New Infrared Transitions in Solid Parahydrogen in the MIR and NIR/VIS Regions

We have studied the infrared spectrum of solid parahydrogen at different orthohydrogen impurity levels in the mid-infrared (MIR) region between 600-2000 cm-1 and in the near infrared/visible (NIR/VIS) region between 10 000-16 500 cm-1. The most important new observations in the MIR region, obtained with a single pass through an absorption cell 4.75 cm in length, are the U0(0) + S0(0) double transition around 1520 cm-1, broadened to about 20 cm-1 by roton delocalization, and the single orthohydrogen transition U0(1) at 1619.12 cm-1, which was previously observed only in normal hydrogen. For the NIR/VIS measurements an internal multireflection cell with 14-cm absorption path length was used. Of particular interest here is the second overtone band of solid hydrogen including double transitions of the type Q2(J) + Q1(J') (J = 0, 1; J' = 0, 1). At 10 241.07 cm-1 the new single transition W2(0) could be observed. For several double transitions in the NIR/VIS region a fine structure is observed, which can be explained by anisotropic interaction in rotationally excited pairs of molecules. The treatment of the fine structure of the Q2(0) + S1(0) transition leads to the prediction of a considerable intensity of the triple transition Q2(0) + Q1(0) + S0(0). Stimulated by this result we have found the triple transitions Q1(0) + Q1(0) + S0(0) at 8660 cm-1 and S1(0) + Q1(0) + S0(0) at 8990 cm-1 in the first overtone region. At 12 788 cm-1 we detected an absorption feature that we have assigned to the triple transition S1(0) + Q1(0) + Q1(0). We explain the infrared activity of this transition in terms of a three-body process, a dipole moment induced within a triplet of hydrogen molecules by successive pairwise induction. Copyright 1998 Academic Press.