Yandell Margaret A, King Sarah B, Neumark Daniel M
Department of Chemistry, University of California, Berkeley, California 94720, USA.
J Chem Phys. 2014 May 14;140(18):184317. doi: 10.1063/1.4875021.
Decay dynamics of nascent dipole bound states of acetonitrile and nitromethane are examined using time-resolved photoelectron imaging of iodide-acetonitrile (I(-)·CH3CN) and iodide-nitromethane (I(-)·CH3NO2) complexes. Dipole-bound anions are created by UV-initiated electron transfer to the molecule of interest from the associated iodide ion at energies just below the vertical detachment energy of the halide-molecule complex. The acetonitrile anion is observed to decay biexponentially with time constants in the range of 4-900 ps. In contrast, the dipole bound state of nitromethane decays rapidly over 400 fs to form the valence bound anion. The nitromethane valence anion species then decays biexponentially with time constants of 2 ps and 1200 ps. The biexponential decay dynamics in acetonitrile are interpreted as iodine atom loss and autodetachment from the excited dipole-bound anion, followed by slower autodetachment of the relaxed metastable ion, while the dynamics of the nitromethane system suggest that a dipole-bound anion to valence anion transition proceeds via intramolecular vibrational energy redistribution to nitro group modes in the vicinity of the iodine atom.
利用碘化物 - 乙腈（I(-)·CH3CN）和碘化物 - 硝基甲烷（I(-)·CH3NO2）配合物的时间分辨光电子成像技术，研究了乙腈和硝基甲烷新生偶极束缚态的衰变动力学。偶极束缚阴离子是通过紫外引发的电子从相关碘离子转移到目标分子而产生的，其能量略低于卤化物 - 分子配合物的垂直脱附能。观察到乙腈阴离子以4 - 900皮秒范围内的时间常数进行双指数衰变。相比之下，硝基甲烷的偶极束缚态在400飞秒内迅速衰变形成价态束缚阴离子。然后，硝基甲烷价态阴离子物种以2皮秒和1200皮秒的时间常数进行双指数衰变。乙腈中的双指数衰变动力学被解释为碘原子的损失以及从激发的偶极束缚阴离子上发生自脱附，随后是弛豫亚稳离子较慢的自脱附，而硝基甲烷体系的动力学表明，偶极束缚阴离子到价态阴离子的转变是通过分子内振动能量重新分布到碘原子附近的硝基模式来进行的。
