Femtosecond 3 Pulse Stimulated Photon Echo Peak Shift Measurements of Organic Polymer Glass from Room Temperature to 30 K

Abstract

Recently it has been shown that the femtosecond 3 pulse stimulated photon echo peak shift (3PEPS) measurement can give accurate dynamic information about the solvation process [1,2]. The peak shift is measured as a function of the population time T by obtaining time zero from the peaks of the two echo signals in phase matching directions - k 1+ k 2 + k 3 and k 1 - k 2 + k .. The decrease of peak shift reflects the decrease in rephasing capability caused by solvation dynamics. In other words, it measures the system’s degree of retained memory of its Bohr frequency after spending some time in a population state. The 3PEPS measurement is an indirect way of measuring the transition frequency correlation function M(t). It is also very sensitive to oscillatory contributions and can pick up intramolecular vibrations. Other features that make 3PEPS attractive are its insensitivity to the population dynamics and its unique capability of picking up very slow or static (inhomogeneous) components. It has shown that the solvation of polar solutes in polar solvents have rather universal characteristics. The initial ultrafast process, usually identified as an inertial response of solvent molecules, occurs on a < 100 fs time scale, and is essentially identical in various solvents [1]. On the other hand, the diffusive regime (> 0.5 ps) of the solvation process shows a strong solvent dependence which may be described satisfactory by dielectric relaxation theories. Here we report temperature dependent 3PEPS data for a dye (IR144) doped organic polymer glass, polymethylmethacrylate (PMMA), which is considered to have a slower dynamics and larger static inhomogeneity compared to room temperature solvents.