Inelastic scattering of positive ions and electrons from water: The 4–6 eV energy loss region

• Published in: The Journal of chemical physics 1978-08, Vol.69 (4), p.1445-1452
• Main Authors: Edmonson, D. A., Lee, J. S., Doering, J. P.
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.436768

Inelastic scattering of positive ions and electrons by water molecules has been investigated with particular emphasis on the 4–6 eV energy loss region. Aside from vibrational excitation of the ground state, no inelastic process was observed below 6 eV with either H+ or H2+ projectile ions although with H2+ impact, singlet–triplet transitions above 6 eV were easily detected. Molecular beam target electron impact experiments using incident energies from 10 to 20 eV showed the previously-observed weak continuum energy loss in the 4–6 eV region. Time of flight measurements established that the continuum signal was from electrons produced in the collision region and not negative ions. An investigation of signals obtained from the apparatus when it was heavily contaminated with water showed a background continuum starting at 3.5 eV energy loss whose intensity varied directly with the recent history of exposure of the apparatus to water vapor and inversely with the collision region temperature. These results, taken together, suggest that there is no bound electronic state of water below 6 eV and that the apparent 4–6 eV energy loss process observed in electron impact experiments actually arises from one or both of two unrelated effects; background scattering of electrons from the water-contaminated apparatus and H− production by dissociative attachment when the incident energy is near 6.5 eV.