Rotational coherence spectroscopy (RCS) by time resolved single resonant degenerate four-wave mixing (DFWM) on the femtosecond time scale was performed in the gas phase. Results are presented for the diatomic molecules O_2 and N_2, for the linear molecule CO_2, and for the symmetric top CHCl_3. For CO_2 additional experiments have been performed at reduced pressure and in the collisionless surrounding of a molecular bean. When delaying the probe pulse a periodic recovery of the DFWM signal is observed. The period of these recurrences can be assigned unambiguously to rotational Raman transitions of the ground state within the laser bandwidth. In the case of the symmetric top, distortion of the molecule due to centrifugal forces becomes apparent in the transient. The decay of the transients yields the relative strength of the collisional dephasing of the Raman-induced polarization. At zero delay also optical-field-induced birefringence of electronic nature contributes to the signal. The different time scales of Raman and electronic effects allow to estimate their relative strength. From the occupation of the rotational levels, a temperature can be estimated.
展开▼
