COLLISIONAL ACTIVATION DYNAMICS OF OH-(H2O)(N) CLUSTER IONS - COMPARISON TO H3O+(H2O)(N)

The collisional activation dynamics of hydrated hydroxide clusters in a fast ion beam have been studied upon multiple glancing collisions wi th argon. Glancing collisions are defined as those involving detectabl e parent or fragment ions at the end of the beam line. Hydrated cluste r ions are ideal for multiple-collision studies because one glancing c ollision will not on average impart enough energy to dissociate the cl uster while two such collisions predominantly lead to dissociation. Th e absolute values of the attenuation cross section of OH-(H2O)(n=0-4) with argon are 31.6(1.8), 29(2), 25.2(1.4), 33(2), and 50(10) Angstrom (2), respectively. The absolute values of the glancing collision induc ed dissociation cross sections are 1.17(0.08) Angstrom(2) for OH-(H2O) (1) and 5.7(0.4) Angstrom(2) for OH-(H2O)2. The fraction of single gla ncing collisions which do not lead to dissociation or loss from the io n beam is 0.92 for OH-(H2O)(1) and 0.25 for OH-(H2O)(2). Single-collis ion branching ratios of 0.88:0.12 and 0.52:0.46:0.02 have been determi ned for successive loss of waters from OH-(H2O)(2) and OH-(H2O)(3), re spectively. Detachment gives way to dissociation upon collisional acti vation as several waters are clustered to OH-. Comparison of the hydra ted hydroxide attenuation cross sections vs cluster size to those of h ydrated hydronium and hard sphere expectations implies that the excess charge of OH- is more localized in water clusters than the proton.