Raman matrix isolation spectroscopy of dihydrogen. II. Aggregation processes in argon.

The vibrational spectra of H2, D2, and HD trapped in solid argon were examined as a function of concentration and sample annealing. Before annealing, increasing the hydrogen concentration only leads to an asymmetrical broadening and a small blue shift of the Q( J) lines assigned to monomeric species. These observations are believed due to formation of low stoichiometry aggregates. After annealing above 35 K, new features—10 cm-1 blue shifted with respect to monomer and small aggregate lines—appear with properties comparable to those reported for solid dihydrogen at 2 K. Force field calculations on aggregates involving two to six molecules (including ortho/para and isotopic mixtures) allow a semiquantitative interpretation of the experimental data, and suggest an aggregation degree larger than six. Finally it is concluded that a possible interpretation of all the data is the formation of microcrystals upon annealing, tied to the low solubility and the high mobility of dihydrogen in solid argon. [ABSTRACT FROM AUTHOR]