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Physical Review A

Doubly excited electron-ion angular momentum transfer in parity-unfavored multiphoton ionization

Meng Han, Peipei Ge, Yiqi Fang, Xiaoyang Yu, Zhengning Guo, Yongkai Deng, Chengyin Wu, Qihuang Gong, Yunquan Liu

Published Jun 1, 2020• 9 citations
Publisher

Abstract

We observe the parity-unfavored photoelectron emission in multiphoton single ionization of krypton atoms in an intense ultraviolet laser field. With systematic experiments of successively varying the light ellipticity and intensity, we identify that this parity-unfavored emission is associated with the first excited ionic state via a resonant pathway. We reveal an abnormal Coulomb asymmetry of the parity-unfavored photoelectron emission in elliptically polarized fields when the ellipticity is less than 0.3. The simulation using the time-dependent Schr\"odinger equation based on single-active-electron approximation fails to reproduce the experiment. We attribute the experimental observations to the angular momentum correlation between the resonantly excited electron and the spin-orbit excited ion. Our results demonstrate that the well-known picture of the angular momentum transfer in multiphoton ionization is broken and instead that the electron shares the spin angular momenta of photons with the ion as an effect of electron correlations.

DOI
10.1103/physreva.101.061401

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We observe the parity-unfavored photoelectron emission in multiphoton single ionization of krypton atoms in an intense ultraviolet laser field. With systematic experiments of successively varying the light ellipticity and intensity, we identify that this parity-unfavored emission is associated with the first excited ionic state via a resonant pathway. We reveal an abnormal Coulomb asymmetry of the parity-unfavored photoelectron emission in elliptically polarized fields when the ellipticity is less than 0.3. The simulation using the time-dependent Schr"odinger equation based on single-active-electron approximation fails to reproduce the experiment. We attribute the experimental observations to the angular momentum correlation between the resonantly excited electron and the spin-orbit excited ion. Our results demonstrate that the well-known picture of the angular momentum transfer in multiphoton ionization is broken and instead that the electron shares the spin angular momenta of photons with the ion as an effect of electron correlations.
When a bound electron is photoionized from an atom or molecule, the remaining multielectron ion with excess internal energy will relax towards lower-energy states. The subsequent transverse angular momentum transfer between the parent ion and the ejected electron can lead to an electron parity-unfavored transition. Here we perform a self-calibrated timing experiment to measure the time delay of the angular momentum transfer of parity-unfavored transitions during the multiphoton ionization. We use a strong linearly polarized 400-nm light field to trigger a parity-unfavored transition in the channel of the spin-orbit excited ionic state of krypton atoms and then selectively probe the phases of the electron wave functions with different magnetic quantum numbers with a weak parallelly (orthogonally) polarized 800-nm light field. The parallel and orthogonal probes serve as the start and the stop of a stopwatch, respectively. Hence, this allows us to access the characteristic time of such an ultrafast intra-atom multielectron process.
Single-photon ionization of an atom or molecule can be subdivided into parity-favored and -unfavored transitions, the latter characterized by electron emission, preferentially perpendicular to the electric vector. The nonresonant existence of these transitions is shown experimentally and studied over an extended energy range for a variety of satellite transitions in atomic argon. The spectra exhibit several clearly resolved satellite lines with strongly negative \ensuremath{\beta} values close to -1, independent of the photon energy. The results confirm the corresponding predictions of angular-momentum transfer theory.
We consider few-photon ionization of atomic lithium by linearly polarized femtosecond laser pulses and demonstrate that asymmetries of the electron angular distribution can occur for initially polarized (2p,(2p,(2p, mmm=+1)+1)+1) target atoms. The dependence of the photoelectron emission angle relative to the electric field direction is investigated at different laser intensities and wavelengths. The experimental spectra show excellent agreement with numerical solutions of the time-dependent Schr\"odinger equation. In the perturbative picture, the angular shift is traced back to interferences between partial waves with mean magnetic quantum number $\ensuremath{\langle}m\ensuremath{\rangle}\ensuremath{\ne}0$. This observation allows us to obtain quantum mechanical information on the final electronic state.
Multiphoton excitation with elliptically polarized light brings out more information in the photodetachment or photoionization processes than linearly polarized light. This information can only be reached, however, by recording photoelectron angular distributions for several values of the ellipticity angle, and fitting the data with a common formula. But the analytic dependence of photoelectron angular distributions on the ellipticity has been unknown up to now, except for the one- and two-photon excitation cases. Here we give a demonstration of a general formula, valid for any number N of exciting photons, with the assumptions of (i) an initially even and momentumless (J=0) atom or ion and (ii) that after excitation of an electron, the residual core is left in an odd state. Slight modifications in the demonstration would make it possible to extend the general formula to any J=0 initial atomic state.
We theoretically study the photoelectron momentum distributions from multiphoton ionization of a model lithium atom over a range of laser wavelengths from 500 nm to 700 nm by numerically solving the time-dependent Schrödinger equation. The photoelectron momentum distributions display many ring-like patterns for the three-photon ionization, which vary dramatically with the change of the laser wavelength. We show that the wavelength-dependent photoelectron energy spectrum can be used to effectively identify the resonant and nonresonant ionization pathways. We also find an abnormal ellipticity dependence of the electron yield for the (2+1) resonance-enhanced ionization via the 4d intermediate state, which is relevant to the two-photon excitation probability from the ground state to the 4d state.
We calculate two-dimensional photoelectron momentum distributions and energy spectra after multiphoton ionization of Li atoms subject to intense laser fields in the one-photon Rabi-flopping regime. The time-dependent Schr\"odinger equation is solved within the single-active-electron approximation using a model potential which reproduces accurately the binding energies and dipole matrix elements of the Li atom. Interaction with the external electromagnetic field is treated within the dipole approximation. We show that the Rabi oscillations of the population between the ground 2s2s2s state and the excited 2p2p2p state in the one-photon resonance regime are reflected in the energy spectra of emitted photoelectrons, which manifest interference structures with minima. Transformations of the interference structures in the photoelectron energy spectra caused by the variation of the laser peak intensity and pulse duration are analyzed.