Pump-probe Spectroscopy

In this experiment, we measure the rate of vibrational energy redistribution in molecular systems in dilute solution. For example, we vibrational excite the acetylenic CH stretch to the first excited state and measure the time scale of vibrational energy localization in the acetylenic CH stretch in dilute carbon tetrachloride (CCl4) solution. This study of intramolecular vibrational energy redistribution (IVR) has been performed in isolated gas-phase using high-resolution infrared spectroscopy so far. Knowing the basic mechanisms of energy redistribution, we are extending the study of IVR into the solution phase, where solvent molecules can modify the relaxation dynamics of the solute that is being studied. For the study of vibrational energy relaxation rate in solution, we use infrared pump-probe spectroscopy, where an intense pump pulse vibrationally excites a molecular bond and a weak probe pulse monitors the decay of the population. The current study shows that two mechanisms are involved in the vibrational energy relaxation process; IVR and solvent-induced IVR.

An interferometric pump-probe transient bleaching experiment is located in the Ultrafast Laser Facility for measuring the rate of energy redistribution in molecular systems. These experiments are currently performed in the solution phase. A parallel project is currently operating at the Thomas Jefferson National Accelerator Facility Free Electron Laser for measuring these IVR lifetimes in gas phase species. The much lower number densities of gas phase molecules makes these measurements much more difficult, and it is hoped that the much higher (75 MHz) repetition rate of the FEL will make the difference. We are currently constructing a new gas/liquid apparatus for the Ultrafast Laser Facility which may greatly enhance the S/N ratio in the gas phase experiments, as well as a dispersed-frequency multi-element detector system to extend the pump-probe lifetime studies to two dimensions. This experiment was greatly enchanced by the recent (6/2000) installation of the 1-2 ps regen/OPA system in the Ultrafast Laser Facility. The bandwidth of the ps is much more suited to the absorption profiles of many of the molecules of interest to these studies, improving the efficiency of the experiment, and the available per-pulse energy (~15 mJ @ 3mm) is much better than for the fs OPA systems (~10 mJ).