近日，美国加州大学的Stephen R. Leone教授及其领导的研究小组取得一项新进展，他们利用X射线能谱研究了甲烷阳离子的飞秒对称性破缺和相干弛豫。相关成果已于2023年5月4日在国际权威学术期刊《科学》上发表。

在这项研究中，研究人员对甲烷阳离子的几何弛豫（即Jahn-Teller扭曲）进行了超快分子对称性破缺的时间分辨研究。他们利用碳K边缘的软X射线进行了阿秒瞬态吸收光谱实验，发现在几飞秒的强场电离甲烷后的10±2飞秒内发生了扭曲。这种扭曲激发了对称性破缺阳离子的不对称剪切振动模式中的相干振荡，并在X射线信号中被检测到。

然而，这些振荡在58±13飞秒内被阻尼，因为振动相干性丧失并且能量重新分配到低频振动模式中。该研究对这个原型示例的分子弛豫动力学进行了完整的重构，并开辟了探索复杂系统的新途径。

据悉，深入理解光激发分子的弛豫途径对于获得光化学方面的原子级见解至关重要。

附：英文原文

Title: Femtosecond symmetry breaking and coherent relaxation of methane cations via x-ray spectroscopy

Author: Enrico Ridente, Diptarka Hait, Eric A. Haugen, Andrew D. Ross, Daniel M. Neumark, Martin Head-Gordon, Stephen R. Leone

Issue&Volume: 2023-05-04

Abstract: Understanding the relaxation pathways of photoexcited molecules is essential to gain atomistic level insight into photochemistry. Herein, we performed a time-resolved study of ultrafast molecular symmetry breaking via geometric relaxation (Jahn-Teller distortion) on the methane cation. Attosecond transient absorption spectroscopy with soft X-rays at the carbon K-edge revealed that the distortion occurred within 10 ± 2 femtoseconds after few-femtosecond strong-field ionization of methane. The distortion activated coherent oscillations in the asymmetric scissoring vibrational mode of the symmetry broken cation, which were detected in the X-ray signal. These oscillations were damped within 58 ± 13 femtoseconds, as vibrational coherence was lost with the energy redistributing into lower-frequency vibrational modes. This study completely reconstructs the molecular relaxation dynamics of this prototypical example and opens new avenues for exploring complex systems.

DOI: adg4421

Source: https://www.science.org/doi/10.1126/science.adg4421