School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh EH9 3FJ, Scotland, United Kingdom.
International Centre for Quantum and Molecular Structures, College of Sciences, Shanghai University, 99 Shangda Road, Shanghai 200444, China.
J Chem Phys. 2017 Aug 21;147(7):074305. doi: 10.1063/1.4998150.
A new synchrotron-based study of the photoelectron spectrum (PES) of difluoromethane is interpreted by an ab initio analysis of the ionic states, which includes Franck-Condon (FC) factors. Double differentiation of the spectrum leads to significant spectral sharpening; the vibrational structure observed is now measured with greater accuracy than in previous studies. Several electronic structure methods are used, including equation of motion coupled cluster calculations with single and double excitations (EOM-CCSD), its ionization potential variant EOM-IP-CCSD, 4th order Møller-Plesset perturbation theory (MP4SDQ) configuration interaction (CI), and complete active space self-consistent-field (CASSCF) methods. The adiabatic ionization energies (AIEs) confirm the assignments as band I, one state 1B (12.671 eV); band II, three states, 1B (14.259) < 1A (15.030) < 1A (15.478 eV); and band III, three states, 2B (18.055) < 2A (18.257) < 2B (18.808 eV). The three ionizations in each of the bands II and III lead to selective line broadening of the PES structure, which is attributed to vibronic overlap. The apparent lack of a vibrational structure attributable to both the 1A and 2A states in the PES arises from line broadening with the preceding states 1B and 2B, respectively. Although these A states clearly overlap with their adjacent higher IE, some vibrational structure is observed on the higher IE. The effects of vibronic coupling are evident since the observed structure does not fit closely with the calculated Born-Oppenheimer FC profiles. Correlation of the lowest group of four AIEs in the PES of other members of the CHX group, where X = F, Cl, Br, and I, clearly indicate these effects are more general.
一项基于同步加速器的二氟甲烷光电子能谱（PES）的新研究通过对离子态的从头分析进行解释，其中包括 Franck-Condon（FC）因子。对光谱进行双重微分导致光谱显著锐化；现在观察到的振动结构比以前的研究更准确地测量。使用了几种电子结构方法，包括单重和双重激发的运动方程耦合簇计算（EOM-CCSD）、其电离势变体 EOM-IP-CCSD、四阶 Møller-Plesset 微扰理论（MP4SDQ）组态相互作用（CI）和完全活性空间自洽场（CASSCF）方法。绝热电离能（AIE）证实了作为带 I 的分配，一个状态 1B（12.671 eV）；带 II，三个状态，1B（14.259）<1A（15.030）<1A（15.478 eV）；和带 III，三个状态，2B（18.055）<2A（18.257）<2B（18.808 eV）。带 II 和 III 中的每个带的三个电离导致 PES 结构的选择性线宽增加，这归因于振子重叠。PES 中 1A 和 2A 态的振动结构明显缺乏归因于前态 1B 和 2B 的线宽增加。尽管这些 A 态明显与它们相邻的较高 IE 重叠，但在较高 IE 上观察到一些振动结构。振子耦合的影响是明显的，因为观察到的结构与计算的 Born-Oppenheimer FC 轮廓不太吻合。在 CHX 基团其他成员的 PES 中，最低的四组 AIE 的相关性，其中 X=F、Cl、Br 和 I，清楚地表明这些影响更为普遍。
