Vibrational Dynamics in Liquids and Glasses Probed with Infrared Photon Echoes Using a Free Electron Laser

Abstract

Molecular vibrations are involved in a vast number of physical, chemical, and biological processes. Coupling between molecular vibrations and external mechanical degrees of freedom (heat bath) is responsible for the flow of energy into and out of molecules and for thermally activated processes. Fluctuations of vibrational energy levels of a molecule caused by interactions with a dynamic environment are important in fundamental chemical and biological processes, such as electron transfer and chemical reactions. In spite of the importance of the coupling of molecular vibrations to a heat bath, relatively little is known about the temperature dependent dynamics of molecular vibrations in liquids and glasses. Here we give an account of the first ps infrared (IR) vibrational photon echo experiments performed on molecular vibrations in liquids and glasses [1,2]. The photon echo experiment is an optical line narrowing experiment that has been applied extensively to the study of electronic excitation dynamics in condensed matter systems [3-6]. However, the study of vibrations in condensed phases using photon echoes has been limited because of the need to work with ps IR pulses. Very recently the first vibrational Raman echoes were performed [7] and a vibrational photon echo study of a group attached to a surface in high vacuum was conducted [8].