Abstract

Vibrational overtones in molecular liquids and multiphonon states in crystals are related to important processes in dense molecular systems. We show here that nonlinear optical techniques can be profitably extended⁽¹⁾ to the selective study of spectral and dynamical properties of overtones and multiphonon states and used to elucidate previously inaccessible features of vibrational motion in condensed matter and crystals in particular. In contrast to conventional spectroscopy, nonlinear techniques, in particular coherent excitation, also allow the creation of multiphonon and large wave vector states in an off equilibrium configuration and the detection of their subsequent evolution. Of particular interest in this context are bound two-phonon states (bi-phonons) in molecular crystals^(2-4) which arise from the lattice anharmonicity and are expected to pocess spectral and dynamical features quite distinct from those of free two-phonon states.

© 1984 Optical Society of America