Solidlike coherent vibronic dynamics in a room temperature liquid: Resonant Raman and absorption spectroscopy of liquid bromine.
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 up to 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 and apparent dissociation limit of the molecule dramatically reduced by approximately 30% in the liquid phase, indicating an attractive cage-molecule interaction. The excited state potentials (A('), B, and C) are extracted from the absorption spectrum. The spectrum is first inverted under assumption of the classical reflection approximation, then corrected by forward simulations through quantum time correlations. The extrapolated B and C potentials are used to simulate RR spectra. Their validity is cross-checked by the interference pattern of the polarized spectra due to two-channel RR scattering. The discrepancy between calculated and observed intensities can be entirely assigned to vibrational dephasing, which is observed to follow the exponential energy gap law-dephasing rates perfectly trace the Birge-Sponer plot of the vibrational progression-suggesting that vibrational dissipation controls the decay of coherence. Despite strong intermolecular electronic interactions and vibrational energy gaps of approximately kT, vibrational coherences are long lived: Coherence times range from >or=25 to >or=2.4 ps between v=1 and v=25. Remarkably, the RR line shapes are skewed toward the red, indicating upchirp in frequencies that develop over a period of 400 fs. Evidently, the molecular vibrations adiabatically follow the solvent cage, which is impulsively driven into expansion during the approximately 20 fs evolution on the electronically excited state. Liquid bromine retains coherence in ordered sluggish local cages with quadrupolar interactions-dynamics akin to molecules isolated in structured cryogenic rare gas solids.
Branigan
Edward T
ET
Department of Chemistry, University of California, Irvine, California 92697-2025, USA.
van Staveren
Marie N
MN
Apkarian
V Ara
VA