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. [1-3] As to the excited atom desorption, two mechanisms, excimer dissociation (ED) and cavity ejection (CE), have been proposed [4] and confirmed experimentally. ED process is similar to the dissociation of an excited dimer (excimer) in the gas phase, i.e., the energetic fragment desorbs from the surface by the dissociation of a molecular-type exciton. Negative electron affinity of the matrix is known to be essential for the CE process to have a repulsive interaction between the excited atom and the surrounding ground state atoms, so that desorbed atoms via the CE mechanism are essentially in excited states. The desorption via CE mechanism can be observed only for solid Ne and Ar, but not for solid Kr and Xe because of their positive electron affinities in the bulk, while ED process is known to occur for all rare gas solids.

Desorption of an excimer from the surface of RGSs has been predicted theoretically for solid Ne [5] , Ar [6] , and Kr [7] , but the experimental evidence has been obtained only for solid Ar [8] . We have previously observed the emission from the desorbed particles with long lifetime from the surface of solid Ne, which was thought to be closely related to Ne₂* desorption [9] . In order to confirm the desorption of Ne₂*, we have measured the spatial distribution of the emitted VUV photons from the desorbed particles initiated by the creation of the surface and bulk excitons.

Experiments have been done at the beam line BL5B in UVSOR. The experimental setup is schematically shown in fig.1, which is similar to the one used in our previous work [3] equipped with a pin hole camera in order to observe the spatial distribution of the emitted light. Briefly, the sample substrate is a Pt(111) attached to the head of a rotatable liquid He cryostat installed in an UHV chamber (base pressure < 10^-8 Pa). The pin hole camera consists of a MCP with 75mm in diameter, two dimensional position sensitive detector, and a pin hole with 5mm in diameter. The distance between the sample and the pin hole is 90mm and the magnification is unity.

Figure 2 shows a typical image of the spatial distribution of the emitted VUV light from the desorbed excited particles measured at the excitation energy of the 1st order bulk exciton (B1, 70.7nm). Photons emitted from the sample are not detected because of a special geometry of the experimental setup where the pin hole camera can not directly see the sample surface. The plume, which is observed just in front of the sample surface, is due to the photons emitted from the desorbed particles, and the others are the background signals mainly due to the photons reflected by the chamber wall. From the size of the plume (~15mm) and assuming that the desorbed particle is an excimer with the kinetic energy of 0.23eV which is a calculated result by Chen et al.[5], the radiative lifetime of the desorbed particle is estimated to be about 10µsec. Considering that this value is consistent with the radiative lifetime of the excimer, Ne₂*(³·_u), in the gas phase (11.9µsec) [10] , and that the radiative lifetimes of the atomic excited states (2p⁵3s) in the gas phase are in the order of 10^-9sec for optically allowed states (³P₁, ¹P₁), and longer than 10sec for optically forbidden states (³P_0,2), we conclude that the plume is due to VUV emissions from the excimer desorbed from the surface of solid Ne. We have done the ESD experiment in Gakushuin University using a similar experimental setup in order to take the decay spectra of the plume intensity, which was not possible in PSD experiment because of very low signal intensity. By using a pulsed electron beam (pulse width ~1µsec, 10kHz repetition) we have obtained time spectra of the plume intensity (not shown). The radiative lifetime measured by the ESD experiment is found to be the same as PSD experiment within an experimental uncertainty.

Figure 3 shows the plume intensity, i.e., the desorption yield of Ne₂*(³·_u), as a function of wavelength of the incident light together with the result for Ne* desorption [1] from the surface of solid Ne. The spectra show that the excimer desorption is also stimulated by the exciton creation on the surface and in the bulk as in the case of excited atom desorption. The main difference in the excimer desorption compared to the atomic desorption is the decrease of the relative intensity of the 1st order surface exciton (S1, 72.3nm). This can be due to the less number of the nearest neighbor atoms on the surface compared to the bulk, and the efficient desorption of excited atoms on the surface, both of which result in the decrease of the efficiency of the excimer formation on the surface. It is interesting to note that the relative intensity of 2p⁵3p-type surface exciton (S', 65.4nm) remains almost constant compared to those of bulk intensities. We do not have a clear explanation for this result at present. Further PSD and ESD experiments are in progress.