Two-color resonant four-wave mixing as a new tool to study state-to-state energy transfer

Abstract

Applying a recently developed ns Two-Colour Resonant Four-Wave Mixing (TC-RFWM) technique, a number of resonance lines forbidden by conventional three-level schemes are observed. A theoretical frequency-domain picture is developed with the help of exact (off-diagonal) relaxation matrices and general four-level schemes. The newly observed extra resonances are induced by collisional transfer of rotational energy. The role of the population, orientation and alignment gratings that are formed in both electronic states coupled by laser fields is elucidated. Relaxation models using the fast-collision approximation are elaborated for Hund's cases a and b giving quantitative arguments for the proposed interpretation. The potential of the TC-RFWM technique to study state-to-state transfer rates is emphasized.