Reaction dynamics within a cluster environment

International audienceThis perspective article reviews experimental and theoretical works where rare gas clusters and helium nanodroplet are used as nanoreactor to investigate chemical dynamics in a solvent environment. A historical perspective is presented first, then specific considerations on the mobility of reactants within these reaction media. The dynamical response of pure clusters and nanodroplets to photoexcitation is shortly reviewed before examining the role of the cluster (or nanodroplet) degrees of freedom on the photodynamics of the guest atoms and molecules

Reaction Dynamics within a Cluster Environment

This perspective article reviews experimental and theoretical works where rare gas clusters and helium nanodroplet are used as nanoreactor to investigate chemical dynamics in a solvent environment. A historical perspective is presented first, then specific considerations on the mobility of reactants within these reaction media. The dynamical response of pure clusters and nanodroplets to photoexcitation is shortly reviewed before examining the role of the cluster (or nanodroplet) degrees of freedom on the photodynamics of the guest atoms and molecules

Direct observation of slow Intersystem crossing in an aromatic ketone, fluorenone

International audienceDirect measurements of Single vibronic Level InterSystem Crossing (SLISC) have been performed on the fluorenone molecule in the gas phase, by time resolved photoelectron and photoion spectroscopy. Vibronic transitions above the S1 nπ* origin were excited in the 432–420 nm region and the decay of S1 and growth of T13ππ* could be observed within a 10 ns time domain. The ionization potential is measured as 8.33 ± 0.04 eV. The energy of the first excited triplet state of fluorenone, T1 has been characterized directly at 18 640 ± 250 cm−1. The internal conversion of S1 to S0 is found to amount to ∼15% of the population decay, thus ISC is the dominant electronic relaxation process. ISC, although favored by the S11nπ*–T13ππ* coupling scheme, is 3 orders of magnitude less efficient than in the similar molecule benzophenone. Thus, the planarity of the fluorenone molecule disfavors the exploration of the configuration space where surface crossings would create high ISC probability, which occurs in benzophenone through surface crossings. The time evolution of S1 fluorenone is well accounted for by the statistical decay of individual levels into a quasi-continuum of T1 vibronic levels

Cluster Isolated Chemical Reactions : Medium effects on reactivity

International audienceCluster Isolated Chemical Reaction (CICR) is a new method developed in our laboratory, which enables quantitative studies of heterogeneous chemical processes at a microscopic level. CICR technique consists in depositing in a controlled way reactants on free nanometric van der Waals clusters, that play the role of a reaction medium. It provides a way to study the influence of the reaction medium on the trapped reactants, on the reaction path and on the products. The present paper reviews some of the CICR results obtained in our laboratory concerning the reactivity of barium atoms and small barium aggregates with molecular reactants like N2O, CO2, SF6... on argon, methane or nitrogen clusters. Keywords: barium, clusters, chemiluminescence, chemical reactions, heterogeneous chemistry, mass spectrometr

Vacuum-Ultraviolet Absorption Spectrum of 3-Methoxyacrylonitrile

International audienc

Time Resolved Observation of the Solvation Dynamics of a Rydberg Excited Molecule Deposited on an Argon Cluster -II: DABCO at Long Time Delays

International audienceThe real-time dynamics of DABCO–argon clusters is investigated in a femtosecond pump–probe experiment where the pump excites DABCO to the S1 state within the argon cluster. The probe operates by photoionization and documents the energy and angular distributions of the resulting photoelectrons. The present work complements a previous work from our group [Awali Phys. Chem. Chem. Phys., 2014, 16, 516−526] where this dynamics was probed at short time, up to 4 ps after the pump pulse. Here, the dynamics is followed up to 500 ps. A multiscale dynamics is observed. It includes a jump between two solvation sites (time scale 0.27 ps) followed by the relaxation of the solvation cage excess vibrational energy (time scale 14 ps) and then by that of DABCO (time scale >150 ps). Polarization anisotropy, double polarization, and angular anisotropy effects are reported also. They are interpreted (quantitatively for the former effect) in terms of decoherence of rotational alignment, driven by the overall rotation of the DABCO–argon clusters. A tomographic view of the DABCO excited orbital, provided by the double anisotropy effect, is discussed on a qualitative basis

Rovibrational laser jet-cooled spectroscopy of SF 6 –rare gas complexes in the ν 3 region of SF 6

