Desorption of excited dimer from solid Ne

appeared in Progress Report, Atomic Collision Research in Japan 23, (1997) 86.

EXCITON INITIATED DESORPTION OF EXCITED DIMER FROM THE SURFACE OF SOLID Ne BY PHOTON EXCITATION

T. Hirayama, T. Adachi, A. Hayama and I. Arakawa
Department of Physics, Gakushuin University, Mejiro, Toshima, Tokyo 171

K. Mitsuke and M. Sakurai
Institute for Molecular Science, Myodaiji, Okazaki 444

We have been studying the desorption of excited particles from the surface of rare gas solids initiated 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), were proposed[4] and have been confirmed experimentally. The desorption via ED process is due to a dissociation of a molecular type self-trapped exciton (m-STE) similar to the dissociation of an excited dimer (excimer) in the gas phase. 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.

Exciton initiated desorption of an excimer has been predicted theoretically for Ne[5], Ar[6] and Kr[7] solids, while experimental evidence has been obtained only for Ar[8]. In our previous work[9], we have observed the emission from the desorbed particles with long life-time from the surface of solid Ne, which is thought to be closely related to Ne₂* desorption. In order to confirm the desorption of Ne₂*, we have measured the spatial distribution of the emitted VUV light from the desorbed particles using a pin hole camera, which consists of a pin hole with 5mm in diameter, a MCP with 75mm in diameter and 2-dimensional position sensitive detector.

Experiments have been done at the beam line BL5B in UVSOR, Institute for Molecular Science, Okazaki. Figure 1 shows the typical 2-dimensional image of the plume, i.e., the emission from the excited particles desorbed from the surface of solid Ne, at the excitation of the 1st order bulk exciton (B1, 70.7nm). The light from the sample is not detected because of a special geometry where the pin-hole camera can not directly see the sample. The radiative life time of the desorbed particle is estimated to be about 10 ľsec using the size of the plume (~15 mm) and assuming that the desorbed particle is an excimer with the kinetic energy of 0.23 eV which is the calculated results by Chen et al.5). Considering that this value is consistent with the radiative life time of the excimer, Ne2*(3u), in gas phase (11.9ľsec) [10], and the radiative life times of the atomic excited states (2p⁵3s) in gas phase are in the order of 10^-9 sec for optically allowed states and 100 sec for optically forbidden states, we conclude that the emission comes from the desorbed excimer. The dependence of the excimer desorption yield as a function of wavelength is shown in fig.2 together with the desorption yield of excited atoms 3) . One can find from the figure that the desorption yield of the excimer by the creation of the 1st order surface exciton is much smaller than those initiated by the creation of bulk excitons (B1, B2) opposed to the atomic desorption case. This can be due to the less number of the nearest neighbor atoms on the surface, and the efficient desorption of excited atoms on the surface, both of which result in the decrease of the efficiency of the dimer formation on the surface.

Fig.1. 2-dimensional image of the emission from the excited particle desorbed from the surface of solid Ne at the excitation of 1st order bulk exciton (B1, 70.7nm). [Jpeg 21kBytes]

Fig2. Desorption yield of Ne2* and Ne* as a function of incident wavelength. [Jpeg 21kBytes]

References

1) I. Arakawa et al., Nucl. Instrum. Meth. Phys. Res. B101, 195 (1995).
2) T. Hirayama et al., J. Electr. Spectr. Rel. Phen. 80, 101 (1996).
3) T. Hirayama et al., Surf. Sci. in press.
4) T. Kloiber and G. Zimmerer, Radiat. Eff. Def. Solids 109, 219 (1989).
5) L. F. Chen et al., Nucl. Instrum. Meth. Phys. Res. B 116, 61 (1996).
6) S. T. Cui et al., Phys. Rev. B 39, 12345 (1989).
7) W. T. Buller and R. E. Johnson, Phys. Rev. B 43, 6118 (1991).
8) C. T. Reimann et al., Phys. Rev. B 45, 43 (1992).
9) E. V. Savchenko et al., Surf. Sci. in press.
10) B. Schneider and J. H. Cohen, J. Chem. Phys. 61, 3240 (1974).