Low-energy peak structure in strong-field ionization by midinfrared laser pulses: Two-dimensional focusing by the atomic potential

Abstract

We analyze the formation of the low-energy structure (LES) in above-threshold ionization spectra first observed by Quan et al. [1] and Blaga et al.[2] using both quasi-classical and quantum approaches. We show this structure to be largely classical in origin resulting from a two-dimensional focusing in the energy-angular momentum plane of the strong-field dynamics in the presence of the atomic potential. The peak at low energy is strongly correlated with high angular momenta of the photoelectron. Quantum simulations confirm this scenario. Resulting parameter dependences agree with experimental findings [1,2] and, in part, with other simulations [3][4][5].

Universal pulse dependence of the low-energy structure in strong-field ionization

“…The fact that the CEP-fixed numerical results agree with the experimental ones, which can be interpreted as an average over the CEP due to the Gouy phase shift along the Gaussian laser focus is not obvious, especially considering the pronounced CEP dependence of the LES for short pulses [15,16].…”

“…However, the observation of new structures [10,11] in the electron spectrum at low energies posed a puzzle: reproduced by numerical solutions of the time-dependent Schrödinger equation as well as by classical simulations [11][12][13][14][15][16][17][18], these now called "low" [10] and "very-low" [13] energy structures (LES and VLES) evade the well established three-step model. Early numerical studies [11][12][13] clearly hinted the importance of the Coulomb potential from the parent ion during formation of the low energy structures.…”

“…During soft recollisions on the other hand, the electron revisits its parent ion with its momentum approaching zero. This makes the electron trajectory susceptible to external influences, leading to energy gain or loss from the binding potential which gives rise to a spectral bunching of electron momenta at the LES/VLES position [14,16].…”

“…The LES peak position is predicted to depend on the carrier envelope phase (CEP) [15,16] and the laser pulse duration [15]. To date, experimental verification was lacking since the required tunable pulse duration in the mid-infrared spectral region, a prerequisite for achieving the strong field limit, is technically quite demanding.…”

Low-energy structure in the ionization of argon: Comparison of experiment with theory

“…[10] It is widely believed that the high-energy electron is related to the rescattering (recollision) mechanism, [11] while the low-energy part is believed to originate in direct photo-ionization. To date, little research [12][13][14][15] has been reported on the intensity dependence of the low-energy structure (LES), especially for a detailed comparison of the current state-of-the-art theory and the experimental data.…”