Department of Chemistry, University of California, Irvine, California 92697-2025, USA.
J Chem Phys. 2010 Jan 28;132(4):044503. doi: 10.1063/1.3291610.
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.
呈现并严格解释了液态溴的紫外可见吸收和共振拉曼（RR）光谱。RR 光谱显示了高达 30 个泛音的非谐振动级数，定义了 2.05 A<r<3.06 A 范围内的基态势。在液相中，X 态势的吸引分支变软，分子的明显离解极限显著降低约 30%，表明存在吸引力的笼-分子相互作用。激发态势（A(')、B 和 C）从吸收光谱中提取。首先在经典反射近似的假设下对光谱进行反演，然后通过量子时间相关进行正向模拟进行修正。外推的 B 和 C 势用于模拟 RR 光谱。通过双通道 RR 散射引起的偏振光谱的干涉图案来检查它们的有效性。计算和观察到的强度之间的差异可以完全归因于振动去相，观察到的振动去相率完全遵循 Birge-Sponer 图的振动级数-表明振动耗散控制相干的衰减。尽管存在强烈的分子间电子相互作用和大约 kT 的振动能隙，但振动相干性的寿命很长：在 v=1 和 v=25 之间，相干时间范围从>or=25 到>or=2.4 ps。值得注意的是，RR 线形状向红色倾斜，表明在大约 400 fs 的时间内频率出现上啁啾。显然，分子振动在溶剂笼中绝热跟随，溶剂笼在大约 20 fs 的电子激发态演化过程中被瞬间驱动膨胀。液态溴在具有四极相互作用的有序缓慢局部笼中保持相干性-类似于在结构化低温稀有气体固体中分离的分子的动力学。
