INDUCED TRANSIENT BIREFRINGENCE OF A RESONANTLY PUMPED MOLECULAR GAS

We present a theoretical study of the induced transient birefringence of a low density homogeneous molecular gas in a resonant pump-probe ex periment, The molecular coherent state induced by the resonant pump fi eld is described by second-order perturbation theory. The induced bire fringence can be detected by a delayed probe pulse propagating through the molecular medium after illumination by the pump pulse. In the cas e of a nonresonant probe, the birefringence is linearly proportional t o the mean value of the electronic polarizability of the molecular gas . The birefringence signal is composed of distinct components due to p opulation change and those of rotational, vibrational, and mixed vibra tional-rotational origins. This is demonstrated by numerical simulatio ns on Li-2 gas. Moreover, the quantum beats contained in the birefring ence, as a function of the time delay between the pump and probe pulse s, is dominated by the pure rotational motion. Finally, the birefringe nce is sensitive to the shape of the applied pump pulse and dependent on the spectral phase of the pump pulse. (C) 1996 American Institute o f Physics.