H2Br/C3H2Br2) of 10.6 eV was extracted from the breakdown diagram. ’ ASSOCIATED CONTENT
bS
Supporting Information. Calculations of the radicals and cations, TPES of C3H2, velocity map images, and full refs 37 and 44. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail: ingo.fi
[email protected].
’ ACKNOWLEDGMENT This work was financially supported by the Deutsche Forschungsgemeinschaft (contract Fi575/7-X). Travel subsidies were provided by SOLEIL through the European Commission program “Transnational access to research infrastructures” and by the German-French binational PROCOPE program. C.A. and B.K.C.M. acknowledge financial support from the RTRA “Triangle de la Physique” (Project “Radicaux” 2009-007T) and from the CAPES-COFECUB program no. 525/06 (France/Brazil). We thank the group of Bernd Engels and the Leibnitz Rechenzentrum for providing computer capacities for calculations. Michael Steinbauer and Manuel Renz are acknowledged for 2229
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The Journal of Physical Chemistry A useful discussions. We would like to thank the staff at SOLEIL for operating the storage ring and Jean-Francois Gil and Laurent Nahon for technical support on the DESIRS beamline.
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