Probing structural dynamics via ultrafast nonlinear infrared spectroscopy.

摘要

This thesis presents the results of a variety of ultrafast nonlinear infrared spectroscopic experiments performed during my tenure as a graduate student at Stanford University in the research group of Professor Michael D. Fayer. The unifying theme of the experiments is that they all served to elucidate the structural dynamics of the condensed phase molecular systems chosen for study. A novel tunable ultrafast mid-infrared laser system developed specifically to perform the types of ultrafast nonlinear infrared spectroscopic experiments presented herein is also described.; The key features of the laser system are its wavelength tunability and its variable spectral bandwidth/pulse duration. The system's output is tunable over the entire MIR region of the spectrum, allowing the user to study virtually any molecular vibration of interest, rather than being limited to only a few types of chemical bonds with similar vibrational frequencies. The variability of the bandwidth/pulse duration allows the user to tailor the laser's pulse characteristics to the molecular system being studied, depending upon whether short time resolution or a narrow range of wavelengths is desirous.; Hydrogen bonding was studied by infrared pump-probe spectroscopy using the hydroxyl (OD) stretch of coexisting monomers and oligomers of methanol- d (MeOD) dissolved in CCl₄ as a probe. Experiments were performed at a variety of wavelengths (corresponding to different MeOD species) and MeOD concentrations. The results suggest that relaxation of the OD stretch leads indirectly to transfer of energy to the hydrogen bond coordinate and breaking of the hydrogen bond, which can then reform on two distinct timescales, corresponding to geminate or non-geminate recombination.; Interconversion between rotational conformers on the ground electronic state potential energy surface was observed for ethyl isocyanate (EIC) dissolved in 2-methylpentane (2MP) using infrared vibrational echo spectroscopy. The results of the echo experiments show that the N=C=O antisymmetric stretching feature is dominated by interactions with the solvent at room temperature, but by changes in the conformation of the ethyl group at intermediate temperatures above the 2MP glass transition temperature. Below T_g, the dephasing of the N=C=O stretch appears to be due to librational motions of the ethyl group within its potential energy well.