Photoelectron imaging of resonance-enhanced multiphoton ionization and above-threshold ionization of ammonia molecules in a strong 800-nm laser pulse

In this work, we mainly investigate the NH₃ molecular multiphoton ionization process by using the photoelectron velocity map imaging technique. Under the condition of femtosecond laser (wavelength at 800 nm), the photoelectron images are detected. The channel switching and above-threshold ionization (ATI) effect are also confirmed. The kinetic energy spectrum (KES) and the photoelectron angular distributions (PADs) are obtained through the anti-Abel transformation from the original images, and then three ionization channels are confirmed successfully according to the Freeman resonance effect in a relatively low laser intensity region. In the excitation process, the intermediate resonance Rydberg states are $C^{\sim }{}^1{A}_1^{{}^{\prime }}$ (6 + 2 photons process), $B^{\sim }{}^1{E}^{{}^{\prime \prime }}$ (6 + 2 photons process) and $C^{\sim }{}^1{A}_1^{{}^{\prime }}$ (7 + 2 photons process), respectively. At the same time, we also find that the photoelectron angular distributions are independent of laser intensity. In addition, the electrons produced by different processes interfere with each other and they can produce a spider-like structure. We also find ac-Stark movement according to the Stark-shift-induced resonance effect when the laser intensity is relatively high. (paper)