Optical heterodyne detected transient grating for the separations of phaseand amplitude gratings and of different chemical species

The sensitivity and stability of an optical heterodyne detected transient g rating (OHD-TG) method without an active phase control system was examined, and the system was used to separate the phase and amplitude gratings. From the absorptive component at 633 nm after the photoexcitation of a photochr omic molecule (a spiropyran molecule), the diffusion constant of only the m erocyanine form is selectively determined. The phase grating component cons ists of both the spiro and merocyanine contributions. However, since the co ntribution of the merocyanine form is still predominant over the spiro form in the phase grating signal, the diffusion process of the spiro form canno t be observed clearly. By adjusting the phase between the local oscillator and probe fields, the contribution of the merocyanine form can be selective ly reduced and the diffusion constant of the spiro form is successfully mea sured. The result shows that the selective reduction of one species by the OHD-TG technique has a great advantage for studying photochemical reactions .