Infrared cavity ringdown spectroscopy of acid–water clusters: HCl–H2O, DCl–D2O, and DCl–(D2O)2

Infrared cavity ringdown laser absorption spectroscopy was used to characterize the gas-phase HCl and DCl stretch modes of three small acid–water clusters at 0.04 cm−1 resolution. The H35Cl stretch of HClH2O at 2723.1 cm−1 and the D35Cl stretch for DClD2O and DCl(D2O)2 were found to be at 1976.0 and 1796.7 cm−1, respectively. The spectral shifts with respect to the HCl and DCl monomers are consistent with theoretical predictions and matrix isolation work. Rotational structure was resolved for DClD2O and spectroscopic constants for both chlorine isotopomers were determined. The spectral shifts and band shapes were similar to those observed for the bonded OH stretch of pure water clusters. Cluster number densities (∼1×1012 cm−3) were slightly lower than found for the pure water clusters under similar conditions. Predissociation and IVR broadening in the acid–water clusters were determined to be qualitatively similar to the case of pure water and DF clusters.