Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, USA.
J Phys Chem B. 2013 Jun 27;117(25):7737-49. doi: 10.1021/jp403726t. Epub 2013 Jun 12.
We present a mixed quantum-classical molecular dynamics study of the nonequilibrium hydrogen-bond dynamics following vibrational energy relaxation of the hydroxyl stretch in a 10 mol % methanol/carbon tetrachloride mixture and pure methanol. The ground and first-excited energy levels and wave functions are identified with the eigenvalues and eigenfunctions of the hydroxyl's adiabatic Hamiltonian and as such depend parametrically on the configuration of the remaining, classically treated, degrees of freedom. The dynamics of the classical degrees of freedom are in turn governed by forces obtained by taking the expectation value of the force with respect to the ground or excited vibrational wave functions. Polarizable force fields and nonlinear mapping relations between the hydroxyl transition frequencies and dipole moments and the electric field along the hydroxyl bond are used, which were previously shown to quantitatively reproduce the experimental infrared steady-state absorption spectra and excited state lifetime [Kwac, K.; Geva, E. J. Phys. Chem. B 2011, 115, 9184; 2012, 116, 2856]. The relaxation from the first-excited state to the ground state is treated as a nonadiabatic transition. Within the mixed quantum-classical treatment, relaxation from the excited state to the ground state is accompanied by a momentum-jump in the classical degrees of freedom, which is in turn dictated by the nonadiabatic coupling vector. We find that the momentum jump leads to breaking of hydrogen bonds involving the relaxing hydroxyl, thereby blue-shifting the transition frequency by more than the Stokes shift between the steady-state emission and absorption spectra. The subsequent nonequilibrium relaxation toward equilibrium on the ground state potential energy surface is thereby accompanied by red shifting of the transition frequency. The signature of this nonequilibrium relaxation process on the pump-probe spectrum is analyzed in detail. The calculated pump-probe spectrum is found to be in reasonable agreement with experiment, thereby providing further credibility to the underlying force fields and mixed quantum-classical methodology.
我们呈现了一项混合量子经典分子动力学研究，研究对象是在 10mol%甲醇/四氯化碳混合物和纯甲醇中，羟基伸缩振动的振动能弛豫后非平衡氢键动力学。基态和第一激发态能级和波函数是通过羟基绝热哈密顿量的本征值和本征函数确定的，因此参数依赖于剩余经典处理自由度的构型。经典自由度的动力学反过来又由通过取力相对于基态或激发振动波函数的期望值获得的力来控制。使用了极化力场和羟基跃迁频率与偶极矩和沿羟基键的电场之间的非线性映射关系，这些先前被证明可以定量再现实验红外稳态吸收光谱和激发态寿命[Kwac，K.；Geva，E. J. Phys. Chem. B 2011，115，9184；2012，116，2856]。从第一激发态到基态的弛豫被视为非绝热跃迁。在混合量子经典处理中，从激发态到基态的弛豫伴随着经典自由度的动量跳跃，这反过来又由非绝热耦合向量决定。我们发现，动量跳跃导致涉及弛豫羟基的氢键断裂，从而使跃迁频率蓝移超过稳态发射和吸收光谱之间的斯托克斯位移。随后在基态势能表面上向平衡的非平衡弛豫因此伴随着跃迁频率的红移。详细分析了泵浦探测光谱中这种非平衡弛豫过程的特征。计算出的泵浦探测光谱与实验结果吻合良好，从而为基础力场和混合量子经典方法提供了进一步的可信度。
