Multiphoton above-threshold ionization in superintense free-electron x-ray laser fields: Beyond the dipole approximation
Zhongyuan Zhou and Shih-I Chu
Accepted
We present an accurate and efficient nondipole computational approach in momentum space for the nonperturbative study of multiphoton above-threshold ionization (ATI) of atoms in superintense and ultrashort-wavelength laser fields. This approach has been successfully used to investigate the multiphoton processes of a hydrogen atom exposed to superintense free electron X-ray laser fields. We find that compared to dipole approximation the nondipole ATI spectra are enhanced substantially in high energy regime and the photoelectron angular distributions (PADs) are distorted significantly in higher-intensity and/or longer-pulse laser fields, in particular two lobes are induced along and against the laser propagation direction. The origin of these phenomena has been explored in details.