presented in the 21th ICPEAC. (July 1999, Sendai)
*Department of Physics, Gakushuin University, Toshimaku, Tokyo 171-8588 JAPAN
#Institute for Molecular Science, Okazaki, Aichi, 444-8585 JAPAN
| Desorption of excited atoms and excimers induced by valence exciton excitation from the surface of solid Ne by low energy electron and photon irradiation will be presented. Excimer (Ne2*3·u) desorption was experimentally confirmed for the first time, and was found to be well described in the frame work of cavity ejection mechanism as in the atomic desorption case. |
When a low energy electron or photon is incident on the surface of rare gas solids, various electronic excitation processes and subsequent de-excitation (relaxation) processes occur.1 Interaction of the excited atom (exciton) with the surrounding ground state atoms plays an essential role in the relaxation process. In order to reveal the dynamical aspects of the excitons, we have studied desorption of excited atoms and excimers from the surface of rare gas solids induced by exciton creation.
As to the neutral atom desorption induced by exciton creation, two mechanisms, excimer dissociation (ED) and cavity ejection (CE), were proposed2 and have been confirmed experimentally.3 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. It is still an open question whether the dissociation occurs before or after the desorption.4 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. Figure 1(a) shows the desorption yield of excited atom in 2p53s (3P0,2) state as a function of wavelength of excitation light.7 Each peak well corresponds to the energy of valence exciton creation.
Desorption of an excimer from solid rare gases has been predicted theoretically for solid Ne, Ar and Kr, while experimental evidence has been obtained only for Ar.5 We have previously observed the emission from the desorbed particles with long lifetime from the surface of solid Ne, which is thought to be closely related to Ne2* desorption.6 In order to confirm the desorption of Ne2*, we have measured the spatial distribution of VUV light emitted from the desorbed excited species using a pinhole camera, which consists of a pin hole with 3 mm in diameter, a MCP with 75 mm in diameter, and 2-dimensional position sensitive detector.
Measured radiative lifetime is in good agreement with that of Ne2* 3·u in gas phase. It is also shown that the excimer desorbs before vibrational relaxation. It is clearly shown from fig.1(b)8 that the desorption of an excimer is also initiated by the formation of an exciton as the atomic desorption case. Measured kinetic energy of desorbed excimer (0.2 ± 0.1 eV) suggests that the mechanism of excimer desorption can be explained also by the cavity ejection model. Detailed discussion will be presented in the conference.
+ e-mail: takato.hirayama@gakushuin.ac.jp
1. N. Schwentner, E.-E. Koch, and J. Jortner, Electronic excitations in condensed rare gases (Springer-Verlag, Berlin, 1985).
2. T. Kloiber and G. Zimmerer, Radiat. Eff. Def. Solids 109, 219 (1989).
3. I. Arakawa, in Molecular Crystals and Liquid Crystals (Gordon and Breach Science Publishers, New York, 1998), Vol. 314, p. 47, and references therein.
4. D. E. Weibel et al., Surf. Sci. 283, 204 (1993).
5. C. T. Reimann et al., Phys. Rev. B 45, 43 (1992).
6. E. V. Savchenko et al., Surf. Sci. 390, 261 (1997).
7. T. Hirayama et al., Surf. Sci. 390, 266 (1997).
8. T. Hirayama et al., to be submitted.
Fig.1. Desorption yield of (a) excited Ne atom7 in 2p53s 3P0,2 states and (b) Ne excimer in 3·u states8 as a function of wavelength of incident light. Assignment of the 1st order surface (S1) and bulk (B1), 2p53p-type surface (S'), 2nd and 3rd bulk (B2, B3) excitons are shown.