Mixed quantum/semiclassical wave-packet dynamical method for condensed-phase molecular spectroscopy signals.

We report the successful application of a recently developed mixed quantum/semiclassical wave-packet dynamical theory to the calculation of a spectroscopic signal, the linear absorption spectrum of a realistic small-molecule chromophore in a cryogenic environment. This variational fixed vibrational basis/Gaussian bath (FVB/GB) theory avails itself of an assumed time scale separation between a few, mostly intramolecular, high-frequency nuclear motions and a larger number of slower degrees of freedom primarily associated with an extended host medium. The more rapid, large-amplitude system dynamics is treated with conventional basis-set methods, while the slower time-evolution of the weakly coupled bath is subject to a semiclassical, thawed Gaussian trial form that honors the overall vibrational ground state, and hence the initial state prepared by its Franck-Condon transfer to an excited electronic state.

Broad-Band Pump-Probe Spectroscopy Quantifies Ultrafast Solvation Dynamics of Proteins and Molecules.

In this work, we demonstrate the use of broad-band pump-probe spectroscopy to measure femtosecond solvation dynamics. We report studies of a rhodamine dye in methanol and cryptophyte algae light-harvesting proteins in aqueous suspension. Broad-band impulsive excitation generates a vibrational wavepacket that oscillates on the excited-state potential energy surface, destructively interfering with itself at the minimum of the surface.

How fissors works: observing vibrationally adiabatic conformational change through femtosecond stimulated Raman spectroscopy.

With the help of a two-dimensional model system comprising a slow conformational degree of freedom and a higher-frequency vibration, we investigate the molecular-level origin and dynamical information content of femtosecond stimulated Raman spectroscopy (fissors) signals. Our treatment avails itself of the time scale separation between conformational and vibrational modes by incorporating a vibrationally adiabatic approximation to the conformational dynamics. We derive an expression for the fissors signal without resort to the macroscopic concepts of light- and phonon-wave propagation employed in prior coupled-wave analyses.