OPTICAL ABSORPTIONS OF LI ATOMS IN MIXED AR XE MATRICES/

We present the results of matrix isolation spectroscopy experiments on mixed Ar/Xe matrices containing Li atoms produced by laser ablation o f solid lithium. Optical absorption spectra of Li/Ar/Xe matrices conta ining similar to 0.01% guest Li atoms and approximate to 0, 3%, 10%, 3 0%, 50%, 70%, 90%, 97%, and 100% Xe as the matrix host an included. In all cases well defined ''triplet'' absorption features (i.e., three m ain peaks) are observed for the Li atom 2p<--2s absorption. We also pr esent new data on the photobleaching of the well known ''red triplet'' absorption in Li/Xe matrices, which show changes to the fine structur e observed on the sharp 655 nm component. In these dilute guest system s, the Li atom absorption line shape is determined completely by guest -host interactions, which depend strongly on the local Li atom trappin g site structure. In single Rg host matrices, it is possible that the trapping site structures may correspond to single or multiple Rg atom substitutional sites in otherwise crystalline regions of the rare gas solid. In these cases, the observed tripler line shapes would be due t o the removal of the threefold degeneracy of the excited Li atom 2p st ate by dynamical distortions of the system away from the high symmetry equilibrium trapping site structures. In the mixed Ar/Xe matrices, th e Li atom trapping sites necessarily have lower equilibrium or static symmetries due to the possibly amorphous nature of these solids, and t o the differences in the Li-Ar and Li-Xe interactions. The observed sp ectra in mixed host matrices thus contain contributions from the many and varied Li atom trapping site structures, yet they still exhibit th e familiar tripler absorption pattern. While we do not settle the long -standing question as to the crystalline vs amorphous nature of the si ngle Rg host matrices, the present observations do provide new data fo r the comparison of the relative importance of static vs dynamic effec ts on the spectra of matrix isolated alkali atoms.