When an atom or molecule interacts with an intense laser pulse, the outmost electron is first tunnel ionized, accelerated in the oscillating electric field of the laser, and may be driven back to re-collide with its parent ion. This re-colliding process, being the core ingredient of the rescattering scenario, has initiated many interesting laser-induced phenomena: if the re-colliding electron recombines with its parent ion, it emits high harmonic photon; if the electron re-collides elastically with the ion, it can absorb more energy from the laser field, and presents the high-order above threshold ionization (HATI) process; if the electron re-collides in-elastically with its parent ion, the other electrons in the ion may be ionized by this collision, and the nonsequential double (multiple) ionization happens. Recently, Associate professor Bingbing Wang and Professor Panming Fu from Institute of Physics, Chinese Academy of Sciences (CAS)/Beijing National Laboratory for Condensed Matter Physics developed the non-perturbation quantum electrodynamics (QED) method to study the re-collision process in strong laser field in collaboration with their coworkers. They successfully extended the QED theory to deal with high harmonic generation and HATI. Especially, they established the relationship between the frequency-domain theory and the time-domain theory in strong-field physics. Furthermore, they demonstrated that an angle-resolved HATI spectrum carries three pieces of important information: the fingerprint of the molecular wave function in the direct above-threshold-ionization amplitude, the geometrical structure of the molecule in the potential scattering between two plane waves, and the interaction between the ionized electron and the laser field, manifested in a phase factor associated with laser-assisted collisions.
  Recently, Associate professor B. Wang cooperated with Prof. Liu’s group of Wuhan Institute of Physics and Mathematics (CAS), Prof. Jing Chen and Prof. Jie Liu’s group of Institute of Applied Physics and Computational Mathematics in Beijing, and Prof. Ya Cheng of Shanghai Institute of Optics and Fine Mechanics (CAS), to study the energy-resolved angular distributions of photoelectrons generated in above-threshold ionization (ATI) of nonaligned diatomic molecules in high-intensity short laser pulses. Note that the role of molecular orbital structure in ionization behavior of nonaligned molecules has been extensively addressed during the last decade. However, most studies were focused on the total ion yield or the low-energy electron part. Recent theoretical study has been performed on angular distribution (AD) of high-energy electrons for aligned molecules and distinct electron emission patterns have been revealed for N2 and O2 due to the different structure of their ground state molecular orbits. On the other side, experiments have been attempted along this direction, however, no clear evidence has been found to be related to the molecular orbital structure effect.
  They present angle-resolved photoelectron energy spectra of nonaligned diatomic molecules N2 and O2, analyzing the effect of molecular orbital structure on high-energy photoelectron emission in strong field molecular ionization. The angular distribution for photoelectrons with energy of 10Up (Up: ponderomotive energy), i.e., the plateau cutoff in ATI spectra, is found to be broader in O2 than in N2. Resorting to the analyses from both an S-matrix theory and an intuitive semiclassical model, we attribute the observation to the effect of the ground state molecular orbital structure on high-energy electron emission in strong field molecular ionization. The result has been published on Phys. Rev. Lett. [PRL 104, 203001] in May 2010.
  This work is supported by NNSF of China (No. 10674153), the National Basic Research Program of China (No. 2006CB806000), and the CAEP Foundation (Nos. 2006z0202 and 2008B0102007).