Infrared spectroscopy of (HCl)m(H(2)O)n clusters in helium nanodroplets: definitive assignments in the HCl stretch region.

Infrared spectra in the HCl stretch region (2600-2900 cm(-1)) are presented for small, mixed (HCl)(m)(H(2)O)(n) clusters solvated in helium nanodroplets. Sharp bands associated with the Cl-H...Cl stretch vibrations in m:n = 1:1, 2:1, 2:2, and 3:1 clusters are superimposed on a broad background that increases in intensity as larger clusters are grown within the droplets. The broad background is determined to be partially due to mixed clusters with m > 3 and n > 2. The sharp bands are assigned to specific cluster compositions, m:n, via pick-up pressure dependence studies and optically selected mass spectrometry. Orientation of the clusters is achieved with the application of a large electric field to the laser/droplet beam interaction region. The intensity of each band is measured as a function of the applied field strength. Simulations of this electric field dependence based on ab initio calculations of permanent dipole moments and vibrational transition moment angles leads to definitive structural assignments for each sharp band. The 2:1 complex is cyclic, and a band associated with the 2:2 cluster is determined to arise from the nonalternating cyclic structure.