High resolution infrared spectroscopy of (HCl) and (DCl) isolated in solid parahydrogen: Interchange-tunneling in a quantum solid.

Infrared spectroscopic studies of weakly bound clusters isolated in solid parahydrogen (pH) that exhibit large-amplitude tunneling motions are needed to probe how quantum solvation perturbs these types of coherent dynamics. We report high resolution Fourier transform infrared absorption spectra of (HCl), HCl-DCl, and (DCl) isolated in solid pH in the 2.4-4.8 K temperature range. The (HCl) spectra show a remarkable amount of fine structures that can be rigorously assigned to vibration-rotation-tunneling transitions of (HCl) trapped in double substitution sites in the pH matrix where end-over-end rotation of the cluster is quenched. The spectra are assigned using a combination of isotopically (H/D and Cl/Cl) enriched samples, polarized IR absorption measurements, and four-line combination differences. The interchange-tunneling (IT) splitting in the ground vibrational state for in-plane and out-of-plane HCl-HCl dimers is 6.026(1) and 6.950(1) cm, respectively, which are factors of 2.565 and 2.224 smaller than in the gas phase dimer. In contrast, the (DCl) results show larger perturbations where the ground vibrational state IT splitting in DCl-DCl is 1.141(1) cm, which is a factor of 5.223 smaller than in the gas phase, and the tunneling motion is quenched in excited intramolecular vibrational states. The results are compared to similar measurements on (HCl) made in liquid helium nanodroplets to illustrate the similarities and differences in how both these quantum solvents interact with large amplitude tunneling motions of an embedded chromophore.