Spectral properties of zero temperature dynamics in a model of a compacting granular column

The compacting of a column of grains has been studied using a one-dimensional Ising model with long range directed interactions in which down and up spins represent orientations of the grain having or not having an associated void. When the column is not shaken (zero 'temperature') the motion becomes highly constrained and under most circumstances we find that the generator of the stochastic dynamics assumes an unusual form: many eigenvalues become degenerate, but the associated multi-dimensional invariant spaces have but a single eigenvector. There is no spectral expansion and a Jordan form must be used. Many properties of the dynamics are established here analytically; some are not. General issues associated with the Jordan form are also taken up.Comment: 34 pages, 4 figures, 3 table

Transition State Spectroscopy of the Photoinduced Ca + CH3F Reaction. 1. A Cluster Isolated Chemical Reaction Study

The "cluster isolated chemical reactions" technique is used to examine the dynamics of the photoinduced reaction producing electronically excited CaF when 1:1 Ca·CH3F complexes are deposited at the surface of large argon clusters. This technique ensures quantitatively that 1:1 complexes are actually at the origin of the observed signals. The reaction is monitored by observing the CaF chemiluminescence while scanning the photoexcitation laser. The resulting action spectrum contains information about the absorption bands of the complex, filtered by the dynamics of the reaction. The observations suggest a profound alteration of the calcium electronic structure and a control of the reaction by the CF stretch in CH3F

Characterization of a seeded pulsed molecular beam using the velocity map imaging technique

International audienceAn experimental study has been performed to characterize the density and the velocity distribution in a pulsed molecular beam generated by a source associating a pulsed valve and an oven placed just downstream. In its operating mode, the flow is alternatively in a supersonic and effusive regime. The Velocity Map Imaging (VMI) technique associated with laser ionization allows measuring the velocity distribution and the density of molecules as a function of time during the expansion. It gives us a very precise insight into the structure of the molecule bunch, and therefore into the nature of the expansion from which the molecular beam is extracted

Transition state in metal atom reactions

The observation of the transition state in metal molecule reactions has been approached by several experimental methods, crossed beams, transition state spectroscopy and more briefly via time dependent femtosecond localization. The chemistry is far richer than the one-dimensional harpoon model involving an instant single electron jump imagined at the origin

Cluster isolated chemical reactions

International audienceThe study of chemical reactions in clusters is a rapidly growing field that is attractive for investigating medium effects in reaction dynamics. Cluster isolated chemical reaction (CICR) is a new direction developed in our laboratory, which enables quantitative studies to be made in that field. CICR experiments consist in depositing a controlled number of reactants on large van der Waals clusters that play the role of a solvent of fairly well known size, structure and temperature. This offers enormous advantages, both on the experimental side and for further theoretical investigations. The present review intends to draw together informations that are pertinent for developing experiments and concepts towards understanding chemical reactions in clusters, through CICR-type experiments. In particular, it reviews the questions first of generating and controlling the properties of large van der Waals clusters and secondly of attaching reactants to clusters and controlling their number and location in the cluster. Besides reviewing thoroughly the CICR work done so far in our laboratory, the present paper also reviews part of the vast literature pertaining to reaction dynamics in large van der Waals clusters

Spectroscopy of the Ca Dimer on Argon and Helium Clusters by Laser Induced Fluorescence at 380 nm

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A bayesian approach for the joint decomposition of a sequence of photoelectron spectra

International audienceThis 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

Chemiluminescent Ba + N2O reaction in molecular clusters of CH4 and mixed ArCH4 clusters

International audienceWe report measurements of visible chemiluminescence from a molecular cluster environment. Chemiluminescence spectra of the Ba + N2O reaction on pure CH4 clusters and on Ar clusters with a small fraction of CH4 have been measured and compared. As in the case of pure Ar clusters, there is a spectral contribution from rovibrationally ‘hot’ BaO∗, which is ejected immediately after the reaction, and a contribution from ‘cold’ solvated BaO∗. When using pure CH4 clusters the vibrational structure of the latter is washed out and a considerable blue-shift observed. From mixed clusters a superposition of pure cluster spectra is observed, and the blue-shift of the component influenced by CH4 is smaller than on pure CH4 clusters. This is interpreted by the formation of a still-luminescent BaO∗CH4 complex solvated on the Ar cluster