Laser-Solid Interactions: Localization of Momentum and of Energy
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LASER-SOLID INTERACTIONS: LOCALIZATION OF MOMENTUM AND OF ENERGY
J. A. VAN VECHTEN IBM Thomas J. Watson Research Center, P. 0. Box 218 Yorktown Heights, New York 10598
ABSTRACT Mechanisms are discussed by which sufficient momentum and energy may be localized on individual atoms, ions or molecules to account for photoablation in different regimes. In order of increasing excitation intensity, these include the roles of: "recoilless emission" (as in Bragg scattering and the M6ssbauer effect) during Auger recombination of a core exciton, particularly in the Knotek-Feibelman effect; the metastable binding of multiple holes earlier proposed by Feibelman, particularly in the data of Itoh and coworkers; the change in the equilibrium interatomic spacing concomitant with excitation of a Frenkel exciton (localized broken bond), particularly in the ablative photodecomposition reported by Srinivasan and co-workers; the interaction of virtual excitons with real excitons in a strongly polarized solid, e.g. one subject to laser annealing pulses; and the interaction of ions with surface plasmons, particularly in the laser sputtering data of Hanabusa et al. A connection is made to rapid annealing and to sub-threshold defect formation. Further support is noted for the bose condensation model of pulsed laser annealing.
INTRODUCTION In order to study the effects of radiation and to distinguish between time varying thermal processes and those that occur before the deposited energy is dissipated as heat, it is helpful to study processes with the greatest activation energies for which data can be obtained. Optical ablation fits this description and appears to be among the most useful processes in the laser-solid interaction field. To understand why the former is so, let us first review the elementary theory of thermal rates. The thermal rate, R, at temperature T of a process connecting initial state i with final state f with an activation free energy AGif = AHf - TASf, where AH and AS are the corresponding enthalpy and entropy, may be written (1) Rif(T) = v0 exp(ASif/k) exp(-AHif/kT). Here v0 is the frequency with which the process is attempted. It is common practice to assume for processes involving atomic rearrangements in a solid that P. is the lesser of the Debye frequency or kT/h, where k and h are the constants of Boltzmann and of Planck. Thus, 0vo/OT = 0 for T > 670 K in Si. Eq. (1) is a consequence of the fact that, for a thermal distribution, the probability that any one of the possible modes of the system be attained is (2) P,(T) cc exp(-Hs/kT), where H, is the enthalpy of that mode, s. One then chooses s to be the bottle-neck, or saddle point mode of the process in question. The effective number of such bottle-neck modes through which the process can proceed is (by definition) exp(ASs/k). Finally, AHif = Hs - Hi
ASif = Ss - Si.
Mat. Pea. Soc. Symp. Proc. Vol. 23 (1984) QElsevier Science Publishing co., Inc.
(3)
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When energy is added to the system in some particular way, certain modes are populated to an extent greate
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