Time-resolved spectroscopies heterodyned

Torre, R., Bartolini, P. and Righini, R. (2004). Structural relaxation in supercooled water by time-resolved spectroscopy. Nature 428 296-299. Constantine, S., Zhou, Y., Morais, J. and Zigler, L.D. (1997). Dispersed optical heterodyne detected birefringence and dichroism of transparent liquids. J. Phys. Chem. A 101 5456-5462. 

Fig. 2 Experimental arrangement for time-resolved FSRS (femtosecond stimulated raman spectroscopy). The femtosecond actinic pump pulse excites the sample electronically. After a delay the femtosecond probe pulse and picosecond Raman pump pulse arrive together to interrogate the instantaneous molecular structure. The self-heterodyned signal is emitted in the probe direction, dispersed, and detected by a kHz readout CCD. Data collection is best performed by division of subsequent Raman pump-on by Raman pump-off laser shots (lower trace), however this has been performed by other groups as a subtraction of subsequent pulses (upper trace). Reproduced from ref 2 with permission from the PCCP Owner Societies (2012).

The dynamics of the collective excitations can be determined conveniently by photon autocorrelation spectroscopy, which is also called self-beating, time-resolved Rayleigh or quasi-elastic light scattering spectroscopy [29]. Here the time autocorrelation function of the scattered light intensity G (t) is measured, which, in the heterodyne detection regime, is given by ... 

The difference between optical nutation and free induction decay should be clear. While the optical nutation occurs at the Rabi frequency which depends on the product of laser field intensity and transition moment, the free induction decay is monitored as a heterodyne signal at the beat frequency 0) 2 which depends on the Stark shift. The importance of these coherent transient phenomena for time-resolved sub-Doppler spectroscopy is discussed in the next section. Its application to the study of collision processes is treated in Chap.12. For more detailed information the excellent reviews of BREWER [11.43,48] are recommended. 

---