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Optics Young Scientist Summit (OYSS)

The intensity dependence of Rydberg state excitation in the strong field ionization of hydrogen atoms by two-color laser fields

Shijia Liu, Zhaoyan Zhou

Published Dec 29, 2023• 0 citations
Publisher

Abstract

To study the physical mechanism of atomic Rydberg state excitation (RSE) in two-color laser fields, we solve the time-dependent Schrödinger equation (TDSE) to calculate the ionization and excitation of hydrogen atoms. The relationship between the excited state population and the excitation rate of hydrogen atoms and the intensity of the fundamental frequency (FW) field can be observed by adjusting the laser intensity of the second harmonic (SH) field. The results demonstrate that multiphoton resonance absorption aligns with the physical mechanism of atomic resonant state excitation in two-color laser fields.

DOI
10.1117/12.3015672

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To study the physical mechanism of atomic Rydberg state excitation (RSE) in two-color laser fields, we solve the time-dependent Schrödinger equation (TDSE) to calculate the ionization and excitation of hydrogen atoms. The relationship between the excited state population and the excitation rate of hydrogen atoms and the intensity of the fundamental frequency (FW) field can be observed by adjusting the laser intensity of the second harmonic (SH) field. The results demonstrate that multiphoton resonance absorption aligns with the physical mechanism of atomic resonant state excitation in two-color laser fields.
This paper studies the multiphoton resonant ionization by two-colour laser pulses in the hydrogen atom by solving the time-dependent Schrodinger equation. By fixing the parameters of fundamental laser field and scanning the frequency of second laser field, it finds that the ionization probability shows several resonance peaks and is also much larger than the linear superposition of probabilities by applying two lasers separately. The enhancement of the ionization happens when the system is resonantly pumped to the excited states by absorbing two or more colour photons non-sequentially.
The process of resonant multiphoton ionization of a hydrogen atom in the ground 1 s state is studied by direct numerical integration of the nonstationary Schrödinger equation for a quantum system in an electromagnetic field. The dependence of photoionization probability on the radiation intensity is found to be nonmonotonic. It is established that the minima of ionization correspond to multiphoton resonances between the ground state and one of the excited (Rydberg) atomic states perturbed by the laser field. It is shown that ionization is suppressed due to rearrangement of Rydberg states in a strong electromagnetic field and is accompanied by efficient Raman Λ transitions, which connect a set of closely lying Rydberg states via the continuum.
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The dynamics of hydrogen atom in the presence of a strong radiation field of the titanium-sapphire laser is studied for the Keldysh parameters γ ≥ 1 and γ ≤ 1. It is demonstrated that the ionization is supplemented with the effective population of the excited states with the principal quantum numbers n = 5–10 in the entire range of variation in the Keldysh parameter. The population of the excited Rydberg states can be interpreted as a consequence of the multiphoton resonance involving the initial 1 s state and a group of excited states in the vicinity of the continuum boundary with the simultaneous repopulation of these states by Λ-type Raman transitions under the action of the laser field. The resulting coherent Rydberg packet appears to be stable with respect to ionization, so that the ionization of the atomic system in the presence of strong electromagnetic field is suppressed. Physical reasons for the stabilization are discussed. An interpretation of the effective population of the Rydberg states in the recent experiments on the ionization of atomic helium by the titanium-sapphire laser is proposed.
A nonperturbative method is used to study the two-color multiphoton ionization of atoms by solving the Schr\"odinger equation in momentum space, which facilitates the extraction of the rapidly varying part of the wave function. Our method generalizes the problem for cases where an arbitrary number of ionization photons are absorbed, and also takes into account the threshold shifts due to both the high- and low-frequency fields. The photoelectron energy spectra of the hydrogen atoms are calculated and the dependence on the laser field strengths and frequencies is discussed. \textcopyright{} 1996 The American Physical Society.