J. M. Mestdagh

OBSERVATION OF AN INDIRECT PATHWAY IN THE FEMTOSECOND STUDY OF ALKYL NITRITE PHOTODISSOCIATION IN THE S1 STATE

The present work has addressed the question of the dissociation time of four alkyl nitrites upon photoexcitation in the S1 state (methyl nitrite, n‐ , and t‐butyl nitrites and i‐amyl nitrite). The time resolved Laser Induced Fluorescence technique has been used in the femtosecond regime under bulk conditions. The photodissociation has been initiated at 351 nm by 150 fs pump pulses, and has been probed using a two‐photon process at 467 nm by 200 fs pulses. The LIF signal has qualitatively the same shape for the four nitrites: it passes through a transient peak before reaching a plateau. The two‐photon process that induces the detected fluorescence is nonresonant for detection of the dissociation product NO through the A←X transition. Conversely, the two‐photon process is resonant or quasiresonant for detection of the excited nitrite molecule in the S1 level before it dissociates. This leads to an enhanced detection efficiency of the non‐dissociated excited molecule versus that of the NO fragment. A simple ...

Spectroscopy, polarization and nonadiabatic dynamics of electronically excited Ba(Ar)n clusters: Theory and experiment

Molecular Dynamics simulations using a surface‐hopping method for transitions between different electronic states are employed to study the dynamics following photoexcitation of the Ba(Ar)125 cluster. The results are used to interpret spectroscopic experiments on large, size‐distributed Ba(Ar)n clusters. The dynamics of the coupled electronic‐nuclear motions in the cluster involves transitions between three potential energy surfaces, corresponding to the nearly‐degenerate p‐states of the excited Ba atom. Ejection of excited Ba atoms, adsorbed on the surface of the cluster, can take place. The focus in comparing theory and experiment is on the emission spectrum from the excited clusters, on the polarization of this radiation, and on the polarization of light emitted by excited Ba atoms ejected from the cluster. Based on the good agreement found between theory and experiment, a comprehensive picture of the excited state dynamics is given. It is found that upon excitation, energy is rapidly redistributed in ...

Photodesorption dynamics of Ba atoms from large Ar clusters

Barium atoms at the surface of large argon clusters are excited into the 6s6p1P1 state leading either to ejection and subsequent fluorescence of the free barium atom or to fluorescence of solvated barium. Relative probabilities for these two processes as a function of the excitation energy and of the cluster size are presented. By comparison with the binding energy from a molecular dynamics (MD) calculation, we find that the barium atom is not excited above the limit for direct dissociation. The nonzero desorption probability is attributed to the gain of surface energy of the argon cluster which is partially transferred to the barium atom. Using linearly polarized laser light for the excitation, a positive polarization of the fluorescence of the ejected barium atoms is observed, whereas the polarization fluorescence of the solvated barium is negative. This indicates that the solvated barium atoms undergo a relaxation process which rotates the orientation of the excited orbital before the fluorescence. The...

Cluster isolated chemical reactions: Reactivity of Ba atoms and small Ba clusters with SF6 and CO2 molecules on large argon clusters

The cluster isolated chemical reaction technique was used to investigate the reactivity of the Ba/CO2 and Ba/SF6 systems in the environment of Ar≈6000 clusters. The method was extended to document several aspects of the reactivity. Notably, mass spectrometry gives insight into the full reactivity of the system deposited on the clusters. Laser induced fluorescence (LIF) and chemiluminescence are also used as detection tools. Unexpectedly, we found that a single barium atom neither reacts with CO2 nor with SF6 at the cluster temperature (32 K). In contrast, the LIF results suggest the formation of a weakly bound covalent Ba…CO2 complex. Finally, Ba2 and larger barium aggregates react with CO2, and Ba3 and larger aggregates react with SF6. The chemiluminescent products are Ba2O in the first case, and BaF in the second. These observations are rationalized on the ground of the harpoon model.

