Laboratoire de Chimie Physique, UMR 8000, CNRS et Université Paris-Sud 11, Bât. 350, Centre Universitaire Paris-Sud, F-91405 Orsay, France.
J Phys Chem A. 2010 Apr 15;114(14):4818-30. doi: 10.1021/jp909422q.
Threshold photoelectron spectra (TPES) of the isotopomers of the methyl radical (CH(3), CH(2)D, CHD(2), and CD(3)) have been recorded in the 9.5-10.5 eV VUV photon energy range using third generation synchrotron radiation to investigate the vibrational spectroscopy of the corresponding cations at a 7-11 meV resolution. A threshold photoelectron-photoion coincidence (TPEPICO) spectrometer based on velocity map imaging and Wiley-McLaren time-of-flight has been used to simultaneously record the TPES of several radical species produced in a Ar-seeded beam by dc flash-pyrolysis of nitromethane (CH(x)D(y)NO(2), x + y = 3). Vibrational bands belonging to the symmetric stretching and out-of-plane bending modes have been observed and P, Q, and R branches have been identified in the analysis of the rotational profiles. Vibrational configuration interaction (VCI), in conjunction with near-equilibrium potential energy surfaces calculated by the explicitly correlated coupled cluster method CCSD(T*)-F12a, is used to calculate vibrational frequencies for the four radical isotopomers and the corresponding cations. Agreement with data from high-resolution IR spectroscopy is very good and a large number of predictions is made. In particular, the calculated wavenumbers for the out-of-plane bending vibrations, nu(2)(CH(3)(+)) = 1404 cm(-1), nu(4)(CH(2)D(+)) = 1308 cm(-1), nu(4)(CHD(2)(+)) = 1205 cm(-1), and nu(2)(CD(3)(+)) = 1090 cm(-1), should be accurate to ca. 2 cm(-1). Additionally, computed Franck-Condon factors are used to estimate the importance of autoionization relative to direct ionization. The chosen models globally account for the observed transitions, but in contrast to PES spectroscopy, evidence for rotational and vibrational autoionization is found. It is shown that state-selected methyl cations can be produced by TPEPICO spectroscopy for ion-molecule reaction studies, which are very important for the understanding of the planetary ionosphere chemistry.
已在 9.5-10.5 eV VUV 光子能量范围内使用第三代同步加速器辐射记录了甲基自由基（CH(3)、CH(2)D、CHD(2)和 CD(3)）的同位素的阈上光电电子能谱（TPES），以在 7-11 meV 的分辨率下研究相应阳离子的振动光谱。基于速度映射成像和 Wiley-McLaren 飞行时间的阈上光电电子-光电离符合（TPEPICO）光谱仪已被用于同时记录由直流闪光照相分解硝基甲烷（CH(x)D(y)NO(2)，x + y = 3）在 Ar 种子射流中产生的几种自由基的 TPES。在分析旋转轮廓时，观察到属于对称伸缩和面外弯曲模式的振动带，并识别了 P、Q 和 R 支。振动构型相互作用（VCI）与通过显式相关耦合簇方法 CCSD(T*)-F12a 计算的近平衡势能面相结合，用于计算四个自由基同位素和相应阳离子的振动频率。与高分辨率红外光谱数据非常吻合，并且进行了大量预测。特别是，计算出的面外弯曲振动的波数，nu(2)(CH(3)(+)) = 1404 cm(-1)、nu(4)(CH(2)D(+)) = 1308 cm(-1)、nu(4)(CHD(2)(+)) = 1205 cm(-1) 和 nu(2)(CD(3)(+)) = 1090 cm(-1)，应该准确到约 2 cm(-1)。此外，计算的 Franck-Condon 因子用于估计自动电离相对于直接电离的重要性。所选模型总体上解释了观察到的跃迁，但与 PES 光谱学相反，发现了旋转和振动自动电离的证据。结果表明，可以通过 TPEPICO 光谱学产生用于离子-分子反应研究的态选择甲基阳离子，这对于理解行星电离层化学非常重要。
