Single photon ionization of van der Waals clusters with a soft x-ray laser: ( S O 2 ) n and ( S O 2 ) n ( H 2 O ) m

van der Waals cluster ( S O 2 ) 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 ( S O 2 ) n cluster. The distribution of ( S O 2 ) n clusters decreases roughly exponentially with increasing cluster size. The photoionization dissociation fraction of I [ ( S O 2 ) n − 1 S O + ] ∕ I [ ( S O 2 ) n + ] decreases with increasing cluster size due to the formation of cluster. The metastable dissociation rate constants of ( S O 2 ) n + are measured in the range of ( 0.6 - 1.5 ) × 10 4 s − 1 for cluster sizes 5 ⩽ n ⩽ 16 . Mixed S O 2 - H 2 O clusters are studied at different experimental conditions. At the condition of high S O 2 concentration (20% S O 2 partial pressure), ( S O 2 ) n + cluster ions dominate the mass spectrum, and the unprotonated mixed cluster ions ( S O 2 ) n H 2 O + ( 1 ⩽ n ⩽ 5 ) are observed. At the condition of low S O 2 concentration (5% S O 2 partial pressure) ( H 2 O ) n H + cluster ions are the dominant signals, and protonated cluster ions ( S O 2 ) ( H 2 O ) n H + are observed. The mixed clusters, containing only one S O 2 or H 2 O molecule, S O 2 ( H 2 O ) n H + and ( S O 2 ) n H 2 O + are observed, respectively.