Infrared Spectra of HCl-H2O Clusters in Helium Nanodroplets

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)(3)Cl- 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.