Cluster isolated chemical reaction (CICR) spectroscopy: Ba atoms and Ba(CH4)n complexes on large neon clusters

The cluster isolated chemical reaction (CICR) technique was applied to neon clusters (Ne≈7000) on which barium atoms and methane molecules were deposited. Clusters carrying barium only were studied first. Qualitatively, the present results on neon clusters are in line with our previous results on argon clusters. In particular, surface location of barium was observed. The central part of the present work concerns neon clusters carrying both one barium atom and one to ten methane molecules. Several types of spectroscopy were performed in the region of the resonance transition (6s2)1S→(6s6p)1P of barium (excitation spectrum of the total fluorescence, emission spectrum, action spectrum for forming (6s6p)3P), and experiments where the number of methane molecules per cluster, which was strictly controlled, was varied systematically. The corresponding results were interpreted on the ground of a model, which transposes both chemical thermodynamics of equilibria and reaction kinetics to CICR experiments. Such an a...

Stabilization of barium dimers on clusters: Reactions of Ba2 with Cl2 and O2 on large argon clusters

Barium dimers have been produced and stabilized by the successive pickup of two barium atoms on large argon clusters (average size of 8000). By adding chlorine and oxygen molecules, chemiluminescent reactions of these dimers have been observed. This technique of dimer formation can be extended to higher order clusters to study reactions of cold metallic clusters.

Dynamics of the deactivation and desorption of Ba atoms from Ar clusters

The Doppler profiles of Ba(3P2) atoms desorbed from the surface of argon clusters following the deactivation of Ba(1P1) have been measured. These measurements have been performed for desorption from pure ArN clusters and as a function of a known average number of CH4 molecules deposited on the cluster. Analysis of the profile widths with respect to the kinetic energy release from deactivation indicates that desorption occurs along a single Ba–Ar and Ba–CH4 coordinate in the former and latter cases, respectively. By comparing the kinetic energy distributions in the desorbed barium with the relative kinetic energy available at the temperature of the cluster it is found that the collisions leading to deactivation in both cases are gas kinetic at the temperature of the cluster (35 K). The residual anisotropies in the Doppler profiles reveal the Ba–Ar deactivation to be a relatively inefficient process allowing the barium to undertake a full migration on the cluster surface before desorbing. This results in an...

Femtosecond study of Xe‐Cl2 reactions in clusters

The reaction of a xenon atom with a chlorine molecule has been studied in the femtosecond regime using the pump (395 nm) and probe (308 nm) technique in Xe‐Cl2 clusters. The electronically excited reaction product XeCl was observed both after a simultaneous and a sequential (with a delay of ≊500 fs) absorption of the pump and probe photons.

Tandem time-of-flight experiment for low energy collision studies

We present an experiment adapted to collisional studies of cluster ions based on a laser vaporization setup coupled to a supersonic expansion. The ions are selected in a first time-of-flight, slowed down to the desired energy, and collided in an octopolar guide. The parent and fragment ions are then reaccelerated in order to be mass analyzed in a reflectron time-of-flight. An original method for the extraction of the ion that uses a double voltage pulse, is proposed. The experiment has been applied to collisions of hydrated cobalt ions. An absolute cross section of 17 Å2 for the loss of one water molecule by Co(H2O)2+ in collision with neon at a center-of-mass energy of 10 eV, has been determined, with an accuracy of 10%. The threshold for this reaction has been measured at 1.5 eV and is in good agreement with the existing literature (Dalleska et al. J. Am. Chem. Soc.1994, 116, 3519). Ions that cannot be formed by conventional ligand exchange methods, can also be studied. As an example, the threshold for dehydration of the Co2(H2O)+ ion has been measured at 1.5 ± 0.2 eV.

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

Abstract We report measurements of visible chemiluminescence from a molecular cluster environment. Chemiluminescence spectra of the Ba + N 2 O reaction on pure CH 4 clusters and on Ar clusters with a small fraction of CH 4 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 CH 4 clusters the vibrational structure of the latter is washed out and a considerable blue-shift observed. From mixed Ar CH 4 clusters a superposition of pure cluster spectra is observed, and the blue-shift of the component influenced by CH 4 is smaller than on pure CH 4 clusters. This is interpreted by the formation of a still-luminescent BaO ∗ ue5f8CH 4 complex solvated on the Ar cluster.