HIGH-RESOLUTION INFRARED-SPECTROSCOPY OF THE J=1 H-2 PAIR IN PARAHYDROGEN CRYSTALS

We have conducted high-resolution laser spectroscopic studies of the o rtho-H-2 impurity-pair Q(1)(1) (upsilon=1<--0, J=1<--1) transitions in solid para-H-2 with the ortho-H-2 concentration of less than or equal to 0.2%. Several hundred lines were observed in the frequency region between 4142.9 and 4150.3 cm(-1), with the linewidth of 7 to 200 MHz h alf width at half maximum. Except for the single-molecule Q(1)(1) tran sition located at 4146.5621 cm(-1), all transitions are due to the pai rs and higher-order clusters of ortho-H-2. Using calculated relative i ntensities for spectral lines of nearest-neighbor (NN) pairs and next- nearest-neighbor (NNN) pair, and the accurate energy levels of the gro und states by microwave spectroscopy [B. W. Statt, W. N. Hardy, and R. Jochemsen, Can. J. Phys. 58, 1326 (1980)], we have assigned 180 spect ral lines that are due to NN and NNN pairs, including both symmetric a nd antisymmetric excited states. The ground-slate frequency-combinatio n differences agree to within 0.001 cm(-1), the accuracy of the measur ement. The agreement of the relative intensities and polarization depe ndences between the observed spectrum and the calculated spectrum is a lso satisfactory. The energy levels in the excited states obtained fro m the assignment demonstrate that the pair splitting due to the first- and second-order electric quadrupole-quadrupole (EQQ) interaction and the crystal-field interaction remains similar between the ground and excited states. Small and similar deviations from the ground slate hav e been noted for the excited states of NN in-plane (IP), NN out-of-pla ne (OP), and NNN pairs. The widths of the observed spectral lines diff er by more than an order of magnitude depending on the levels. We noti ced that only the F=2, M=0 level, which has the highest energy among a ll the F,M components due to the EQQ interaction, has significant homo geneous broadening on the order of 100 MHz due to relaxation to the lo wer F,M levels, The inhomogeneous broadening due to the randomly distr ibuted ortho-H-2 is approximately proportional to the difference in EQ Q energy between the ground and excited F,M levels. These observations were useful in conducting spectral assignments. Complete energy level patterns for the six excited states: NN IP (s,a), NN OP (s,a), and NN N (s,a) pairs have been determined and discussed. A comprehensive anal ysis using the Hamiltonian is left for a future work. [S0163-1829(98)0 2325-X].