Raman matrix isolation spectroscopy of hydrogen trapped in rare gases: from monomer to high aggregation states

Abstract

The rotational and rovibrational spectra of H 2, HD and D 2 trapped in solid rare gases were examined as a function of concentration and sample annealing. Monomer frequencies were measured within ±0.3 cm −1 and compared with the results of recent calculations; the agreement between the observed and calculated matrix shifts is satisfactory but requires the introduction of a large rotation—translation coupling correction to account for the blue shift of the S o(O) line of HD. Two kinds of aggregates were identified in Ar and Kr, one before, and the other after annealing. Before annealing, a concentration increase in hydrogen leads either to the broadening (Ar) or to the appearance of new components very close to the monomer frequency (Kr). These modifications are attributed to the formation of small aggregates (dimer,trimer). After annealing above 35 K new spectra are observed, with properties comparable to those reported for solid hydrogen. The latter results suggest that hydrogen, because of its low solubility and its high mobility, easily migrates within the host crystal to form microcrystals.