T. Adachi$^1$*, T. Hirayama$^1$, and I. Arakawa$^{1,2}$

$^1$ Department of Physics, Gakushuin University, Toshima, Tokyo 171-8588,Japan
$^2$ Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan

We have been studying the desorption of excited particles from the surface of rare gas solids (RGSs) induced by exciton creation using photon- and electron- stimulated desorption (PSD and ESD) techniques.[1] As to the excited atom desorption, two mechanisms, excimer dissociation (ED) and cavity ejection (CE), have been proposed and confirmed experimentally. Desorption of an excimer from the surface of RGSs has been predicted theoretically for solid Ne, Ar, and Kr, but the experimental evidence has been obtained only for solid Ar[2]. In the present study we have confirmed the desorption of Ne$_2$* ($^{3}\Sigma_{u}$) from the surface of solid Ne induced by the surface and bulk excitons using low energy electron and monochromatic VUV radiation as excitation sources.
PSD and ESD experiments have been done at the beam line BL5B in UVSOR (Institute for Molecular Science, Okazaki), and at Gakushuin University, respectively. The experimental setup is similar to the one used in our previous work[1] equipped with a pin hole camera in order to observe the spatial distribution of the emitted light. Emission lifetime of the desorbed excited particles has been estimated by measuring decay curves using a pulsed electron beam.
In 2-dimensional spatial distribution of the emission, we have observed a "plume" clearly apart from the sample surface, indicating the emission from the desorbed excited particles. By measuring the decay curves of the emission, the lifetime was estimated to be in the order of 10$^{-6}$s, which is consistent with the radiative lifetime of Ne$_2$* ($^{3}\Sigma_{u}$) in gas phase. Detailed analysis of the decay curve shows that the desorbed excimer initiated by a creation of a surface exciton is in highest vibrational state. This can be understood if we consider the fast desorption process via CE mechanism ($\sim10^{-11}$s [4]) and slow vibrational relaxation in solid Ne ($\ge10^{-7}$s [5]). Estimated kinetic energy of desorbed Ne$_2$* (0.2 $\pm$ 0.1 eV) also supports the CE mechanism for excimer desorption.

[1] I. Arakawa, {\em Molecular Crystals and Liquid Crystals} (Gordon and Breach Science Publishers, New York, 1998), Vol.314, p47, and references therein.
[2] C. Reimann et al., Phys. Rev. B {\bf 37}, 1455 (1988).
[3] E.V. Savchenko et al., Surf. Sci. {\bf 390}, 261 (1997).
[4] L.F. Chen et al., Nucl. Instrum. Meth. Phys. Res. B {\bf116}, 61 (1996).
[5] F. Coletti et al., J. Chem. Phys. {\bf 83}, 49 (1985).