Garcia Gustavo A, Dossmann Héloïse, Nahon Laurent, Daly Steven, Powis Ivan
Synchrotron SOLEIL, L'Orme des Merisiers, St. Aubin BP 48, 91192, Gif sur Yvette, France.
Current address: Sorbonne Universités, UPMC Univ Paris 06, CNRS, Institut Parisien de Chimie Moléculaire (IPCM), 4 Place Jussieu, 75252, Paris Cedex 05, France.
Chemphyschem. 2017 Mar 3;18(5):500-512. doi: 10.1002/cphc.201601250. Epub 2017 Jan 23.
Electron-ion coincidence imaging is used to study chiral asymmetry in the angular distribution of electrons emitted from randomly-oriented enantiomers of two molecules, methyloxirane and trifluoromethyloxirane, upon ionization by circularly polarized VUV synchrotron radiation. Vibrationally-resolved photoelectron circular dichroism (PECD) measurements of the outermost orbital ionization reveal unanticipated large fluctuations in the magnitude of the forward-backward electron scattering asymmetry, including even a complete reversal of direction. Identification and assignment of the vibrational excitations is supported by Franck-Condon simulations of the photoelectron spectra. A previously proposed quasi-diatomic model for PECD is developed and extended to treat polyatomic systems. The parametric dependence of the electronic dipole matrix elements on nuclear geometry is evaluated in the adiabatic approximation. It provokes vibrational level dependent shifts in amplitude and phase, to which the chiral photoelectron angular distributions are especially sensitive. It is shown that single quantum excitation of those vibrational modes, which experience only a relatively small displacement of the ion equilibrium geometry along the normal coordinate and which are then only weakly excited in the Franck-Condon limit, can be accompanied by big shifts in scattering phase; hence the observed big fluctuations in PECD asymmetry for such modes.
电子 - 离子符合成像用于研究两种分子（甲基环氧乙烷和三氟甲基环氧乙烷）的随机取向对映体在圆偏振真空紫外同步辐射电离后发射电子的角分布中的手性不对称性。对最外层轨道电离的振动分辨光电子圆二色性（PECD）测量揭示了前后电子散射不对称性大小中意想不到的大幅波动，甚至包括方向的完全反转。光电子能谱的弗兰克 - 康登模拟支持了振动激发的识别和归属。先前提出的用于PECD的准双原子模型得到了发展并扩展以处理多原子系统。在绝热近似下评估了电子偶极矩矩阵元对核几何结构的参数依赖性。它引发了与振动能级相关的振幅和相位变化，手性光电子角分布对其特别敏感。结果表明，那些沿着法向坐标仅经历离子平衡几何结构相对较小位移且在弗兰克 - 康登极限下仅被微弱激发的振动模式的单量子激发，可能伴随着散射相位的大幅变化；因此观察到此类模式的PECD不对称性存在大幅波动。
