Three-wave-mixing spectroscopy is used to determine the dispersive and absorptive parts of a strongly allowed twophoton transition in a series of polydiacetylene solutions. The data analysis yields the energy, width, symmetry assignment, and oscillator strength for the two-photon transition, The data conclusively demonstrate that strong two-photon absorption is a fundamental property of the polydiacetylene backbone. The remarkably large twophoton absorption coeAicients are explained by large oscillator strengths for both transitions involved in the twophoton absorption combined with strong one-photon resonance effects. The experimental results are shown to be consistent with a simple theoretical model for the energies and oscillator strengths of the oneand two-photonallowed transitions.

A comprehensive study of the spatially isotropic component of the laterally averaged molecular hydrogen/Ag(111) physisorption potential is presented. Diffractive selective adsorption scattering resonances for rotationally state-selected H2 and D2 on Ag(111) have been mapped out as a function of incident polar angle for several crystal azimuths and beam energies. These resonances have been used to determine the bound eigenvalues, and subsequently the shape, of the potential well. Best fit Lennard-Jones, Morse, variable exponent, and exponential-3 potentials having well depths of ∼32 meV are derived from the data. These measurements are supported by rotationally inelastic scattering measurements for HD and exact close-coupled quantum scattering calculations. Debye–Waller attenuation measurements are also presented for H2, D2, and HD. The ability to detect these diffractively coupled resonances on a closest-packed metallic surface, i.e., a surface of extremely low corrugation, suggests that such measurements can be carried out on a much wider class of surfaces than previously envisioned.