Resonance-like enhancement in high-order above-threshold ionzation of argon at different wavelengths

Abstract

Quantum S-matrix theory and “uniform approximation” method are used to study the resonance-like enhancement (RLE) structures in photoelectron spectrum of high-order above-threshold ionization (HATI) for argon atoms subjected to strong laser fields at different wavelengths. Our results show that both in the near infrared and mid-infrared fields, the RLE structures in the photoelectron spectra will appear, which manifests as a group of adjacent HATI peaks that show a significant enhancement when the laser intensity increases only a few percent. The RLE occurs precisely when the laser intensity satisfies the channel-closing (CC) condition, and this further confirms the explanation of CC mechanism of the RLE. More importantly, we find that with increasing laser wavelength, the resonance-like enhancement and suppression will appear alternately in the photoelectron energy spectrum, and this alternation phenomenon will be more pronounced as the intensity increases. This phenomenon may be attributed to the interference of “quantum orbital” of electrons which collide with the core at different return time. Since in the condition of long wavelength, the alternation phenomenon of the RLE is more pronounced, the RLE is distributed from the low-energy regime to the cutoff-regime in the photoelectron energy spectrum, thus making the RLE broader than that in the case of short wavelength. This may be used to explain the experimentally observed extension of the RLE energy region at longer wavelength. In addition, it is also shown that similar to the case of the near infrared laser fields, two types of RLE structures are also found in strong mid-infrared laser fields, where type-Ⅰ enhancement occurs in the region 5%-10% below even CC for Ar atom whose ground state has an odd parity, and its intensity dependence is comparatively smooth; and type-Ⅱ enhancement appears exactly at the channel closing and has a particularly sharp intensity dependence. And both types of enhancements are due to the constructive interference of a large amount of quantum orbits.