Zhou, ZhongyuanChu, Shih-I2014-12-012014-12-012013-02-13Zhou, Zhongyuan; Chu, Shih-I. (2013). "Multiphoton above-threshold ionization in superintense free-electron x-ray laser fields: Beyond the dipole approximation." Physical Review A, 87(2):023407. http://dx.doi.org/10.1103/PhysRevA.87.023407.1050-2947https://hdl.handle.net/1808/15979This is the publisher's version, also available electronically from http://journals.aps.org/pra/abstract/10.1103/PhysRevA.87.023407.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 results of the dipole approximation, the nondipole ATI spectra are enhanced substantially in the high-energy regime, and the photoelectron angular distributions are distorted significantly in higher-intensity and/or longer-pulse laser fields; in particular two lobes are induced, one along and one against the laser propagation direction. The origin of these phenomena has been explored in detail.Article10.1103/PhysRevA.87.023407openAccess