Investigating Ultrafast Condensed-Phase Chemical Dynamics with Coherent Multidimensional Spectroscopy.

Abstract

Experimental and theoretical studies of ultrafast chemical dynamics in solutionare presented in this work. Condensed-phase chemical reactions are investigated with newly-developed experimental techniques, including non-equilibrium two-dimensional infrared spectroscopy, transient-dispersed vibrational echo spectroscopy, and vibrational Stark-effect spectroscopy. The experiments are aimed at elucidating the complex relationship between molecules and their solvent environment under equilibrium and non-equilibrium conditions. The orientational relaxation rates of hot radical molecules in non-polar solvents were measured with transient-two-dimensional infrared spectroscopy to obtain the vibrational energy relaxation (cooling) rates following a homolytic bond cleavage reaction. Experimental studies of the asymmetric, solvent-caged radical recombination reactions offered new insightsinto the solvent role in determining the branching ratiosand recombination rates in these asymmetric reactions. Dynamic vibrational Stark-effect spectroscopy is demonstrated as a new probe of molecular dynamics in solution. Within this method, a charge-transfer reaction is optically triggered, causing a change in the electric field at the nearby solvent molecules. The vibrational response of the solvent molecules serves to map the electrostatic changes at the chromophore as well as elucidate the dynamics of the molecules within the first solvation shell. The solvent response is measured upon optically triggering an electron-transfer reaction in the solvatochromic dye Betaine-30. The rate of the back-electron transfer, which returns the dye molecules to the ground state, has been measured from the solvent response.

In addition to molecular dynamics, two-dimensional infrared spectroscopy can directly access theone- and two-quanta energy levels of the system which directly reports on the anharmonic potential of the molecules. The potential surface of dimanganese decacarbonyl and its photoproducts has been modeled up to fourth order in the normal-mode coordinates using ab-initio electronic structure methods. The energy levels are found to be in agreement with experiment. The vibrational dynamics of dimanganese decacarbonyl are modeled using a Markovian quantum master equation with bilinear system-bath coupling. The model accounts for vibrational relaxation, coherence dephasing, coherence transfer and coherence-population coupling. The transport rates are computed using input from molecular dynamics simulations.