Abstract
Optical second-harmonic generation on interfaces between two media has been very actively investigated in recent years for it promises to be an effective tool for time resolved surface studies [1]. The influence of adsorbates on the intensity of the second harmonic (SH) generated has been observed during adsorption of O, CO, and alkali metals on Rh(111) surfaces [2]. In these cases the presence of the adsorbates was detected indirectly through their influence on the density of free electrons in the metal substrate. In this contribution we report an enhancement of the SH signal through an electronic resonance of the adsorbate itself, showing the possible adsorbate specifity of the harmonic signal. The second-harmonic was generated by sulfur atoms adsorbed on a Pd(100) single crystal surface in ultrahigh vacuum (2×10-10 Torr). As p polarized fundamental radiation, incident on the sample at an angle of 45°, served the output of a Q-switched Nd:YAG laser at λ = 1064.2 nm (pulse duration 8 ns, pulse energy 0.65 m), bandwidth 0.1 cm-1). The laser beam diameter of about 0.1 mm resulted in an intensity of about 950 MW/cm2 on the sample. The p polarized SH was detected after proper filtering by a photomiltiplier. The sulfur atoms were supplied by segregation from the bulk, and the coverage was monitored by Auger spectroscopy [3].
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