频率分辨超快电子衍射：在频域和实空间中可视化振动动力学

中文摘要 AI 辅助

超快电子衍射（UED）可直接获取光激发后分子结构变化的信息，但要识别出对该动力学有贡献的单个振动运动仍颇具挑战。本文提出频率分辨UED技术，通过对随时间变化的差分布函数（ΔPDF）进行傅里叶变换，得到结构动力学的二维频率-距离表征。我们将该方法应用于丙二烯（allene）和1,2-丁二烯，二者在200 nm波长下被光激发至S₁(ππ*)态，使用的数据来自已发表的从头算多产生产物轨迹（S. P. Neville等人，《化学物理杂志》，2016年，第144卷，014305）模拟的电子散射信号。我们识别出C=C伸缩振动、CCC弯曲振动及其作用的核间距，并分离出重叠的CH₂振动运动。通过改变傅里叶变换所用的泵浦-探测延迟范围，可确定特定频率分量在激发态动力学过程中的贡献时机。甲基取代会降低C=C伸缩振动和CCC弯曲振动的频率，并在1,2-丁二烯的前90 fs内引入额外的频率分量。我们进一步证明，亚20 fs、最终为几飞秒的时间分辨率对获取这些频率分量至关重要。因此，频率分辨UED可提供与光诱导结构动力学相关的振动频率、核间距及反应时间。

英文摘要

Ultrafast electron diffraction (UED) provides direct information on changes in molecular structure following photoexcitation, but identifying the individual vibrational motions contributing to these dynamics remains challenging. Here, we introduce frequency-resolved UED, where Fourier transformation of the time-dependent difference pair distribution function ($Δ$PDF) gives a two-dimensional frequency-distance representation of the structural dynamics. We apply this approach to allene and 1,2-butadiene following photoexcitation to the $S_1(ππ^*)$ state at 200 nm, using electron scattering signals simulated from previously-published ab initio multiple spawning trajectories (S. P. Neville et al., J. Chem. Phys., 2016, 144, 014305). We identify C=C stretching and CCC bending motions and the internuclear distances over which they contribute, and separate overlapping CH$_2$ vibrational motions. By changing the pump-probe delay range used for the Fourier transform, we identify when specific frequency components contribute during the excited-state dynamics. Methyl substitution reduces the C=C stretching and CCC bending frequencies and introduces an additional frequency component in 1,2-butadiene during the first 90 fs. We further show the importance of sub-20-fs, and ultimately few-femtosecond, temporal resolution for retrieving these frequency components. Frequency-resolved UED therefore provides the vibrational frequencies, internuclear distances, and reaction times associated with photoinduced structural dynamics.

发表机构

• Max-Born-Institut（马克斯·玻恩研究所）
• National Research Council Canada（加拿大国家研究委员会）
• University of Ottawa（渥太华大学）

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