Single photon ionization of van der Waals clusters with a soft x-ray laser:: (SO2)n and (SO2)n(H2O)m

van der Waals cluster (SO2)(n) is investigated by using single photon ionization of a 26.5 eV soft x-ray laser. During the ionization process, neutral clusters suffer a small fragmentation because almost all energy is taken away by the photoelectron and a small part of the photon energy is deposited into the (SO2)(n) cluster. The distribution of (SO2)(n) clusters decreases roughly exponentially with increasing cluster size. The photoionization dissociation fraction of I[(SO2)(n-1)SO+]/I[(SO2)(n)(+)] decreases with increasing cluster size due to the formation of cluster. The metastable dissociation rate constants of (SO2)(n)(+) are measured in the range of (0.6-1.5)x10(4) s(-1) for cluster sizes 5 <= n <= 16. Mixed SO2-H2O clusters are studied at different experimental conditions. At the condition of high SO2 concentration (20% SO2 partial pressure), (SO2)(n)(+) cluster ions dominate the mass spectrum, and the unprotonated mixed cluster ions (SO2)(n)H2O+ (1 <= n <= 5) are observed. At the condition of low SO2 concentration (5% SO2 partial pressure) (H2O)(n)H+ cluster ions are the dominant signals, and protonated cluster ions (SO2)(H2O)(n)H+ are observed. The mixed clusters, containing only one SO2 or H2O molecule, SO2(H2O)(n)H+ and (SO2)(n)H2O+ are observed, respectively. (c) 2006 American Institute of Physics.