High-resolution spectroscopy of HCl and DCl isolated in solid parahydrogen: Direct, induced, and cooperative infrared transitions in a molecular quantum solid

Abstract

The infrared spectroscopy and rovibrational dynamics of HCl and DCl dopants in solid parahydrogen (pH2) were investigated using high-resolution spectroscopic methods. The absorption spectra of HCl and DCl monomers in solid pH2 closely resemble the corresponding low temperature gas phase spectra, indicating that the gas phase vibrational and rotational quantum numbers of the dopant are conserved within the pH2 solid. Small deviations from gas phase behavior are observed, however, such as a reduced HCl rotational energy level spacing and splitting of the fivefold orientational degeneracy of HCl rotational states with J=2. In addition, the pure vibrational Q1(0) (v=1←0, J=0←0) H2 transition, which is infrared inactive in pure solid pH2, is detected in the HCl doped sample. We propose that this transition is induced in pH2 molecules by neighboring HCl molecules through a weak “overlap induction” mechanism that is the only induction mechanism operative for J=0 impurities in pH2. Rovibrational transitions are also detected near the induced Q1(0) H2 absorption; these are attributed to cooperative transitions involving single photon excitation of pH2–HCl pairs. Detailed isotopic analysis reveals that these cooperative transitions involve pure vibrational excitation of the pH2 and pure rotational excitation of the HCl. Two-molecule transitions have long been studied for isotopic and rotational dopants (e.g., D2, HD, orthohydrogen) in solid pH2, but this is the first time such cooperative transitions have been attributed to a chemical impurity in pH2 matrices.