[论文解读] Time-resolved chiral X-Ray photoelectron spectroscopy with transiently enhanced atomic site-selectivity: a Free Electron Laser investigation of electronically excited fenchone enantiomers

D. Faccialà, M. Devetta+29 more|arXiv (Cornell University)|Feb 28, 2022

Advanced Chemical Physics Studies被引用 9

一句话总结

本研究利用自由电子激光实现了时间分辨的圆二色性X射线光电子能谱，以探测光激发fenchone对映体的飞秒动力学。通过利用瞬态增强的位点选择性核心能级光电子发射，实现了对圆二色性电子结构演化的化学与位点特异性检测，观察到3s反常态激发后PECD不对称性增加了5.81%，证实了激发态动力学中增强的圆二色性敏感性。

ABSTRACT

Chiral molecules are widespread in nature, playing a fundamental role in bio-chemical processes and in the origin of life itself. The observation of dynamics in chiral molecules is crucial for the understanding and control of the chiral activity of photo-excited states. One of the most promising techniques for the study of photo-excited chiral systems is time-resolved photoelectron circular dichroism (TR-PECD), which offers an intense and sensitive probe for vibronic and geometric molecular structure as well as electronic structures, and their evolution on a femtosecond timescale. However, the non-local character of the PECD effect, which is imprinted during the electron scattering off the molecule, makes the interpretation of TR-PECD experiments challenging. In this respect, core-photoionization is known to allow site- and chemical-sensitivity to photelectron spectroscopy. Here we demonstrate that TR-PECD utilising core-level photoemission enables probing the chiral electronic structure and its relaxation dynamics with atomic site sensitivity. Following UV pumped excitation to a 3s Rydberg state, fenchone enantiomers (C 10 H 16 O) were probed on a femtosecond scale using circularly polarized soft X-ray light pulses provided by the free-electron laser FERMI. C 1s binding energy shifts caused by the redistribution of valence electron density in this 3s-valence-Rydberg excitation allowed us to measure transient PECD chiral responses with an enhanced C-atom site-selectivity compared to that achievable in the ground state molecule. These results represent the first chemical-specific and site-specific, enantio-sensitive observations on the electronic structure of a photo-excited chiral molecule and pave the way towards chiral femtochemistry probed by core-level photoemission.

研究动机与目标

• 实现对光激发分子中圆二色性电子结构的原子位点选择性探测。
• 通过利用核心能级光电子发射以增强位点敏感性，克服TR-PECD的非局域性问题。
• 在飞秒时间尺度上研究fenchone在3s反常态激发下圆二色性电子结构的演化。
• 建立一种基于核心能级光谱学的化学特异性、对映体敏感的激发态动力学检测方法。

提出的方法

• 利用自由电子激光（FERMI）产生圆偏振软X射线脉冲，实现时间分辨的核心能级光电子能谱。
• 采用50 fs时间分辨率的紫外光泵浦/X射线探针实验，将fenchone激发至3s反常态，并探测其电子响应。
• 应用时间依赖的PES与PECD形式化：PES(E,t) = (1−r(t))PES^g.s.(E) + r(t)PES^3s(E)，其中r(t) = r₀f(t)且f(t) = exp(−t/τ)，τ = 3.3 ps。
• 使用TDDFT与CMS-Xα方法计算结合能与截面，对C1、C2与C3原子施加弛豫局域化约束。
• 将时间依赖的PECD建模为加权平均：PECD(E,t) = [ (1−r(t))PECD^g.s.PES^g.s. + r(t)PECD^3s.PES^3s ] / [ (1−r(t))PES^g.s. + r(t)PES^3s ]
• 聚焦于仅C1（基态）与C2/C3（激发态）显著贡献的能区E₂，实现可靠的理论比较。

实验结果

研究问题

• RQ1 核心能级光电子发射是否能增强时间分辨圆二色性X射线光电子能谱中的原子位点选择性？
• RQ2 光激发手性分子的弛豫过程中，圆二色性光电子圆二色性（PECD）响应如何演化？
• RQ3 特定碳原子（C1、C2、C3）在fenchone 3s反常激发态下的瞬态PECD信号中贡献如何？
• RQ4 3s-反常态中价电子密度的重分布与基态相比，对圆二色性响应的改变程度如何？
• RQ5 采用弛豫局域化约束的理论建模是否能准确预测复杂手性分子的时间依赖PECD？

主要发现

• 实验首次通过核心能级光电子发射，实现了对光激发手性分子中圆二色性电子结构动力学的化学与位点特异性观测。
• 估算在紫外激发后，fenchone中约12.5%的分子处于3s反常态，其衰减时间为3.3 ps。
• 在最大激发概率时刻（t ≈ 50 fs），E₂能区计算得到的PECD达到−11.47%，相比基态（−17.3%）不对称性增加了5.81%。
• 采用弛豫局域化约束的理论建模预测c₃³ˢ/c₁ᵍ.ˢ比值为1.67，c₂³ˢ/c₁ᵍ.ˢ比值为1.08，支持对位点特异性贡献的实验指认。
• 计算得出3s态中C3的PECD不对称性为+10.2%，表明该位点在激发后表现出显著的圆二色性响应。
• 实验与理论在E₂能区（t ≈ 50 fs时PECD为−11.47%）的PECD结果高度一致，验证了将核心能级光谱与时间分辨TR-PECD结合用于手性动力学研究的方法的有效性。