Molecular orbital effect in above-threshold ionization of vinyl bromide induced by strong laser fields

Ionization is a fundamental process that may induce a variety of highly nonlinear physical phenomena in strong femtosecond laser fields. Compared to that of atoms, strong-field ionization (SFI) of polyatomic molecules is far from being understood because of their diverse geometric and orbital structures. In this study, we experimentally investigate SFI of the vinyl bromide (C₂H₃Br) molecule in near ultraviolet and near-infrared strong laser fields employing the electron velocity map imaging (VMI) method. The distinct above-threshold ionization (ATI) structure of the molecule is clearly observed in both strong laser fields. The electron kinetic energy as well as the angular distributions is derived from the electron VMI measurements, and their dependence on the laser intensity is investigated and discussed. Analysis reveals that owing to the small energy difference between the low-lying electronic orbitals of the C₂H₃Br molecule, multiple orbital effect plays a pivotal role in the SFI of the molecule. The present study adds our knowledge on SFI and ATI of the C₂H₃Br molecule, and sheds some light on the interaction of polyatomic molecules with ultrafast strong laser fields.