DYNAMICS OF THE DEACTIVATION AND DESORPTION OF BA ATOMS FROM AR CLUSTERS

The Doppler profiles of Ba(P-3(2)) atoms desorbed from the surface of argon clusters following the deactivation of Ba(P-1(1)) have been meas ured. These measurements have been performed for desorption from pure ATN clusters and as a function of a known average number of CH4 molecu les deposited on the cluster. Analysis of the profile widths with resp ect to the kinetic energy release from deactivation indicates that des orption occurs along a single Ba-Ar and Ba-CH4 coordinate in the forme r and latter cases, respectively. By comparing the kinetic energy dist ributions in the desorbed barium with the relative kinetic energy avai lable at the temperature of the cluster it is found that the collision s leading to deactivation in both cases are gas kinetic at the tempera ture of the cluster (35 K). The residual anisotropies in the Doppler p rofiles reveal the Ba-Ar deactivation to be a relatively inefficient p rocess allowing the barium to undertake a full migration on the cluste r surface before desorbing. This results in an essentially isotropic d istribution of recoil velocities. In contrast Ba-CH4 deactivation is s ufficiently fast to preserve some degree of anisotropy in the desorbed barium velocity distribution. The anisotropy results from the depolar ization of the barium orbital due to bath the migration of the barium on the cluster surface and axial relaxation of the orbital by collisio ns with neighboring argon atoms. Calculations of the anisotropies resu lting from both reorientating mechanisms show a significant degree of relaxation and migration to occur before the barium is desorbed. (C) 1 997 American Institute of Physics.