Loh Zhi-Heng, Leone Stephen R
Departments of Chemistry and Physics, University of California, Berkeley, California 94720, USA.
J Chem Phys. 2008 May 28;128(20):204302. doi: 10.1063/1.2925268.
Femtosecond time-resolved soft x-ray transient absorption spectroscopy based on a high-order harmonic generation source is used to investigate the dissociative ionization of CH(2)Br(2) induced by 800 nm strong-field irradiation. At moderate laser peak intensities (2.0 x 10(14) Wcm(2)), strong-field ionization is accompanied by ultrafast C-Br bond dissociation, producing both neutral Br ((2)P(32)) and Br() ((2)P(12)) atoms together with the CH(2)Br(+) fragment ion. The measured rise times for Br and Br() are 130+/-22 fs and 74+/-10 fs, respectively. The atomic bromine quantum state distribution shows that the BrBr(*) population ratio is 8.1+/-3.8 and that the Br (2)P(32) state is not aligned. The observed product distribution and the time scales of the photofragment appearances suggest that multiple field-dressed potential energy surfaces are involved in the dissociative ionization process. At higher laser peak intensities (6.2 x 10(14) Wcm(2)), CH(2)Br(2) (+) undergoes sequential ionization to form the metastable CH(2)Br(2) (2+) dication. These results demonstrate the potential of core-level probing with high-order harmonic transient absorption spectroscopy for studying ultrafast molecular dynamics.
基于高次谐波产生源的飞秒时间分辨软X射线瞬态吸收光谱被用于研究800nm强场辐照诱导的CH₂Br₂的解离电离。在中等激光峰值强度（2.0×10¹⁴Wcm⁻²）下，强场电离伴随着超快的C-Br键解离，产生中性Br（²P₃/₂）和Br*（²P₁/₂）原子以及CH₂Br⁺碎片离子。测得的Br和Br的上升时间分别为130±22fs和74±10fs。原子溴量子态分布表明，Br/Br的布居比为8.1±3.8，且Br的²P₃/₂态未取向。观察到的产物分布和光碎片出现的时间尺度表明，多个场修饰势能面参与了解离电离过程。在更高的激光峰值强度（6.2×10¹⁴Wcm⁻²）下，CH₂Br₂⁺经历顺序电离形成亚稳的CH₂Br₂²⁺双离子。这些结果证明了用高次谐波瞬态吸收光谱进行芯能级探测在研究超快分子动力学方面的潜力。
