Vibrations of acrylonitrile in N 1s excited states

The N 1s NEXAFS spectra of 1s acrylonitrile gas is accurately reproduced by a complete ab-initio multi-dimensional vibrational analysis. The role of the pi*-orbital localization and hybridization on vibrations accompaning core-excitation is discussed. Transition to the pi*-perpendicular(C=C-C equiv N) delocalized orbital excites mostly streching vibrations of the whole spinal column of the molecule. Promoting a core-electron to the localized pi*-parallel (C equiv N) produces C equiv N stretching vibration combined with two bending modes of the C-C equiv N end of the molecule, related to the change of carbon hybridization

Vibrationally resolved N 1s absorption spectra of the acrylonitrile molecule

Besides two strong π∗⊥1 and π∗∥ resonances in the 398- to 400-eV energy range, N 1s near edge x-ray absorption fine structure spectra of acrylonitrile molecules have a less intense 401- to 403-eV doublet. The two components correspond to the N 1s → π∗⊥2 and N 1s → D∗∥ transitions, where D∗∥ is a diffuse state with strong Rydberg character. The vibrational analysis shows that in the D∗∥ excited state, two low-energy out-of-plane normal modes are strongly excited. The π∗⊥2 excitation triggers a set of in-plane vibrations, in particular, two C=C–C≡N bendings of the molecule

κ-(BEDT-TTF)2Cu2(CN)3 spin liquid : beyond the average structure

We present here the first accurate determination of the exact structure of κ-(BEDT-TTF)2Cu2(CN)3. Not only did we show that the room temperature structure used over the last twenty years was incorrect, but we were also able to correctly and precisely determine it. The results of our work provide evidence that the structure presents a triclinic symmetry with two non-equivalent dimers in the unit cell, which implies a charge disproportionation between the dimers. However, structural refinement shows that the charge disproportionation is quite weak at room temperature

Soft-phonon driven hexagonal-orthorhombic phase transition in BaVS3

The lattice dynamics of low-dimensional BaVS3 is reported across the hexagonal-orthorhombic phase transition occurring at T-S = 250 K using a combination of the thermal diffuse scattering of x rays, inelastic x-ray scattering, and density functional theory calculations. We observed the occurrence of strongly temperature-dependent diffuse scattering upon approaching the transition, centered at the F points associated with weak Bragg reflections with odd L. Inelastic scattering experiments related this observation to the condensation of a low-lying overdamped optical phonon at the Brillouin zone center, evidenced by significant variations of quasielastic scattering. These results unravel the dynamical origin of the zigzag distortion of the V chains at T-S, which is probably a prerequisite of the Peierls anomaly driving the metal-insulator transition at T-MI = 70 K

Role of sulfur in BaVS3 probed by S K-edge absorption spectroscopy

International audienceWe show that the quasi-1D behavior of BaVS 3 can be understood analyzing the X-ray absorption near edge spectra at the sulfur K edge. Linear dichrosm experiments, analyzed with the help of ab initio calculations, reveal two strong and polarization dependent pre-edge features, induced by the band character of the 3d vanadium levels. They are related to crystal field split t 2g and e g states. When the temperature is lowered, the t 2g feature shifts progressively to higher energy, and its intensity increases for the polarization along the c-axis, stacking direction of the V-S face sharing octahedra. This behavior points to the depletion of sulfur states and thus the lack of S 3p -V 3d hybridization in the direction of V-S chains

Order-disorder type of Peierls instability in BaVS3

Lattice dynamics of low-dimensional BaVS3 is reported across the metal-insulator Peierls transition occurring at T-P = 69 K using a combination of the thermal diffuse scattering of x rays, inelastic x-ray scattering, and density-functional theory calculations. The nondetection of a Kohn anomaly points to a unique situation of an order-disorder Peierls instability with a quasielastic critical scattering which has been fully characterized. These observations are discussed in the scope of a Peierls instability dominated by strong electron-phonon coupling and/or nonadiabatic effects

