# Surface specific desorption spectroscopy

## Alan G. Joly\*, Kenneth M. Beck, Matthias Henyk, and Wayne P. Hess

# Pacific Northwest National Laboratory[^1], P.O. Box 999, Richland, WA 99352 USA

Peter V. Sushko and Alexander L. Shluger

University College London, London WC1E 6BT, UK

Photo-induced surface reactions are important in many fundamental and
technological processes, such as desorption, ablation, surface
photochemistry, and materials modification \[1\]. Some of these
processes require sufficient energy to be localized at the surface or
near surface region to induce reaction. Therefore, the efficiency of
these reactions can be greatly enhanced by techniques that specifically
excite the surface. In alkali halides and other materials, the lowest
photon energy necessary for efficient excitation corresponds to the
transitions of surface excitons. Surface excitons also provide one of
the most effective means of localizing energy at a surface. Once formed,
the surface exciton can induce particle desorption, or dissipate through
other mechanisms. Recently, it has been proposed that hyperthermal
halogen-atom emission from alkali halide surfaces is a *signature* of
formation and decomposition of surface excitons \[2-5\]. Several
properties of surface excitons have been investigated by room
temperature halogen-atom desorption experiments on KBr \[3-5\] using
photon excitation energies below the bulk exciton maximum. However, no
surface excitons have so far been detected on these surfaces by other
techniques. Using the predictions based on embedded quantum cluster
techniques \[2,3\], combined with photo-desorption measurements, we
provide unique spectroscopic evidence of the existence of surface
excitons in single crystal KI. Using continuously tunable laser
excitation of KI we measure the velocity profiles and the yield of
desorbing hyperthermal iodine atoms as a function of photon energy.
Based on our theoretical desorption model we demonstrate that these
spectra display the signature of a surface exciton and constitute a new
sensitive method of *surface specific desorption spectroscopy*. Our
results demonstrate that creation of surface excitons can be a much more
general phenomenon than was previously thought based on extant
spectroscopic measurements.

\[1\] Proceedings of the Sixth International Conference on Laser
Ablation, EPOCHAL Tsukuba, Tsukuba, Ibaraki, Japan, October 1--5, 2001,
J.T. Dickinson, C. Fotakis, J. S. Horowitz, K. Murakami, and A. Yabe
Eds. Springer-Verlag, Berlin, Germany, 2002, and preceding volumes in
the COLA series.

\[2\] V. E. Puchin, A. L. Shluger, and N. Itoh, Phys. Rev. B **47**, 10
760 (1993).

\[3\] W. P. Hess, A. G. Joly, D. P. Gerrity, K. M. Beck, P. V. Sushko,
and A. L. Shluger, J. Chem. Phys. **115**, 9463 (2001).

\[4\] W. P. Hess, A. G. Joly, D. P. Gerrity, K. M. Beck, P. V. Sushko,
and A. L. Shluger, J. Chem. Phys. **116**, 8144 (2002).

\[5\] K.M. Beck, A. G. Joly, and W. P. Hess, Phys. Rev. B **63**, 125423
(2001).

[^1]: ^\#^ Pacific Northwest National Laboratory is operated by Battelle
    for the U.S. Department of Energy.

    \*Presenting Author: Alan.Joly@pnl.gov; Telephone: 509 376 7707;
    Fax: 509 376 6066