M. Akbulut et al., TRANSMISSION OF LOW-ENERGY (LESS-THAN-10 EV) O-16(+) IONS THROUGH CONDENSED AMMONIA AND WATER OVERLAYERS, The Journal of chemical physics, 103(6), 1995, pp. 2202-2215
We have studied the transmission of low energy (<10 eV) O-16(+) ions t
hrough ultrathin films of condensed molecular solids, NH3 and (H2O)-O-
18, in order to address the fundamental scattering processes that occu
r in the desorption of ions from below the surface of solids. O-16(+)
ions with a peak energy of similar to 7 eV and a narrow angular distri
bution [full-width at half-maximum (FWHM) similar to 15 degrees] are g
enerated by means of electron stimulated desorption (ESD) from an O-16
oxidized W(100) surface and their yield, energy and angular distribut
ion are measured with a digital ESDIAD (ESD ion angular distribution)
detector. Ultrathin NH3 and (H2O)-O-18 films of known thickness are co
ndensed on the oxidized surface at 25 K and changes in the ion yield,
energy and angular distribution are observed as a function of coverage
. We find that adsorption of only 0.5 monolayer of (H2O)-O-18 is enoug
h to suppress the O-16(+) ion emission by a factor of 100, while three
monolayers of NH3 are necessary for equivalent suppression of the O-1
6(+) ion emission. The angular distribution of the ions increases slig
htly with increasing overlayer coverage. We also fmd that a small perc
entage of (H2O)-O-18 dissociates upon adsorption. We suggest that one
electron charge transfer between O-16(+) and (H2O)-O-18, and between O
-16(+) and the dissociation product OH are the main reasons for the st
rong attenuation of O-16(+) ions by only a fraction of a monolayer of
(H2O)-O-18. Charge transfer is also believed to be the main process th
at causes suppression of O-16(+) ions by ultrathin NH3 films. Other el
astic and inelastic processes are not believed to contribute significa
ntly to O-16(+) attenuation in NH3 or (H2O)-O-18 films. (C) 1995 Ameri
can Institute of Physics.