High resolution infrared spectroscopy of (HCl) 2 and (DCl) 2 isolated in solid parahydrogen: Interchange-tunneling in a quantum solid.

Infrared spectroscopic studies of weakly bound clusters isolated in solid parahydrogen (pH ₂ ) 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) ₂ , HCl-DCl, and (DCl) ₂ isolated in solid pH ₂ in the 2.4-4.8 K temperature range. The (HCl) ₂ spectra show a remarkable amount of fine structures that can be rigorously assigned to vibration-rotation-tunneling transitions of (HCl) ₂ trapped in double substitution sites in the pH ₂ matrix where end-over-end rotation of the cluster is quenched. The spectra are assigned using a combination of isotopically (H/D and ³⁵ Cl/ ³⁷ 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 H ³⁵ Cl-H ³⁷ Cl dimers is 6.026(1) and 6.950(1) cm ^-1 , respectively, which are factors of 2.565 and 2.224 smaller than in the gas phase dimer. In contrast, the (DCl) ₂ results show larger perturbations where the ground vibrational state IT splitting in D ³⁵ Cl-D ³⁷ Cl is 1.141(1) cm ^-1 , 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) ₂ 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.