MANY-BODY POTENTIALS OF AN OPEN-SHELL ATOM - SPECTROSCOPY OF SPIN-ORBIT TRANSITIONS OF IODINE IN CRYSTALLINE XE AND KR

Temperature dependent emission spectra of spin excited iodine in cryst alline Xe and Kr are presented and analyzed in terms of nonadditive an isotropic pair interactions. In the octahedral trap site, the atomic P -2 states split into E(1/2) and G(3/2) groups of the double valued rep resentation. The fourfold degenerate G(3/2) state is subject to strong Jahn-Teller instability and further splits by coupling to phonons int o E(1/2) and E(3/2) Kramers pairs. Accordingly, the observed emission spectra are composed of two bands: 2E(1/2)-->1E(1/2) and 2E(1/2)-->E(3 /2) transitions. Two pairs of bands are observed each in Xe and Kr. Th e long-lived pairs (at 15 K, tau=250 mu s and 930 mu s in Xe and Kr, r espectively) are assigned to the isolated atom, while a short lived pa ir of bands (at 15 K, tau<1 mu s in Xe, and tau=2.2 mu s in Kr) are as signed to I atoms trapped as nearest neighbor to a localized charge, i dentified as (HRg)(+). The isolated atom spectra are simulated by Mont e Carlo methods which assume classical statistics in the heavy atom co ordinates, and adiabatic following of the electronic coordinate. Angle dependent, gas phase pair interactions are used as a starting point. Minor modifications, to the pair interactions, and a temperature depen dent spin-orbit splitting constant, adequately reproduce the experimen tal spectra. Many-body contributions to the effective pair potentials can be estimated to change pair parameters by less than similar to 3%.