MONTE-CARLO SIMULATIONS OF THE STRUCTURES AND OPTICAL-ABSORPTION SPECTRA OF NA ATOMS IN AR CLUSTERS, SURFACES, AND SOLIDS

Optical absorption spectra of Na/Ar systems are calculated by combinin g the classical Monte Carlo simulation method with a quantum mechanica l first-order perturbation scheme [Balling and Wright, J. Chem. Phys. 79, 2941 (1983)] for estimating the energies of the Na- 3p(P-2) excit ed states. The model incorporates many drastic approximations, but con tains no adjustable parameters. Our Na/Ar matrix simulations generated relaxed structures for several candidate trapping sites based on vari ous sized vacancies in face-centered-cubic (fee) solid Ar. Trapping si tes for which the equilibrium structures belong to the O-h or T-d poin t groups yielded the experimentally well-known ''triplet'' absorption line shape; for these cases, the splitting of the degeneracy of the ex cited Na- 3p(P-2) state is due solely to fluctuations away from the e quilibrium structures. Simulations of Na/Ar clusters, surfaces, and ma trix sites possessing a strong permanent axial asymmetry yielded a wid ely split ''doublet plus singlet'' absorption line shape. Despite our success at reproducing several qualitative aspects of the absorption s pectroscopy of Na/Ar matrices? our simulations failed to quantitativel y reproduce the experimental data. We discuss the major limitations of our model, as well as several possible improvements.