Spectra of the $S_0\to S_2$ electronic transition of rhodamine dyes adsorbed at submonolayer coverages on a fused silica substrate have been obtained by resonant second-harmonic generation. Polarization dependences of the second-harmonic signal have been studied and are used in the construction of a model for the orientation of the adsorbates. A value for the molecular second-order nonlinear polarizability at resonance is deduced.

• Received 7 December 1981

DOI:https://doi.org/10.1103/PhysRevLett.48.478