The roles of adsorbed water, acidity, terminal OH groups, and surface morphology in surface-assisted laser desorption/ionization (SALDI) of amino acids from porous graphite and silicon substrates are examined. The SALDI yields and relative intensity ratios of protonated arginine, tryptophan, histidine, methionine, glutamine, and glycine are found to be very similar using porous graphite and porous silicon, despite the large differences in substrate electronic structure and surface chemistry. SALDI does not occur using initially pristine borondoped Si(100) substrates. However, adsorption of water at 130 K to Si(100) containing adsorbed amino acids produces a SALDI signal similar to that observed from porous graphite and porous silicon surfaces containing aqueous amino acid solutions adsorbed at 300 K. The SALDI yields from all substrates are greatly reduced after removal of physisorbed and chemisorbed water and are completely quenched from Si(100) and porous Si once the surface terminal hydroxyls are removed via recombinative desorption. The ion yields of all amino acids increased greatly with reduction of the solution pH, indicating important roles of surface/interface layer acidity and proton affinity of the desorbing amino acid. Multiphoton-induced ionization of interfacial water and terminal-OH-derived surface states may be important in SALDI. Surface morphologies that lead to the adsorption of water matrices with dispersed analytes are most effective because they enhance and maximize protonated complex formation and escape.

Electron beam induced production and desorption of H + , H 2 + , OH + , and H + (H 2 O) n has been studied from water-covered zirconia surfaces. The proton yield is strongly dependent upon the form of water present with large yields mainly from multilayers or clusters. The high proton kinetic energy distributions and the linear yield vs dose are indicative of proton formation and desorption by 2-hole and 2-hole, 1-electron final states. These localized states are produced either directly or via Auger decay pathways, and desorption occurs from the vacuum surface interface as a result of a Coulomb explosion. The proton yield increases from 80 to 150 K and then drops dramatically as the temperature exceeds 150 K. The increased yield is associated with structural and physical changes in the adsorbed water and longer excited-state lifetimes. The decreased yield is correlated with water desorption. Above 180 K, a small proton signal is observable which we associate with electron-stimulated dissociation of hydroxyl groups present on the zirconia surface. At temperatures above 225 K, this yield drops and gives rise to electron-stimulated desorption of OH + ions. H 2 + is also formed from adsorbed water and involves direct dissociative ionization channels. Some reactive scattering of energetic protons and 2-hole Coulomb interactions produce H 3 O + and other higher mass clusters such as H + (H 2 O) n , where n ) 2-8. Though this is a minor channel, it yields information concerning 2-hole screening distances.