Effects of pressure on the trapping site structures and absorption spectraof Li in solid H-2: A path integral Monte Carlo study

Simulation studies for single atomic lithium in solid hydrogen using the pa th integral Monte Carlo (PIMC) method with a constant pressure ensemble are reported. The pressure dependencies of the absorption spectra of Li in sol id H-2 for various trapping sites are extensively investigated at T=5 K and different pressures in the range from 0 to 4.8 GPa. The local structure ar ound the Li atom cannot be rearranged with respect to the presence of press ure. Effects of pressure on the trapping site structures refer only to the decrease in the Li-H-2 and H-2-H-2 separations. The general pressure-induce d effects on Li atomic absorption spectra associate with an increase in hal f-width and a centroid spectral shift (CSS), which experiences a blue shift below a turning pressure (P-t) and a red shift above P-t. For higher symme trical trapping site structures, the peak splitting of the highly symmetric al triplet absorption pattern is mainly ascribed to the perturbation of the H-2 molecules around the Li atom. For lower symmetrical trapping site stru ctures, application of pressure also results in a change in absorption line shape from a low symmetrical triplet feature to a doublet plus singlet pat tern, in which the doublet to singlet splitting increases remarkably with t he increasing of pressure, while the perturbation of the H-2 around the dop ant largely determines the peak splitting for the "doublet." We demonstrate that the pressure provides a powerful means to probe the properties, which cannot be observed at zero pressure. (C) 2001 American Institute of Physic s.