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

Abstract

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 the cluster and no direct ejection of Ba occurs. Electronic relaxation to the lowest P-state occurs, and the latter dominates the cluster emission spectrum and polarization. The electronic state relaxation is mostly complete within tՇ10 ps. Ejection of Ba atoms occurs as a rare and delayed event when a dynamical fluctuation creates a ''hot spot'' at the Ba site, with a non-adiabatic excitation to the highest electronic level. The results show the feasibility of near-quantitative understanding of non-adiabatic processes in large clusters.

Spectroscopy and Dynamics of K Atoms on Argon Clusters

“…Contrary to Ba 12,21 or Ca 14 on an Ar surface, for which the ejection of the metal atom is a minor process, it occurs systematically for excitation of potassium in this energy range.…”

“…In both cases, the P manifold near the surface of an argon cluster results in two bonding Π-like states and one Σ-like state. However, the Σ state is associative for alkaline earth metals 12,14 whereas it is 15 9.9/90.9 9.04/117 dissociative in the case of potassium, as we shall see in this paper. This difference reflects the relative difference of the spatial extension of the atomic p orbitals between these two kinds of atoms.…”

Simulation of the photodissociation of Ar+3

“…The TSH method requires the diagonalization of the Hamiltonian matrix at each time step in order to determine the adiabatic states "on the fly" while no diagonalization is involved in the HWD method. This fact is not crucial when only few states are involved, as in the recent study [28] of Ba(Ar) n where only the three electronic states of the barium atom are involved in the dynamics. However for larger Hamiltonian matrices the TSH method may become prohibitive since the computer time for the diagonalization grows as N 3 .…”

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

“…Excitation of BaAr N into the ⌺-like state is followed by a fast relaxation into the ⌸-like state as evident from a single broad band observed in the BaAr N fluorescence spectra. 8,9 Indeed the molecular dynamics simulations of Krylov et al 27 have shown this ⌺Ϫ⌸ relaxation to be complete within about 10 ps. This rate of relaxation is evidently much more rapid than that of the barium deactivation by both argon and methane.…”

“…In fact a recent molecular dynamics simulation of electronically excited state BaAr N clusters has shown that this relaxation time is of the order of 10 ps. 27 However, excitation of the ⌺-like branch with a sufficiently high energy leads to the appearance of the atomic Ba( 1 S 0 Ϫ 1 P 1 ) line in the emission spectra. 8,9 This is a direct observation of the competition that exists between energy disposal into a Ba-Ar dissociation coordinate, whereupon the barium is desorbed resulting in the resonance line, and energy disposal in the cluster which acts as a thermal bath, giving rise to the broad emission.…”