appeared in UVSOR Activity Report, UVSOR Facility, Institute for Molecular Science, UVSOR-24 (1996) 148.


Absolute desorption yield of metastable atoms from the surface of solid rare gases induced by exciton creation



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

K. Mitsuke.
Institute for Molecular Science, Myodaiji, Okazaki 444 JAPAN.

M. Sakurai.
Department of Physics, Kobe University, Nada, Kobe 657 JAPAN.

E.V. Savchenko
Verkin Institute for Low Temperature Physics and Engineering, Kharkov 310164 UKRAINE.


When an exciton is created in the bulk or on the surface of the rare gas solids (RGS), a desorption may occur after various relaxation processes. The desorbed particle can be the atoms in the ground or excited states, singly and multiply charged ions, neutral and ionic clusters, etc. As to the neutral atom desorption, two mechanisms, excimer dissociation (ED) and cavity ejection (CE), were proposed [1] 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 CE mechanism are essentially in excited states. We have measured for the first time the absolute desorption yield of metastable excited atoms via cavity ejection (CE) mechanism from the surface of RGSs by photon impact in excitonic excitation region.

Experiments have been done using an UHV chamber (P ˛ 10-8Pa) at a beam line BL5B in UVSOR, Institute for Molecular Science. Detail of the experimental setup has been described elsewhere [2, 3] . Briefly, the Pt substrate is attached to the head of a liquid He cryostat which is cooled down to 6K or less. Monochromatized synchrotron radiation is pulsed using a mechanical chopper whose width and interval are 15µsec and 2.5msec, respectively. The photon beam is incident at 20 deg. from the normal direction of the sample surface. Thickness of the RGS is 500 atomic layers or more. The desorbed metastable atoms (Ne*, 2p53s 3P0,2) are detected by an open electron multiplier tube (EMT, HAMAMATSU, R595) with a CuBe dynode as a first electrode. The diameter of the EMT opening is 8mm which corresponds to the detection solid angle of 3.1´10-5sr. Charged particles are rejected by applying suitable potentials on the first dynode of the EMT and the retarding grids in front of the EMT. The detector is fixed at the distance of 360mm from the sample in the normal direction of the sample.

Figure 1 shows the time-of-flight (TOF) spectra of desorbed particles measured at 72.3nm, 70.7nm, 65.4 nm, and 61.1nm, which corresponds to the excitation energies of the 1st order surface (S1) and bulk (B1) excitons, 2p53p type surface exciton (S'), and the 2nd order bulk exciton, respectively. Detailed analysis of these spectra has already been given in ref [4] .

In order to obtain the absolute yield, we have to know the efficiency of the metastable atom detector, absolute intensity of the incident photon beam, and the angular distribution of the desorbed atoms. Absolute detection efficiency of EMT for the metastable rare gas atoms with thermal energy has been reported by several authors [5, 6] to be 0.12, 0.035, 0.020, and 0.0050 for Ne*, Ar*, Kr* and Xe*, respectively. The intensity of the incident photon beam is estimated by measuring the current of photoelectron emitted from a thick Au plate [7] . Typical result for the absolute intensity of the incident photon normalized by the ring current (mA) as a function of wavelength is shown in Fig.2. This spectrum has been measured using grating No.3 and mirror No.4, and with the width of the monochromator slit, 300µm, and the diameter of the beam defining aperture, 3mm. It should be noted that the intensity of the pulsed photon beam is plotted in the figure. The duty ratio of the pulse is about 0.5%, i.e., the photon intensity in DC mode is about 200 times higher. Figure 3 shows the absolute Ne* intensity normalized by the number of incident photon as a function of wavelength. The assignment of the each peaks and the position of the band gap energy (Eg) are shown in the figure. Total desorption yield is then determined absolutely taking the desorption angular distribution [3] and the geometrical condition into account. Absolute yield for Ne* from pure solid Ne by the creation of the 1st order surface (S1) and the 1st order bulk (B1) exciton by photon excitation are found to be 1.3´10-3 and 8.2´10-4 (Ne*/photon), respectively.

Fig.1.Time of flight spectra of desorbed particles from the surface of solid Ne at the excitation of 72.3nm (S1), 70.7nm (B1), 65.4nm (S'), and 61.1nm (B2).

Fig.2. Absolute photon intensity normalized by the ring current (mA) as a function of the wavelength. Photon beam is pulsed using a mechanical chopper. The width and the interval of the pulsed light is 15µsec and 2.5msec, respectively.

Fig.3. Absolute Ne* intensity desorbed from the surface of solid Ne normalized by the number of incident photon as a function of the wavelength. The assignments for each peaks and the position of the band gap energy (Eg) are shown.

REFERENCES.

1 T. Kloiber and G. Zimmerer, Radiat. Eff. Def. Solids 109, (1989) 219.
2 T. Hirayama, T. Nagai, M. Abo, I. Arakawa, K. Mitsuke and M. Sakurai, J. Electr. Spectr. Rel. Phen. 80, (1996) 101.
3 M. Sakurai, T. Nagai, M. Abo, T. Hirayama and I. Arakawa, J. Vac. Soc. Jpn. 38, (1995) 298.
4 I. Arakawa, D. E. Weibel, T. Nagai, M. Abo, T. Hirayama, M. Kanno, K. Mitsuke and M. Sakurai, Nucl. Instrum. Meth. Phys. Res. B101, (1995) 195.
5 Borst, Rev. Sci. Instrum. 42, (1971) 1543.
6 J. M. Alvari–o, C. Hepp, M. Kreiensen, B. Staudenmayer, F. Vecchiocattivi and V. Kempter, J. Chem. Phys. 80, (1984) 765.
7 J. A. R. Samson, J. Opt. Soc. Am. 54, (1964) 6.