Ultrafast dynamics of strong-field dissociative ionization of CH2Br2 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 CH2Br2 induced by 800 nm strong-field irradiation. At moderate peak intensities (2.0 × 10 W/cm), strong-field ionization is accompanied by ultrafast C–Br bond dissociation, producing both neutral Br (P3/2) and Br* (P1/2) atoms together with the CH2Br 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 P3/2 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 ( ) 2 2 ~ B X , ( ) 1 2 ~ B B and ( ) 1 2 ~ A C states, respectively. At higher peak intensities (6.2 × 10 W/cm), CH2Br2 undergoes sequential ionization to form the metastable CH2Br2 dication. These results demonstrate the potential of core-level probing with high-order harmonic transient absorption spectroscopy for studying ultrafast molecular dynamics. ∗ Corresponding author. Electronic mail: [email protected]