Orientation-insensitive methodology for second harmonic generation. 1. Theory
The use of second harmonic generation as a means to probe either adsorption isotherms or the kinetics of adsorption/desorption is limited by the fact that the detected signal is dependent upon both surface coverage and molecular orientation. Thus, the second harmonic intensity must be tediously corrected if either the mean orientation angle or the width of the orientation distribution changes as a function of surface coverage. In this study, a new mathematical view of the second harmonic intensity as a function of excitation polarization is developed. This new approach predicts an experimental geometry that is relatively insensitive to molecular orientation, such that appropriate choice of the excitation polarization rotation angle allows for direct measurement of surface coverage. The theory is presented for the three common dominant hyperpolarizability tensor elements, beta(z'z'z'), beta(x'x'x'), and beta(x'x'z').
Simpson
GJ
Department of Chemistry and Biochemistry, University of Colorado, Boulder 80309, USA.
Rowlen
KL