UV pump-extreme UV (XUV) probe femtosecond transient absorption spectroscopy is used to study the 268 nm induced photodissociation dynamics of bromoform (CHBr3). Core-to-valence transitions at the Br(3d) absorption edge (∼70 eV) provide an atomic scale perspective of the reaction, sensitive to changes in the local valence electronic structure, with ultrafast time resolution. The XUV spectra track how the singly occupied molecular orbitals of transient electronic states develop throughout the C-Br bond fission, eventually forming radical Br and CHBr2 products. Complementary ab initio calculations of XUV spectral fingerprints are performed for transient atomic arrangements obtained from sampling excited-state molecular dynamics simulations. C-Br fission along an approximately C S symmetrical reaction pathway leads to a continuous change of electronic orbital characters and atomic arrangements. Two timescales dominate changes in the transient absorption spectra, reflecting the different characteristic motions of the light C and H atoms and the heavy Br atoms. Within the first 40 fs, distortion from C 3 v symmetry to form a quasiplanar CHBr2 by the displacement of the (light) CH moiety causes significant changes to the valence electronic structure. Displacement of the (heavy) Br atoms is delayed and requires up to ∼300 fs to form separate Br + CHBr2 products. We demonstrate that transitions between the valence-excited (initial) and valence + core-excited (final) state electronic configurations produced by XUV absorption are sensitive to the localization of valence orbitals during bond fission. The change in valence electron-core hole interaction provides a physical explanation for spectral shifts during the process of bond cleavage.

中文翻译：

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用核-价瞬态吸收光谱法在溴仿的紫外光化学中探索超快C-Br键裂变。

紫外泵极紫外（XUV）探针飞秒瞬态吸收光谱法用于研究268 nm诱导的溴仿（CHBr3）的光解离动力学。Br（3d）吸收边缘（〜70 eV）的核价转变提供了反应的原子尺度视角，对局部价电子结构的变化敏感，具有超快的时间分辨率。XUV光谱跟踪瞬态电子态的单个占据的分子轨道在整个C-Br键裂变中如何发展，最终形成自由基Br和CHBr2产物。XUV光谱指纹的补充从头算是对通过采样激发态分子动力学模拟获得的瞬态原子排列进行的。沿近似CS对称反应路径的C-Br裂变导致电子轨道特征和原子排列的连续变化。两个时标支配了瞬态吸收光谱的变化，反映了轻C和H原子以及重Br原子的不同特征运动。在最初的40 fs内，由于（轻）CH部分的置换，从C 3 v对称性变形而形成准平面CHBr2导致价电子结构发生重大变化。（重）Br原子的位移被延迟，并且需要约300 fs才能形成单独的Br + CHBr2产物。我们证明了由XUV吸收产生的价态激发态（初始）和价态+核心激发态（最终）态电子构型之间的过渡对键裂变过程中的价态轨道的定位很敏感。价电子-核-空穴相互作用的变化为键断裂过程中的光谱位移提供了物理解释。