Picosecond time-resolved dispersion spectroscopy using the interference between population and thermal phase gratings

A new method is presented for measuring directly the dispersion spectrum of short-lived transients, based on heterodyne transient grating spectroscopy. The light diffracted from a preformed thermal phase grating is used as a local oscillator, in phase with the light diffracted from the population phase grating. Time-resolved dispersion spectra measured upon photoinduced electron transfer reaction between 9,10-dicyano-anthracene and anisole in acetonitrile are presented. These spectra and the corresponding absorption spectra obtained by Kramers-Kronig transformation are compared with those deduced from the homodyne transient grating spectra. Potential applications of this technique are discussed.