Time-Resolved Measurement of Surface Diffusion Induced by Femtosecond Laser Pulses

Diffusion of atomic oxygen on a vicinal Pt(111) surface induced by femtosecond laser pulses has been studied using optical second-harmonic generation as a sensitive in situ probe of the step coverage. Time-resolved studies of the hopping rate for step-terrace diffusion with a two-pulse correlation scheme reveal a time constant of 1.5 ps for the energy transfer from the electronic excitation of the substrate to the frustrated translations of the adsorbate.

Figure 1

Scheme of the experiment. Step sites are decorated selectively by dissociative adsorption. Intensive ultrashort laser pulses induce diffusion onto the terraces, which is monitored by second-harmonic generation of a probe beam.

Figure 2

Second-harmonic response of the Pt sample during dissociative adsorption of ${\mathrm{O}}_2$ at the steps (dosing) and diffusion of atomic oxygen induced by femtosecond laser pulses of various absorbed fluences $F_{\mathrm{abs}}$. The pump pulses had a repetition rate of 1 kHz and were switched on at the time marked by the vertical arrow. The right $y$ scale gives the conversion of the SHG signal into the relative step coverage [17].

Figure 3

Hopping probability $p_{\mathrm{dif}}$ per laser shot as a function of delay between the $p$- and $s$-polarized pump beams with absorbed fluences of 2.3 and $2.8\mathrm{mJ}/{\mathrm{cm}}^2$, respectively, (symbols). At positive delays, the weaker excitation precedes the stronger one (inset). The thick solid line is a guide to the eye. The thin line shows the SHG cross correlation of the two pump pulses generated at the sample surface.

Figure 4

Calculated hopping probability $p_{\mathrm{dif}}$ (a) as a function of absorbed fluence and (b) as a function of time delay between two pump pulses for two constant friction coefficients (thin dotted and dashed lines) and an electron temperature dependent ${\eta }_{\mathrm{e}}$ (thick dashed line). Experimental values are indicated by symbols (a) and the solid line (b).