Toulson Benjamin W, Borgwardt Mario, Wang Han, Lackner Florian, Chatterley Adam S, Pemmaraju C D, Neumark Daniel M, Leone Stephen R, Prendergast David, Gessner Oliver
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Stanford, California 94025, USA.
Struct Dyn. 2019 Oct 11;6(5):054304. doi: 10.1063/1.5113798. eCollection 2019 Sep.
UV pump-extreme UV (XUV) probe femtosecond transient absorption spectroscopy is used to study the 268 nm induced photodissociation dynamics of bromoform (CHBr). Core-to-valence transitions at the Br(3) 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 CHBr products. Complementary 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 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 symmetry to form a quasiplanar CHBr 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 + CHBr 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.
紫外泵浦-极紫外（XUV）探测飞秒瞬态吸收光谱用于研究268纳米光诱导的溴仿（CHBr₃）光解离动力学。在溴（Br）的（3）吸收边（约70电子伏特）处的芯到价跃迁提供了该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光谱追踪了瞬态电子态的单占据分子轨道在整个C-Br键断裂过程中的发展情况，最终形成自由基Br和CHBr₂产物。对从激发态分子动力学模拟采样得到的瞬态原子排列进行了XUV光谱指纹的补充计算。沿着近似对称的反应路径进行的C-Br断裂导致电子轨道特征和原子排列的持续变化。两个时间尺度主导了瞬态吸收光谱的变化，反映了轻的C和H原子以及重的Br原子的不同特征运动。在最初的40飞秒内，由于（轻的）CH部分的位移使对称性发生畸变形成准平面CHBr₂，导致价电子结构发生显著变化。（重的）Br原子的位移被延迟，需要长达约300飞秒才能形成单独的Br + CHBr₂产物。我们证明，由XUV吸收产生的价激发（初始）和价+芯激发（最终）态电子构型之间的跃迁对价轨道在键断裂过程中的局域化敏感。价电子-芯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。
