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 isotopi c impurities D-2 and HD in solid parahydrogen. Each impurity has a spe ctrum 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 s pectra make clear: (1) the infrared Q(1)(0) transitions of J = 0 isoto pic impurities are induced by the quadrupolar fields of nearby impurit y J = 1 molecules; and (2) the spectral pattern of strong Q(1)(0) line s 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 th e aid of several theoretical works, the strong lines in the D-2 and HD spectra can be individually and unambiguously assigned as specific qu antum 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 transit ions of nn impurity pairs containing the o-H-2 are confirmed by combin ation differences which agree to within 5 x 10(-4) cm(-1), the instrum ental 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 embedde d 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 h ave significant contributions from the changes in renormalization and lattice constant around the heavier isotopic impurity. We have also as signed a few of the weaker spectral features as Q(1)(0) transitions of more distant impurity pairs, but the bulk of these transitions are ye t to be assigned, They do form a distinctive pattern and are thought t o 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 spe ctroscopy 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.