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Physical Review A

Coherence in multistate resonance-enhanced four-photon ionization of lithium atoms

Michael Schuricke, Klaus Bartschat, Alexei N. Grum-Grzhimailo, Ganjun Zhu, Jochen Steinmann, R. Moshammer, J. Ullrich, Alexander Dorn

Published Aug 30, 2013• 15 citations
Publisher

Abstract

A joint experimental and theoretical study of four-photon ionization of Li is performed using a pump-probe setup with the laser frequency tuned in between the excitation energies of the $2s$--$4p$ and $2s$--$4f$ three-photon transitions. The three-dimensional photoelectron momentum pattern is recorded as a function of the delay time between two otherwise identical laser pulses. The process is modeled by numerically solving the time-dependent Schr\"odinger equation. While complicated interference patterns arise in the electron spectrum when the two pulses partially overlap in time, the expected quantum-beat phenomenon between Li atoms in either the $4p$ or the $4f$ state are seen when the fast oscillations involving the $2s$ initial state are averaged over. The qualitative agreement between the predicted and observed beating patterns is very good.

DOI
10.1103/physreva.88.023427

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We investigate the resonance-enhanced few-photon ionization of atomic lithium by linearly polarized light whose frequency is tuned near the $2s\text{\ensuremath{-}}2p$ transition. Considering the direction of light polarization orthogonal to the quantization axis, the process can be viewed as an atomic ``double-slit experiment'' where the 2p2p2p states with magnetic quantum numbers ${m}_{\ensuremath{\ell}}=\ifmmode\pm\else\textpm\fi{}1$ act as the slits. In our experiment, we can virtually close one of the two slits by preparing lithium in one of the two circularly polarized 2p2p2p states before subjecting it to the ionizing radiation. This allows us to extract the interference term between the two pathways and obtain complex phase information on the final state. The experimental results show very good agreement with numerical solutions of the time-dependent Schr\"odinger equation. The validity of the two-slit model is also analyzed theoretically using a time-dependent perturbative approach.
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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.
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We present numerical simulations of the reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) on lithium by solving the three-dimensional time-dependent Schr\"odinger equation. In our scheme, the infrared (IR) field couples the 2s2s2s and 2p2p2p states of lithium and leads to the Rabi oscillations of populations between these two states. We analyzed the RABBITT phases of the two peaks in the Rabi-oscillation-induced Autler-Townes splittings. Our results show that the relative phase between these two peaks changes with the photoelectron energy and depends on the intensity of the IR field. Moreover, in the angle-resolved RABBITT measurement, the phases of the two peaks depend differently on the emission angle of photoelectrons. These behaviors are traced back to the different initial phases of the electron wave packets emitted from the dressed 2p2p2p states and the competition among different ionization channels.
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