Ultrafast Dynamics of Chromophores in Isolated State, High Pressure and Liquid Environment Probed by Absorption and Resonant Emission Spectroscopy.
Vibronically specific subrotational period lifetimes of the predissociated 2^2B _2 state of NO_2 vapor are determined by spontaneous resonance Raman scattering polarization measurements. The lifetimes of the vibronic symmetric stretching (nu_1^ '), bending (nu_2 ^') asymmetric stretching overtone (2nu_3^') and the combination band (nu_1^ '+nu_2^') are 75 +/- 10 femtosecond fs, 2500 +/- 500 fs, 155 +/- 15 fs and 125 +/- 15 fs respectively. The lifetimes of the CH_3I X to B Rydberg transition in high pressure CH _4 and Ar (800-1600 psi), gas phase, supercritical fluid and liquid CO_2 are determined by absolute absorption and resonant emission cross section measurements. In contrast to CH _3I in CH_4 and Ar where the B state lifetimes are independent of pressure (900 +/- 100 fs), the B state lifetime of CH_3I in CO_2 decreases as the CO_2 pressure increases and is about 100 times shorter as CO _2 approaches the condensed phase density. These lifetime studies are valuable in understanding the curve -crossing dynamics of electronic state potential energy surfaces and photochemical activities in solution. The resonance fluorescence polarization ratio of CH_3I in high pressure gases (CH_4, CO_2) is given by a density matrix representation of the third order polarization. It is shown that the population decay rate and the rovibronic excited state coherence, not collision -induced reorientation, are the major factors determining the resonance fluorescence polarization. A set of potential parameters, which can best describe the solvent-solute interactions of CH_3I in high pressure nonpolar gases (Ar and CH_4), are obtained by molecular dynamics simulation of the absorption lineshape of the CH_3I B state. This method is also used to describe the solvent effects on the corresponding resonant emission characteristics such as the redistribution between resonance Raman and resonance fluorescence, band shape changes, and the excitation wavelength dependence of these characteristics. These studies address the question of how the photodissociation dynamics of an isolated molecule are affected by a well characterized bath, and in turn, how we can use the short lived excited state of the chromophore as a measure of bath-solute dynamics.
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- Chemistry: Physical; Physics: Molecular