International audienceHigh resolution infrared spectroscopy combining an external cavity quantum cascade laser with a pulsed pin hole supersonic jet is used to investigate small van der Waals (vdW) hetero clusters containing SF 6 and rare gas (Rg) atoms in the 3 region of SF 6. In a first analysis, the rovibrational band contours of parallel and perpendicular transitions of 1:1 SF 6-Rg hetero dimers (Rg = Ar, Kr, Xe) are simulated to derive ground and excited state parameters and hence ground state and equilibrium S-Rg distances with a precision better than 0. 5 pm. These values are used to assess quantum chemistry calculations (DFT-D method) as well as semi-empirical predictions (combination rules). In a second step, the spectral signatures of the 1:1 heterodimers and of larger heteroclusters containing up to three Rg atoms have been identified by considering reduced vibrational red shifts, i.e., shifts normalized to the average 1:1 red shift. The reduced vibrational red shifts within the series of bands observed and assigned to 1:1 and 1:2 complexes are found to be independent upon the Rg atom, which suggests similar 1:1 and 1:2 structures along the Rg series. In addition, the increasing number of bands when going from monomer to 1:2 complexes illustrates the increased lifting of vibrational degeneracy induced by Rg solvation. Finally, the vibrational shifts of the 1:1 SF 6-Rg hetero dimers are found to fit an intermolecular interaction model in which long-range attractive and short-range repulsive contributions to the vibrational shift are found to partially compensate, the former being dominant. From the same model, well depths are obtained and are found to compare well with quantum chemistry calculations and semi-empirical combination rules

Approche bayésienne pour la décomposition conjointe d’une séquence de spectres de photo-électrons

Ce travail traite de la décomposition d'une séquence temporelle de spectres de photoélectrons en une somme de raies dont on estime les positions, amplitudes et largeurs. Comme les raies évoluent (lentement) dans le temps, la décomposition est effectuée conjointement sur toute la séquence afin de prendre en compte cette information temporelle. A cette fin, nous avons développé un modèle bayésien où un champ de Markov favorise une évolution douce des raies. L'approche est non supervisée et un échantillonner de Gibbs couplé à un schéma de recuit simulé permet d'estimer le maximum a posteriori. Nous montrons la pertinence de cette approche par rapport à une méthode dans laquelle les spectres sont décomposés séparément et présentons une application sur données réelles de photo-électrons.This work deals with the decomposition of a temporal sequence of photoelectron spectra into a sum of peaks whose positions, amplitudes and widths are estimated. Since the peaks exhibit a (slow) evolution with time, the decomposition is performed jointly on the whole sequence to take this temporal information into account. To this end, we have developed a Bayesian model where a Markov random field favors a smooth evolution of peaks. The approach is unsupervised and a Gibbs sampler within a simulated annealing scheme enables to estimate the maximum a posteriori. We show the relevance of this approach compared with a method in which the spectra are decomposed separately and present an application on real photoelectron data

Dynamics of desorption of reaction products from reactions on large clusters

International audienceThe reaction of barium trimers with molecules of SF6 has been investigated on large argon clusters (around 4000 atoms). The reaction is strongly exoergic (by approximately 5 eV) and leads to several reaction products; among which, the desorption of ground state BaF molecules is observed. The energetics of these molecules has been analysed by laser-induced fluorescence with subDoppler resolution. The rotational, vibrational and translational energy distributions are found to be thermal, with temperatures in the range 1600–2400 K. These temperatures are lower than those found in a preceding analysis of chemiluminescent desorbed products but remain much higher than the temperature of the cluster. This is interpreted by different time scales of interaction of the products with the cluster substrate: short times leading to highly internally excited products, long times to solvated products, with the ground state desorbed products appearing as intermediates in between

High-resolution vacuum ultraviolet absorption spectra of 2,3- and 2,5-dihydrofuran

International audienceUsing a synchrotron-based Fourier-transform spectrometer, the high-resolution absorption spectra of the C1-symmetric 2,3-dihydrofuran (23DHF) and C2v-symmetric 2,5-dihydrofuran (25DHF) have been measured from 5.5 eV to 9.4 eV with an absolute absorption cross section scale. Oscillator strengths and vertical excitation energies of the lowest 18 states have been computed using the average of the second- and third-order algebraic diagrammatic construction polarization propagator method and the equation-of-motion coupled-cluster method at the level of singles and doubles model. These show that the bright valence transitions of ππ*-character are embedded into Rydberg transitions, whose oscillator strengths are at least one order of magnitude lower. To account for intensity borrowing, the first broad valence transition between 5.5 eV and 6.8 eV was simulated using a nuclear ensemble, and the agreement between experiment and theory is excellent. Whereas 23DHF only exhibits one broad valence transition followed by d/f Rydberg series converging to the ionization energy, the absorption spectrum of 25DHF has four bands, attributed to a valence nπσ → π*-transition, nπσ → 3px,z/3dxz transitions, a second valence nπ → π*-transition followed by d/f Rydberg series converging to the ionization energy, respectively. All Rydberg series converging to the ionization energy have been characterized in terms of their quantum defects