Berlin 2005 – scientific programme

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O: Oberflächenphysik

O 5: Zeitaufgelöste Spektroskopie I

O 5.8: Talk

Friday, March 4, 2005, 12:30–12:45, TU EB107

Mechanism of femtosecond laser induced diffusion of oxygen on vicinal Pt(111) — •K. Stépán, J. Güdde, and U. Höfer — Fachbereich Physik und Zentrum für Materialwissenschaften, Philipps Universität Marburg, D-35032 Marburg

Laser-induced diffusion of atomic oxygen from step edges onto terraces of a vicinal Pt(111) surface has been studied by exploiting the sensitivity of optical second-harmonic generation (SHG) on surface symmetry. The excitation mechanism of the diffusion process has been investigated by measuring the diffusion rate as a function of fluence and delay between two femtosecond pump pulses. The strong nonlinear fluence dependence (∝ F¹⁵) and a width of 1.5 ps in the two-pulse-correlation measurements show that the diffusion process can be understood within the same framework as the well studied phenomena of desorption induced by multiple electronic transitions (DIMET). For a quantitative modeling of the data we approximate the detailed coupling of adsorbate modes and excited substrate electrons with an electronic friction coefficient [1]. In contrast to the situation in most laser-induced desorption experiments, a friction coefficient that depends on the excitation density is required in to describe the whole data set. We interpret this dependence in terms of an indirect electronic excitation mechanism of the frustrated translation leading to diffusion. We suggest that the electronic excitation of the substrate couples primarily to the O-Pt stretch vibrations which then excite frustrated translations via an anharmonic coupling of modes.

[1] M. Brandbyge et al. Phys. Rev. B. 52, 6042 (1995)