Observation of Rovibrational Transitions of HCl, (HCl)2, and H2O−HCl in Liquid Helium Nanodroplets

We report the infrared spectra of HCl, (HCl)2, and H2O−HCl in liquid helium nanodroplets in the frequency region between 2680 and 2915 cm-1. For the HCl monomer a line width of 1.0 cm-1 (H35Cl) corresponding to a lifetime of 5.3 ps was observed. The line broadening indicates fast rotational relaxation similar to that previously observed for HF. For (HCl)2 the free HCl as well as the bound HCl stretching band has been observed. The ν2 + bands of (HCl)2 could be rotationally resolved, and rotational constants were deduced from the spectra. We observed both the allowed and the symmetry forbidden transition. However, the forbidden “broken symmetry” tunneling transition of the mixed dimer shows an intensity that is considerably enhanced compared to the gas phase. Upon the basis of the present measurements we were able to calculate the tunneling splitting in the excited state. The tunneling splitting is found to be reduced by 28% compared to the gas phase. Transitions from the ground state to the K a = 1 level of the free HCl stretch (ν1) are recorded and show considerable line broadening with a line width of 2 cm-1. The excited state K a = 1 has an additional rotational energy of about 10 cm-1, thereby allowing fast rotational relaxation by coupling to helium excitations. In addition we observed the HCl stretch of the HCl−H2O dimer, which exhibits an unusually large width (1.7 cm-1 for H35Cl)) and large red shift (8.5 cm-1), compared to the gas-phase values. The large-amplitude motion originating from the libration mode of the HCl−H2O complex is supposed to act as a fast relaxation manifold.