High-resolution infrared spectroscopy of isotopic impurity Q(1)(0) transitions in solid parahydrogen

We have made a high-resolution infrared spectroscopic study of the Q(1)(0) (v = 1 <-- 0, J = 0 <-- 0) vibrational transitions of the isotopic impurities D-2 and HD in solid parahydrogen. Each impurity has a spectrum composed of similar to 100 sharp lines spread over similar to 0.4 cm(-1). The linewidths vary, but are on the order of 10 MHz. These spectra make clear: (1) the infrared Q(1)(0) transitions of J = 0 isotopic impurities are induced by the quadrupolar fields of nearby impurity J = 1 molecules; and (2) the spectral pattern of strong Q(1)(0) lines is due to the splitting of the M-orientational levels of J = 1/J = 0 o-D-2 or J = 1/J = 0 HD nearest-neighbor (nn) impurity pairs. With the aid of several theoretical works, the strong lines in the D-2 and HD spectra can be individually and unambiguously assigned as specific quantum state Q(1)(0) transitions of nn impurity pairs containing p-D-2/o-D-2 or o-H-2/o-D-2, and o-H-2/HD, respectively. The assigned transitions of nn impurity pairs containing the o-H-2 are confirmed by combination differences which agree to within 5 x 10(-4) cm(-1), the instrumental precision. These assignments yield complete Q(1)(0) energy level diagrams for the nn impurity pairs o-H-2/o-D-(2) and o-H-2/HD embedded in solid parahydrogen. The experimental energy level splittings are fit to a two parameter model which describes anisotropic interactions in the parahydrogen crystal. These experimental parameters appear to have significant contributions from the changes in renormalization and lattice constant around the heavier isotopic impurity. We have also assigned a few of the weaker spectral features as Q(1)(0) transitions of more distant impurity pairs, but the bulk of these transitions are yet to be assigned, They do form a distinctive pattern and are thought to be the Q(1)(0) transitions of impurity triples and larger clusters, This study is one of the few cases for which high-resolution laser spectroscopy has been successfully applied to the condensed phase and for which many of the transitions have rigorous quantum state assignments. (C) 1996 American Institute of Physics.