Monitoring of the Early Stages of Thin Film Growth by the Generation From Second Harmonic Radiation of Supported Metal P
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Introduction In the past, the linear optical properties of metal clusters and small particles have been studied in detail [1,2]. Little is known about their nonlinear optical behavior. Very recently, the second and third order nonlinear susceptibilities X(2) and X(3) of clusters have found increasing interest [3-7]. Experimental and theoretical investigations are motivated by a variety of reasons, the most essential ones being the ultrafast electronic response of particles with small dimensions and the possibility to vary the electronic structure and therefore the susceptibilities in a well defined manner by changing the particle size. Possible applications include the exploitation of third order processes for switching of light on a sub-picosecond timescale. On the other hand, the high surface specifity of second harmonic (SHG) or sum frequency generation (SFG) opens up the possilbility of monitoring catalytic reactions on the surface of metal clusters. Another argument is that particles of variable size and shape can be regarded as building blocks of rough metal films making the study of their nonlinear properties a precondition for a deeper understanding of the susceptibilities X(2) and zV3) of real metallic surfaces in general. The present paper addresses the second order nonlinear susceptibility X(2) of small metal particles supported on a dielectric substrate and reports first exploratory experiments by optical SHG. The study of SHG on small metal particles without adsorbate molecules is an important precondition for a full understanding of how frequency doubled light is generated at the particle surface. Hence, all experiments reported here were performed under ultrahigh vacuum conditions. It is demonstrated that SHG provides a sensitive tool to investigate in situ the growth 477 Mat. Res. Soc. Symp. Proc. Vol. 354 01995 Materials Research Society
process of metal clusters on surfaces in the course of nucleation, to characterize them with respect to their size and shape and thus observe the initial stages of thin films formation. Since SHG only occurs if the inversion symmetry is broken, i. e. on the surface of supported metal particles, it can be exploited to monitor adsorption and chemical reactions on the particle surface with very high sensitivity. As will be described in the course of this paper in more detail the main advantages of our experiments as compared to earlier work is the following combination of experimental features: preparation of the particles on a single crystal surface at low and constant number density, in situ monitoring of the SHG signal during cluster growth, characterization of the particles with regard to their size and shape, use of well defined conditions, i.e. ultrahigh vacuum, and study of the influence of adsorbates on the SH intensity.
Method and Experimental Within the dipole approximation ideal, spherical metal clusters in a homogeneous matrix do not generate a surface or bulk SH signal since the electronic structure as well as the geometrical shape possess inversion sy
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