Infrared Spectra of HCl−H₂O Clusters in Helium Nanodroplets

Abstract

Small (HCl)m(H2O)n clusters have been assembled in He droplets, and their spectra in the HCl stretch range (2500−3000 cm−1) have been obtained. In a recent He droplet study, a band at 2670 cm−1 was assigned to the dissociated H3O+(H2O)3Cl− ion pair. In this work, we have revised the assignment of this band to a cyclic hydrogen-bonded form of the (HCl)2(H2O)2 cluster based on careful measurements of the pickup pressure dependence as well as the transition moment angles associated with the HCl stretch vibrations. A number of vibrational bands due to small mixed clusters have also been observed. As the number of the captured water molecules increases, a broad feature appears that spans the 2550−2800 cm−1 range. The possible origin of this spectral broadening in large (HCl)m(H2O)n clusters is discussed.