TIME-RESOLVED SURFACE 2ND HARMONIC-GENERATION - A TEST OF THE METHOD AND ITS APPLICATION TO PICOSECOND ISOMERIZATION IN ADSORBATES

The application of time-resolved surface second harmonic generation (TRSSHG) as a surface-specific probe of picosecond reaction kinetics has been studied. The method was tested by comparing the measured dynamics with those obtained from the time-resolved fluorescence (TRF) technique. The agreement between the two methods was good. It is concluded that the TRSSHG method is a useful probe of picosecond surface chemistry. Small differences between fluorescence and second harmonic data are explained by a consideration of the different natures of the two experiments. The surface-specific method is essentially an analogue of transient absorption spectroscopy, except that induced changes in hyperpolarizability are measured. The combined results show that large-scale intramolecular motions can occur on a picosecond time scale in molecules adsorbed on solid surfaces. The rates of the isomerization reactions are contrasted with those measured in solution. It is concluded that the details of the adsorbate-substrate interaction, especially the effect on ground-state structure, determines the rate of isomerization. © 1990 American Chemical Society.