OPTICAL 2ND-HARMONIC GENERATION BY SUPPORTED METAL-CLUSTERS - SIZE AND SHAPE EFFECTS

Optical Second Harmonic Generation (SHG) by metal clusters has been in vestigated. For this purpose clusters were generated by the deposition and nucleation of metal atoms on a LiF(100) single crystal surface un der ultrahigh vacuum conditions. The size and shape of the metal parti cles was characterized by optical transmission spectroscopy. The SHG i ntensity was detected in situ as a function of cluster size during the nucleation. Fundamental wavelengths of lambda = 1064 and 532 nm were used and the SHG signal was measured for different polarization combin ations of the incident and registered light. SH radiation is detectabl e for particles as small as approximately 1 nm. The signal grows monot onically as a function of particle size, passes a maximum and finally drops off. This behavior is discussed in terms of resonant enhancement of the signal by surface plasmon excitation and changes of chi((2)) a s a function of particle size and shape. In further experiments the ch emisorption of oxygen on the surface of the metal particles was studie d. The SH signal decreases as a function of oxygen coverage and amount s to only about 15% of the initial value upon chemisorption of one mon olayer. This indicates that the SH signal originates almost exclusivel y from the surface of the clean clusters and that higher order bulk co ntributions are negligible.