Ultrafast Dynamics of Strong-Field Dissociative Ionization of CH2Br2 Probed by Femtosecond Soft X-Ray Transient Absorption Spectroscopy

摘要

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 CH2Br2 induced by 800 nm strong- field irradiation. At moderate peak intensities (2.0 x 1014 W/cm2), strong- field ionization is accompanied by ultrafast C-Br bond dissociation, producing both neutral Br (2P3/2) and Br* (2P1/2) atoms together with the CH2Br+ fragment ion. The measured rise times for Br and Br* are 130 x 22 fs and 74 x 10 fs, respectively. The atomic bromine quantum state distribution shows that the Br/Br* population ratio is 8.1 x 3.8 and that the Br 2P3/2 state is not aligned. The observed product distribution and the timescales of the photofragment appearances suggest that multiple field-dressed potential energy surfaces are involved in the dissociative ionization process. In addition, the transient absorption spectrum of CH2Br2 + suggests that the alignment of the molecule relative to the polarization axis of the strong-field ionizing pulse determines the electronic symmetry of the resulting ion; alignment of the Br-Br, H-H, and C2 axis of the molecule along the polarization axis results in the production of the ion.