Photodissociation dynamics of water containing clusters. I. Kr⋅H2O+

Abstract

The mass selected Kr⋅H2O+ cluster is photodissociated in the range 514 to 357 nm using lines from an argon ion laser. Product branching ratios are measured and shown to be a strong function of photon wavelength; Kr+/H2O products dominate at 357 nm (90%) but are equal in intensity to H2O+/Kr products at 514 nm. A small KrH+/OH product is observed at all wavelengths (∼5%), representing the first observation of a photoinduced, intracluster proton transfer reaction. The total cross section is estimated to be ∼2×10−19 cm2 at 514 nm. Laser polarization studies indicated the Kr+/H2O products come from direct accessing of a repulsive upper state (intracluster charge–transfer reaction). Both Kr+(2P3/2) and Kr+(2P1/2) spin–orbit states are formed, but their branching ratio is very strongly dependent on wavelength: 100% Kr+(2P3/2) at 514 nm, 100% Kr+(2P1/2) at 357 nm, and variable amounts of each in between. Analysis of the kinetic energy distribution of Kr+/H2O products indicates H2O is strongly rotationally excited (0.18 to 0.23 eV). This fact, coupled with analysis from an impulsive model for Kr+–H2O dissociation suggests the Kr atom is above (or below) the H2O+ plane in the Kr⋅H2O+ ground state, situated closer to the O end of the molecule. Further analysis of the Kr+/H2O kinetic energy distribution yields the binding energy D00(Kr–H2O+) =0.33± 0.1 eV. Polarization studies indicate H2O+/Kr products arise from a bound upper state. Phase space theory modeling of the kinetic energy distribution indicates the H2O+ product is formed with ∼1.3 eV internal energy. Two models are discussed, one that suggests H2O+(Ã 2A1) is formed and a second that suggests H2O+ is the chromophore, internally converts to vibrationally hot H2O+(X̃ 2B1) and slowly leaks vibrational energy to the c