Shigeto Shinsuke, Pang Yoonsoo, Fang Ying, Dlott Dana D
School of Chemical Sciences, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
J Phys Chem B. 2008 Jan 17;112(2):232-41. doi: 10.1021/jp074082q. Epub 2007 Aug 9.
Anti-Stokes Raman scattering is used to monitor vibrational energy redistribution in the ambient temperature liquids nitromethane (NM-h3) and perdeuterated nitromethane (NM-d3) after ultrafast IR excitation of either the symmetric or asymmetric CH- or CD-stretch transitions. The instantaneous populations of most of the fifteen NM vibrations are determined with good accuracy, and a global fitting procedure with a master equation is used to fit all the data. The pump pulses excite not only CH- or CD-stretches but also certain combinations of bending and nitro stretching fundamentals. The coupled vibrations that comprise the initial state are revealed via the instantaneous rise of the anti-Stokes transients associated with each vibrational fundamental. In contrast to many other polyatomic liquids studied previously, there is little energy exchange among the CH-stretch (or CD-stretch) excitations, which is attributed to the nearly free rotation of the methyl group in NM. The vibrational cooling process, which is the multistep return to a thermalized state, occurs in three stages in both NM-h3 and NM-d3. In the first stage, the parent CH- or CD-stretch decays in a few picoseconds, exciting all lower-energy vibrations. In the second stage, the midrange vibrations decay in 10-15 ps, exciting the lower-energy vibrations. In the third stage, these lower-energy vibrations decay into the bath in tens of picoseconds. The initial excitations are thermalized in approximately 150 ps in NM-h3 and there is little dependence on which CH-stretch is excited. VC is somewhat faster in NM-d3 with more dependence on the initial CD-stretch, taking approximately 100 ps with symmetric CD-stretch excitation and approximately 120 ps with asymmetric CD-stretch excitation. Comparison is made with earlier nonequilibrium molecular dynamics simulations of VC [Kabadi, V. N.; Rice, B. M. Molecular dynamics simulations of normal mode vibrational energy transfer in liquid nitromethane. J. Phys. Chem. A 2004, 108, 532-540]. The simulations do a good job of reproducing the observed VC process and in addition they predicted the slow interconversion among CH-stretch excitations and the slower relaxation of the asymmetric CH-stretch now observed here.
反斯托克斯拉曼散射用于监测在超快红外激发对称或不对称的 CH 或 CD 伸缩跃迁后，环境温度下的液体硝基甲烷（NM-h3）和全氘代硝基甲烷（NM-d3）中的振动能量重新分布。十五种 NM 振动中的大多数的瞬时布居数能够被精确测定，并且使用带有主方程的全局拟合程序来拟合所有数据。泵浦脉冲不仅激发 CH 或 CD 伸缩，还激发弯曲和硝基伸缩基频的某些组合。通过与每个振动基频相关的反斯托克斯瞬态的瞬时上升，揭示了构成初始态的耦合振动。与之前研究的许多其他多原子液体不同，CH 伸缩（或 CD 伸缩）激发之间几乎没有能量交换，这归因于 NM 中甲基几乎自由的旋转。振动冷却过程，即多步回到热平衡态的过程，在 NM-h3 和 NM-d3 中均分三个阶段发生。在第一阶段，母体 CH 或 CD 伸缩在几皮秒内衰减，激发所有较低能量的振动。在第二阶段，中间范围的振动在 10 - 15 皮秒内衰减，激发较低能量的振动。在第三阶段，这些较低能量的振动在几十皮秒内衰减到溶剂中。在 NM-h3 中，初始激发在大约 150 皮秒内达到热平衡，并且对激发哪个 CH 伸缩几乎没有依赖性。在 NM-d3 中振动冷却稍快一些，并且对初始 CD 伸缩的依赖性更大，对称 CD 伸缩激发时大约需要 100 皮秒，不对称 CD 伸缩激发时大约需要 120 皮秒。与早期关于振动冷却的非平衡分子动力学模拟[Kabadi, V. N.; Rice, B. M. Molecular dynamics simulations of normal mode vibrational energy transfer in liquid nitromethane. J. Phys. Chem. A 2004, 108, 532 - 540]进行了比较。这些模拟在重现观察到的振动冷却过程方面做得很好，此外，它们还预测了 CH 伸缩激发之间缓慢的相互转换以及现在在此处观察到的不对称 CH 伸缩较慢的弛豫。
