School of Chemical Sciences, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
J Phys Chem A. 2009 Feb 26;113(8):1445-52. doi: 10.1021/jp809679y. Epub 2009 Feb 2.
Ultrafast Raman spectroscopy with infrared (IR) excitation is used to study vibrational energy dynamics of ambient temperature liquids benzene and benzene-d(6). After IR pumping of a CH-stretch or CD-stretch parent excitation, the redistribution of vibrational energy is probed with anti-Stokes Raman. Ten benzene or 12 benzene-d(6) vibrations out of 30 total have large enough cross sections to be observed. The pathways, quantum yields, and lifetimes for energy transfer among these vibrations are quantified. Using a CCl(4) molecular thermometer, we demonstrate an ultrafast Raman calorimetry method which allows measurement of the rate that benzene vibrational energy is dissipated into the bath. On the basis of energy conservation, we then determine the time-dependent dissipation of aggregate vibrational energy from the unobserved, "invisible" vibrations. During the approximately 1 ps IR excitation process, vibrational energy is coherently redistributed to several vibrational modes ("coherently" means the rate is faster than (T(2))(-1) of the pumped transition). This energy is then further redistributed in an incoherent intramolecular vibrational relaxation process with a 6 ps T(1) time constant. The subsequent dynamics involve energy transfer processes accompanied by vibrational energy dissipation to the bath. This vibrational cooling process has a half-life of 30 ps in benzene and 20 ps in benzene-d(6), and thermalization is complete in approximately 100 ps. The observed strongly Raman-active vibrations have about the same amount of energy per mode as the invisible vibrations. The invisible vibrational energy in benzene decays somewhat faster than the observed energy. These two decay rates are about the same in benzene-d(6).
采用红外（IR）激发的超快拉曼光谱研究了环境温度下液体苯和苯-d(6)的振动能动力学。在 CH 伸缩或 CD 伸缩母体激发的 IR 泵浦之后，用反斯托克斯拉曼探测振动能的再分配。在总共 30 个振动中，有 10 个苯或 12 个苯-d(6)振动具有足够大的截面来进行观察。量化了这些振动之间能量转移的途径、量子产率和寿命。使用 CCl(4)分子温度计，我们展示了一种超快拉曼量热法，允许测量苯振动能向浴中耗散的速率。基于能量守恒，我们然后从未观察到的“不可见”振动确定总振动能量随时间的耗散。在大约 1 ps 的 IR 激发过程中，振动能量被相干地重新分配到几个振动模式（“相干”意味着速率比泵浦跃迁的 (T(2))(-1)快）。然后，通过具有 6 ps T(1)时间常数的非相干分子内振动弛豫过程进一步重新分配能量。随后的动力学涉及能量转移过程，伴随着振动能量向浴的耗散。这个振动冷却过程在苯中半衰期为 30 ps，在苯-d(6)中半衰期为 20 ps，并且大约在 100 ps 内完全热化。观察到的强拉曼活性振动在每个模式上具有与不可见振动相同的能量。苯中的不可见振动能量的衰减速度比观察到的能量稍快。在苯-d(6)中，这两个衰减速率大致相同。
