Simulations of wavepacket motion at the ultrafast time scale in condensed media.

摘要

UV-visible absorption and resonance Raman (RR) spectra of liquid bromine are presented and rigorously interpreted. The RR spectra, which show an anharmonic vibrational progression of 30 overtones, define the ground state potential in the range 2.05 A < r < 3.06 A. The attractive branch of the X-state potential is softened, indicating an attractive cage-molecule interaction. The excited state potentials (A', B, and C) are extracted from the absorption spectrum using first the reflection approximation and then quantum time correlations. The extrapolated B and C potentials are used to simulate RR spectra. Vibrational dephasing follows the exponential energy gap law, suggesting that vibrational dissipation controls the decay of coherence. Despite strong intermolecular electronic interactions and vibrational energy gaps of ∼kT, vibrational coherences are long lived: coherence times range from ≥25 to ≥2.4 ps between nu = 1 and nu = 25. Remarkably, the RR line shapes are skewed toward the red, indicating upchirp in frequencies which develops over a period of 400 fs. Evidently, the molecular vibrations adiabatically follow the solvent cage, which is impulsively driven into expansion during the ∼20 fs evolution on the electronically excited state.;Dynamically skewed spectral lines arise for chirped damped oscillators. As such, the concept is broadly applicable. To the extent that optical transitions reflect frequency differences between periodic motions---electronic, vibrational, rotational, magnetic---skewed lines should appear whenever the frequencies evolve during observation. We extend our treatment to the chirped damped rotor, and applied it to treat the ro-vibrational line shapes of a small molecule---OCS---isolated in superfluid helium. This simple analysis generates valuable physical insights on the prevailing dynamics of a rotor coupled to its environment, in this case to a quantum fluid which can coherently follow the motion of a molecular rotor.;Spectrally resolved transient grating measurements (SRTG) performed on bromine frozen in amorphous ice reveal a wealth of information about the nature of the prepared state and the coupling between bromine and ice. The SRTG signal follows the oscillation of the density on the B state, probed through a charge transfer to solvent state T. Time constants for the electronic and vibrational dephasing, as well as the motion on both B and T states, are determined. I show that, while a classical simulation reproduces the major experimental features of spectrally integrated signals, the resulting state is unphysical. Classical simulations completely fail in the case of SRTG. As a most general classical state, we use coherent squeezed states with time dependent squeeze, which represent an evolving classical distribution with time dependent variance in its momentum and position. The SRTG data can only be reproduced through wave mechanics in which the evolution is in terms of nonlocal state wavefunctions. This is most rigorously demonstrated by firstly noting that the signal can be treated to represent the evolving density (psi*psi(t)), reducing distortions due to measurement, and then demonstrating that the Wigner representation of the signal contains a negative hole. A well defined Wigner negative area in the Wigner quasi distribution is a hallmark of wave interference, which is a quantum phenomenon. The time-dependent phase space portrait of the system is extracted, and used to characterize the instantaneous state space composition of the packet. Two-time correlation simulations allow determination of the potential parameters, and show that the system can be adequately modeled as a 1-dimensional oscillator, with a small but well defined signature of the bath to which it is coupled. The latter is extracted through a careful decomposition of the spatiotemporal coherences. Frequency analysis allows the creation of maps of the various coherences, showing the evolution of these terms of the density in space and in time. A coherence map corresponding to a clathrate phonon is observed, and leads to the determination that the bath is alternately compressing and extending the molecular bond.