On the time resolution of picosecond phase grating spectroscopy in tracing structural relaxation dynamics

A theoretical model of fast density changes in a solution of species undergoing photoinduced structural relaxation is presented. The model is used to explain a very fast rise time of the signal observed in a picosecond phase grating experiment on heme proteins. The time resolution of the structural relaxation dynamics is found to be limited only by the laser pulse width and not by the acoustic propagation time over the fringe spacing of the grating as in the thermal relaxation detection. A convolution equation to restore the dynamics of the fast initial density changes driven by the structural relaxation is given.