Photodesorption Dynamics in Threshold-Fluence UV Laser Induced Surface-Decomposition of Bi 2 Sr 2 CaCu 2 O 8
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PHOTODESORPTION DYNAMICS IN THRESHOLD-FLUENCE UV LASER INDUCED SURFACE-DECOMPOSITION OF Bi 2Sr2 CaCu 2O8 t
LAWRENCE WIEDEMAN AND HENRY HELVAJIAN* Mechanics and Materials Technology Center, The Aerospace Corporation, P.O. Box 92957, Los Angeles, California 90009.
ABSTRACT In this article, we review experimental measurements taken in our laboratory for the low-fluence UV pulsed laser photodecomposition of the perovskite ceramic Bi 2Sr2Ca Cu 20 8. We present the measured photoejected product kinetic energy (KE) distributions, including that for the ion and the neutral, the atomic and polyatomic species. For the specific case of the neutral species desorption, we present new experimental results that elucidate the change in the photophysical desorption process as a function of the incident laser fluence. The results show the transition from the electronic-induced desorption to thermal evaporation as the laser fluence is increased. The KE distribution data suggests that for low-fluence laser excitation, the primary photodesorption process is two-hole localization at a binding site followed by coulomb repulsion. A fraction of the escaping ions acquire the full coulomb KE (e2/ro; ro internuclear distance). The neutral species acquire a partial fraction of the available KE (i.e., the KE gained prior to the neutralization). The appearance of polyatomic species desorption is explained via the spectator-stripping model. INTRODUCTION In the past few years, there have been numerous measurements that show electronicinduced desorption is possible from the low-fluence laser irradiation of a surface [1-3]. The observations include both ion [21 and neutral [3] atomic species ejection where the desorption is via a nonthermal excitation process. The laser-induced species evaporation model, commonly used to describe laser desorption, fails to explain the nonthermal character of the ejected species KE distribution and the observed wavelength dependence in the population distribution [4]. These phenomena have been primarily observed at very low fluences, and with UV lasers. The irradiated targets have included metals [5], semiconductors [6], perovskite ceramics [3], and insulating materials [2]. To date, no theory can explain all aspects of the laser-induced electronic desorption process. However, it is strongly believed that surface and bulk defects are necessary to enhance the energy pooling at local sites [7]. At most laser fluences, the interaction of a pulsed laser with a surface commonly results in abrupt heating of the surface layers. The surface heating may be direct via excitation of phonon modes, but most commonly it is indirect, resulting from the quenching of hot carriers in the bulk [8]. In the latter scheme, there exists also the probability for electronic excitations by which surface atoms may be ejected. The desorption may occur via (a) direct excitation to antibonding states [9], (b) adatom charge transfer excitation followed by image potential attraction and subsequent charge neutralization [10], and (c) two-hole loca
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