Kossel X-ray standing-waves within a Cr/B4C/Sc multilayer excited by protons (Orale)

International audienceA characteristic X-ray line emitted from an atom within a periodic structure can be diffracted by the (emitting) structure itself according to the Bragg law. The subsequent Kossel [1] interferences lead to a modulation of the x-ray line intensity as a function of the detection angle in the vicinity of the Bragg angle value [2]. Standing-wave mechanism and Kossel diffraction can be viewed as space reversed processes by virtue of the reciprocity theorem. Kossel interferences have been yet observed using incident X-ray radiation [3-4], electrons [1-2, 5] and ions [6], in crystals [2,5-6] and in periodic multilayers [2-4].In the present work, we have studied the characteristic Cr and Sc K emissions produced by a periodic Cr/B4C/S multilayer exposed to a beam of 2 MeV-protons. The period of the multilayer is close to 2 nm. The intensity of these two emission lines is measured as function of the grazing exit angle, i.e. the angle between the direction of the detector and that of the surface of the multilayer. In the case of the Sc K emission, in Figure 1 we compare the experimental results to those calculated combining a classical recursive approach to the reciprocity theorem. To our knowledge, it is the first time that ions are used to induce Kossel diffraction in a multilayer. Refined details about the structure of the stack could be obtained, especially the profile and nature of the interfaces.Figure 1: Measured (red) and calculated (blue) intensity of the Sc K emission as a function of the detection angle.[1] W. Kossel, V. Loeck and H. Voges, Z. Für Phys. 94, 139 (1935).[2] P. Jonnard P., J.-M. André, C. Bonnelle, F. Bridou and B. Pardo, Appl. Phys. Lett. 81 (8), 1524 (2002).[3] J.-P. Chauvineau and F. Bridou, J. Phys. IV 6, C7 (1996).[4] Y. Tu, Y. Yuan, K. Le Guen, J.-M. André, J. Zhu, Z. Wang, F. Bridou, A. Giglia and P. Jonnard, J. Synchrotron Radiat. 22, 1419 (2015).[5] V.V. Lider, Crystallogr. Rep. 56, 169 (2011).[6] V. Geist and R. Flagmeyer, Phys. Status Solidi A 26, K1 (1974)

(BEDT-TTF)2Cu2(CN)3 Spin Liquid: Beyond the Average Structure

International audienceWe present here the first accurate determination of the exact structure of κ-(BEDT-TTF) 2 Cu 2 (CN) 3. Not only did we show that the room temperature structure used over the last twenty years was incorrect, but we were also able to correctly and precisely determine it. The results of our work provide evidence that the structure presents a triclinic symmetry with two non-equivalent dimers in the unit cell, which implies a charge disproportionation between the dimers. However, structural refinement shows that the charge disproportionation is quite weak at room temperature

New insights into the structural properties of j-(BEDT-TTF)2Ag2(CN)3 spin liquid

Here, the first accurate study is presented of the room-temperature and 100 K structures of one of the first organic spin liquids, -(BEDT-TTF)2Ag2(CN)3. It is shown that the monoclinic structure determined previously is only the average one. It is shown that the exact structure presents triclinic symmetry with two non-equivalent dimers in the unit cell. But surprisingly this does not lead to a sizeable charge disproportionation between dimers. The difference from the analogue compound -(BEDT-TTF)2Cu2(CN)3 which also presents a spin liquid phase is discussed in detail. The data provided here show the importance of the anionic layer and in particular the transition metal position in the process of symmetry breaking. The possible impact of the symmetry breaking, albeit weak, on the spin-liquid mechanism and the influence of various disorders on the physical properties of this system is also discussed.Work in Spain was supported by MICIU (PGC2018-096955-BC44 and PGC2018-093863-B-C22), MINECO through the Severo Ochoa (SEV-2015-0496) and Maria deMaeztu (MDM- 2017-0767) Programs and Generalitat de Catalunya (2017SGR1506 and 2017SGR1289). Work in Japan is supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (JP23225005 and JP18H05225).Peer reviewe