Mechanisms of electron-stimulated desorption of protons from water: Gas, chemisorbed and ice phases

The stimulated desorption of ions from gas phase and condensed phase H 2 O on Ni(111) has been examined theoretically and experimentally for the near threshold excitation region, 15 to 40 eV. The excited state potential energy curves have been calculated using configuration interaction for H 2 O and a restricted Hartree-Fock (RHF) approach for a variety of small clusters including (H 2 O) 5 and NiH 2 O. Both proton yield and kinetic energy distributions have been measured for chemisorbed, ice phase, and gas phase water and are discussed in terms of specific electronic excitations corresponding to possible desorption pathways. For condensed phase water, the major proton desorption threshold occurs at 20–21 eV and is due to surface predissociation. The final state potential energy curves reached in this process are, in general, described by two electron excitations from the ground state and are thus not dipole allowed. At threshold, these potential energy curves correspond to the excited states of the neutral rather than the ionized molecule. Above 28–29 eV, predissociation or shake-up involving excitations from the O 2s orbital contributes to the ion yield and can give rise to protons of high (7–8 eV) kinetic energy.