Quantum molecular dynamics and spectral simulation of a boron impurity in solid para-hydrogen

Using path-integral molecular dynamics, we investigate the equilibrium properties of a boron impurity trapped in solid para-hydrogen. Because of its singly filled 2p orbital, the B atom interacts anisotropically with the pH(2) molecules in the matrix. To assess the effect of this electronic anisotropy, we compare with similar simulations in which an orientation-averaged B-H-2 potential is used. We investigate three matrices: (a) a single B atom site substituted for a pH(2) molecule, (b) a similar site-substituted matrix with a nearest-neighbor vacancy, and (c) a B atom site substituted not in the bulk but near the pH(2) surface. It is found that small distortions of the lattice occur to permit an energetically favorable orientation of the 2p orbital, even in the absence of a vacancy. When the B impurity is located near the surface, the spherically-averaged potential provides a noticeably different description from the case of the anisotropic potential. The 3s <-- 2p absorption spectra of the B chromophore is also predicted by means of a semiclassical Franck-Condon technique using path integrals to sample the quantum lattice configurations. These spectral simulations provide additional insight into the interpretation of experimental observations of trapped B in a solid pH(2) matrix. (C) 2000 American Institute of Physics. [S0021-9606(00)00743-1].