Vibrational Dynamics in Liquids and Glasses: Picosecond Infrared Vibrational Photon Echoes and Other IR Experiments

Abstract

The first vibrational photon echo experiments conducted in the liquid and solid phases [1,2] are used to examine the vibrational dynamics of solute molecules in liquids and glasses. . These experiments are performed using the Stanford superconducting linac pumped free electron laser which provides a source of tunable ~0.7 psec IR pulses. The psec IR vibrational photon echo experiment directly examines the interactions of a particular vibration with phonons and other vibrational modes of the system. In an IR vibrational echo experiment, the laser is tuned to the vibration of interest. Two pulses, one delayed in time, excite the sample. The echo is a third pulse of light that emerges from the sample in a unique direction. The intensity of the echo pulse is measured as a function of the delay between the two excitation pulses. The decay of the echo is a direct measure of the homogeneous dephasing of the vibration. In condensed matter systems, vibrational lines are generally inhomogeneously broadened. Therefore, taking a spectrum does not provide the dynamical information that is available from analysis of the homogeneous spectrum. The echo decay is the Fourier transform of the homogeneous vibrational line. The echo pulse sequence removes inhomogeneous broadening and permits the dynamical information that is contained in the homogeneous line to be obtained.