J. Berlande

Experiment versus molecular dynamics simulation: Spectroscopy of Ba–(Ar)n clusters

This work presents a quantitative comparison between experiment and molecular dynamics simulations for the excitation spectra of large van der Waals clusters. The emission and excitation spectra of mixed Ba(Ar)n clusters have been obtained for average cluster sizes ranging between 300 and 4000. The simulation is performed by using classical dynamics and pairwise additive potentials for two cases corresponding to the barium atom at the surface or inside the argon cluster. A very good agreement with the experiment is found when the barium atom is at the surface.

Reaction between (N2O, Ar) binary clusters and barium atoms

Abstract Chemiluminescence is observed when barium atoms collide with N 2 O(Ar) n clusters in a crossed-beam experiment (200 n n suggests two different reaction products: one is similar to that observed in the gas phase (“hot” BaO*), and the other to that observed in matrices (“cold” BaO*). The formation of the “matrix-like” product is of increasing importance as the size of the cluster increases. The following mechanism is proposed: barium atoms are captured with a large cross section by the cluster; they migrate on or in the cluster to find the N 2 O molecule and react. Because N 2 O molecules may be located at the surface, or inside the cluster, the BaO molecules give different emission spectra.

A simple method to determine the mean cluster size in a molecular beam

A method based upon the tandem use of the Time-Of-Flight and Surface-Induced-Dissociation techniques is proposed for estimating the average cluster size in a neutral molecular beam. It consists of sending the beam through a buffer gas and measuring the variations of the average beam velocity as a function of the buffer gas pressure. The clusters are detected at the mass of the monomer by surface induced dissociation in the ionization source. This method has been applied to an argon cluster beam and the results are in good agreement with determinations using high energy electron diffraction. This technique appears to be a simple alternative for estimating mean cluster sizes in the range of 100 to a few 1000 monomers.

Spectroscopy of Ba—(Ar)n clusters

Abstract The absorption and emission of barium atoms trapped onto large argon clusters (about 2000 atoms) have been observed. The emission consists in a single peak to the red of the barium resonance line and the absorption of two bands, one on each side of the resonance line. The existence of two absorptions seems characteristic of a configuration where the barium atom remains at the surface of the argon cluster.

A molecular dynamics simulation of rebound, capture and diffusion in collisions at thermal energies between a rare gas atom and an argon cluster (n=125)

Molecular dynamics calculations have been performed to simulate the low energy collision (0.2 eV) of a rare gas atom (He, Ar, Xe) with a cluster of 125 argon atoms. Depending on its relative mass to argon, the projectile is either deflected (He) or captured (Ar, Xe) by the argon cluster. We have determined the deflection function of the He projectile that is scattered, and for Xe we have determined wether it stays near the surface of the cluster or migrates inside. These results have been discussed in the light of very simple models.

Reaction between Ba and N2O in large Arn clusters

The collision between a Ba atom and an Arn cluster carrying N2O molecules has been investigated under crossed molecular beam conditions. The argon cluster acts as a solvent for the Ba+N2O reaction, which is monitored through its chemiluminescent channel forming electronically excited BaO. The effects of cluster size and the number of N2O molecules per cluster have been investigated systematically as have the effects of extra molecules present upon the cluster (CH4). It has been shown that (i) the BaO reaction product either stays solvated in the cluster or is lost from the cluster; (ii) the reaction probability between Ba and N2O is approximately unity for the clusters considered here; (iii) the chemiluminescence quantum yield decreases as the number of N2O molecules per cluster is increased. The effect of a thermal bath (the argon cluster) on the dynamics of the well studied gas phase reaction Ba+N2O is discussed.

Chemiluminescent channels in reactions of Ba(1P1) with water, alcohols, and ethers

The present paper investigates chemiluminescence in reactions of excited barium atoms (6s6p 1P1) with water and a series of alcohols and ethers. The electronically excited product molecule from the reaction with H2O (and D2O) is BaOH (and BaOD) in the A 2Π, A’ 2Δ, and B 2Σ levels. The product molecule is always Ba–ORx in reactions with alcohols ROH, whatever the size of the alcohol (methanol to butanol), and whatever its class (primary to tertiary). By comparison, no chemiluminescence was observed when the reactant was dimethyl and diallyl ether although allowed energetically. The nature of the product molecules in reaction with alcohols, and the absence of reaction with ethers that were found here are remarkably close to what was found by Davis et al. (submitted to J. Chem. Phys.) for the formation of ground state products in reactions of Ba(6s5d 1D2) with water, methanol, and dimethyl ether. The present work thus allows us to extend the model of Davis et al. for the reactivity of Ba(6s5d 1D2) as a power...

Near‐resonant electronic‐to‐rotational energy transfer in Rb(7S→5D)–H2, D2 collisions at thermal energy

The intermultiplet transfer Rb (7S→5D) induced by H2, D2, and He has been studied as a function of collision velocity using a crossed beam apparatus for which the initial rotational distribution of the molecular perturber was determined. A calibration using the blackbody induced redistribution of the excited states populations in the Rb beam allowed the absolute value of the cross section to be obtained. A situation where a near resonance exists (perturber H2) is compared with situations where near resonances do not occur (perturbers D2 and He). For low collision velocities (VR≤4000 m/s), the near‐resonant process Rb(7S)+H2( j=1)→Rb(5D)+H2( j=3) has a much larger cross section than competing nonresonant processes (rotationally elastic or inelastic). Its cross section decreases with the relative velocity as ∼1/V2R. This behavior is compared with the predictions of two theoretical models (the impulse approximation model, which leads to a quantitative comparison with the measured velocity dependence, and the...

Energy dependence of the inelastic process Ba(6s6p1P1)+Ar,He→Ba(6s6p3P1,2)+Ar,He

The cross section of the inelastic transfer from Ba(6{ital s}6{ital p} {sup 1}{ital P}{sub 1}) to Ba(6{ital s}6{ital p} {sup 3}{ital P}{sub 1,2}) induced by collision with argon or helium is measured as a function of the collision energy. The existence of a 0.15 eV energy barrier in the formation of Ba(6{ital s}6{ital p} {sup 3}{ital P}{sub 1}) is observed and allows to explain why Ba(6{ital s}6{ital p} {sup 3}{ital P}{sub 2}) was the only product formed in a previous cell experiment.The cross section of the inelastic transfer from Ba(6s6p 1P1) to Ba(6s6p 3P1,2) induced by collision with argon or helium is measured as a function of the collision energy. The existence of a 0.15 eV energy barrier in the formation of Ba(6s6p 3P1) is observed and allows to explain why Ba(6s6p 3P2) was the only product formed in a previous cell experiment.

Collisions of excited alkali atoms with O2. I. Intermultiplet transfer

The cross sections for the Rb(7S→5D), Rb(5D→7S), and Na(4D→5S) collisional transfers induced by O2 have been measured in absolute values as a function of the collision energy using a crossed beam apparatus. The experimental data have been compared with the predictions given by a multiple curve‐crossing model where the symmetries of the states formed by the colliding alkali‐metal atom–O2 system as well as the molecular orientation are considered. Good agreement is observed between experimental and calculated cross sections. This tends to show that in spite of its approximations, a multiple curve‐crossing model would be useful to obtain fairly accurate informations about collisional processes involving alkali atoms with as much as 4 eV electronic excitation.