Ultrafast strong-field dissociative ionization dynamics of CH₂ Br₂ probed by femtosecond soft x-ray transient absorption spectroscopy

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 C H2 Br2 induced by 800 nm strong-field irradiation. At moderate laser peak intensities (2.0× 1014 W cm2), strong-field ionization is accompanied by ultrafast C-Br bond dissociation, producing both neutral Br (P 32 2) and Br* (P 12 2) atoms together with the C H2 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 Br Br* population ratio is 8.1±3.8 and that the Br P 32 2 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× 1014 W cm2), C H2 Br2+ undergoes sequential ionization to form the metastable C H2 Br2 2+ dication. These results demonstrate the potential of core-level probing with high-order harmonic transient absorption spectroscopy for studying ultrafast molecular dynamics.