Book Description

Ultrafast spectroscopy is one of the most active areas of research in chemical physics. Understanding the ultrafast behavior of molecules, and influencing it with the help of laser fields may lead to the ultimate goal of controlling the flow and outcome of chemical reactions. This thesis work focuses on the investigation and control of molecular dynamics using time-resolved four-wave mixing (FWM) techniques in gas phase samples. Femtosecond FWM spectroscopy has established itself as a powerful tool to uncover molecular dynamics on ultrafast time scales. The femtosecond time resolution combined with a background-free, highly collimated coherent signal makes the approach unique for studying the dynamics especially under conditions of low concentrations. In gas phase, the elementary reaction paths can be directly probed without the interference of solvent molecules. A comparative study of the molecular dynamics in the gas phase and condensed phase gives a detailed picture on the effect of environment. Utilization of Raman as well as optical resonances in FWM spectroscopy provided high selectivity with respect to the type of molecular dynamics observed in the transient signal. New schemes of FWM spectroscopy, which employs an initial pump pulse along with a fully time resolved FWM process, were devised to monitor also the dynamics of higher lying excited states. A feed back controlled optimization process in a FWM scheme is used to selectively excite or suppress vibrational modes in gas phase and liquid phase of the same molecule. The results of the experiments performed using different FWM techniques to monitor, understand, and also to control diverse aspects of molecular dynamics in different systems are presented and discussed.