Dimer desorption from solid argon films electronically excited by MeV He ions.

Abstract

Electronic excitation of solid Ar films by MeV ${\mathrm{He}}^{+}$ ions gives rise to broad luminescence bands centered at 1110 and 1265 \AA{}. We have examined the spatial and temporal characteristics of these dimer luminescence bands. Observing all the luminescence, we measure a spectrum that is dominated by the 1265-\AA{} band. This band is from decay of vibrationally relaxed ${\mathrm{Ar}}_{2}^{\mathrm{*}}$${(}^{3}$${\mathrm{\ensuremath{\Sigma}}}_{\mathit{u}}^{+}$) trapped in the bulk of the Ar film. Observing only the luminescence from a plume of desorbed particles in front of the excited Ar film, we observe a long-lived 1110-\AA{} luminescence band with a long-wavelength tail extending down to the region around 1265 \AA{}. This band is from decay of gas-phase ${\mathrm{Ar}}_{2}^{\mathrm{*}}$${(}^{3}$${\mathrm{\ensuremath{\Sigma}}}_{\mathit{u}}^{+}$;${1}_{\mathit{u}}$,${0}_{\mathit{u}}^{\mathrm{\ensuremath{-}}}$) in various degrees of vibrational relaxation. The ${\mathrm{Ar}}_{2}^{\mathrm{*}}$ ejection kinetic energy (KE), as well as the degree of vibrational relaxation, are important clues to the mechanisms of dimer formation and desorption.We observe a delayed onset of the luminescence under pulsed excitation if the base of the luminescent plume is hidden from view. The average delayed onset time is a time of flight, the value of which indicates that the average KE of the desorbed ${\mathrm{Ar}}_{2}^{\mathrm{*}}$ is 0.1 eV but ranges as low as 0.06 eV for dimers in highly vibrationally excited states. The temporal width of the delayed onset is consistent with a hemispherical plume shape. We have fitted the observed 1110-\AA{} luminescence band with a linear combination of quantum mechanically calculated luminescence spectra of the vibrational levels of ${\mathrm{Ar}}_{2}^{\mathrm{*}}$${(}^{3}$${\mathrm{\ensuremath{\Sigma}}}_{\mathit{u}}^{+}$;${1}_{\mathit{u}}$,${0}_{\mathit{u}}^{\mathrm{\ensuremath{-}}}$).The resulting vibrational-state distribution is non-Boltzmann and increases sharply with decreasing vibrational quantum number, indicating a considerable degree of vibrational relaxation. These results are discussed in terms of simple models of the ${\mathrm{Ar}}_{2}^{\mathrm{*}}$-desorption process. We hypothesize that ${\mathrm{Ar}}_{2}^{\mathrm{*}}$ is ejected with a KE around 0.1 eV after dissociative recombination of an electron with a near-surface self-trapped hole, ${\mathrm{Ar}}_{2}^{+}$, and subsequent dimerization of the energetic ${\mathrm{Ar}}^{\mathrm{*}}$. Many-body collisions occurring during dimerization effectively relax the vibrational state of the dimer.