presented in DIET-8, NJ, USA, Sept. 1999.
OHP in PDF format

I. Arakawa^1,2, T. Adachi¹, T. Hirayama¹ and M. Sakurai^3
1Department of Physics, Gakushuin University, Mejiro, Tokyo 171-8588, Japan.
²Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan.
³Department of Physics, Kobe University, Kobe 657-8501, Japan

Photo-induced desorption at the surface of solid Ne has been investigated. In the present study, "absolute" yield of "total" photo desorption and its excitation energy dependence were measured between 30 nm and 100 nm of the wavelength of the incident light. The sample was a solid Ne film with a thickness between monoatomic layer and 200 atomic layers, which was condensed on a Pt(111) substrate at the temperature of about 6 K. Absolute desorption yield was calculated from the rise of the partial pressure of Ne in a vacuum chamber at the irradiation of the sample by monochromatic light from the synchrotron (UVSOR in IMS, Okazaki) after the careful calibration of the light intensity, the sensitivity of a mass spectrometer, and the pumping speed of pumps.

For a film with a few atomic layers or thinner, the desorption yield is about 1 Ne atom per photon and shows no specific dependence on the excitation energy. In this regime, the Ne film is indirectly excited by the secondary electrons from the substrate. When the film thickness exceeds 10 atomic layers, the peaks caused by the excitonic excitation develop in the desorption spectra. By the excitation of the bulk excitons, the absolute yields reach to almost 1 Ne/photon, while, by the excitation of the surface excitons, they are less than detection limit (0.1 Ne/photon) of the experimental system. Above the band gap energy of solid Ne, the absolute yield has remained almost constant, 1 Ne/photon, and runs up to 2-3 Ne/photon at about twice of the band gap energy.

It can safely be said that the main component in the desorbed species is neutral Ne molecules in the ground state; the absolute yield of metastable desorption at the excitonic excitation is the order of 10$^{-3}$ metastable/photon[1]. The absolute yield of about unity for the total desorption can quantitatively be understood by the following model of desorption. From optical absorption data, the number of excitons created per photon per layer is estimated at about 0.1. The kinetic energies of the particles desorbed through the cavity ejection mechanism is about 0.2 eV and those by the excimer dissociation one 1 eV. Because the cohesive energy of Ne is 0.019 eV, the desorbing paticle, which is originated from the 2nd or 3rd layer under the surface after the bulk exciton creation, can blow 10 or more neutral Ne atoms in the overlayer off. The product of these values results in an order of unity of absolute yield of the total desorption.

[1] T. Hirayama et al., Surf. Sci. {\bf 390}, 266 (1997).