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Quantum Electronics and Laser Science Conference

Coherent control of autoionization in a two-electron atom

Robert van Leeuwen, M.I. Bajema, R. R. Jones

Published Jan 20, 2003• 0 citations
Publisher

Abstract

Summary form only given. We present an experimental demonstration of differential process control in the time domain. In our experiment Ca Rydberg atoms (4 sns J=0) are exposed to a pair of phase-coherent laser pulses (I/sub p/=0.4 ps, /spl lambda/_=393 nm). Each laser pulse performs an "isolated core excitation" (ICE) producing a nonstationary autoionizing wavepacket at an energy just below the Ca/sup +/ 4p/sub 1/2/ ionization limit.

DOI
10.1109/qels.1999.807312

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•Jan 1, 2003•0 citations
Summary form only given. We present an experimental demonstration of differential process control in the time domain. In our experiment Ca Rydberg atoms (4 sns J=0) are exposed to a pair of phase-coherent laser pulses (I/sub p/=0.4 ps, /spl lambda/_=393 nm). Each laser pulse performs an "isolated core excitation" (ICE) producing a nonstationary autoionizing wavepacket at an energy just below the Ca/sup +/ 4p/sub 1/2/ ionization limit.
Coherent short-pulse laser excitation has been used to control the approximate energy and relative proximity of two valence electrons within the same alkaline-earth atom, thereby providing insight into the dynamical evolution of a three-body Coulomb system. Our time-domain experiments enable direct experimental study of the electron dynamics at the classical limit of a two-electron atom. As an example, we look at the mechanism of autoionization for one two-electron configuration class and find that the doubly excited atom decays through a single violent electron-electron collision rather than a gradual exchange of energy between the electrons.
We demonstrate that atomic wavepackets can serve as sensitive detectors for investigating atomic and molecular dynamics. In concert with parametric four-wave mixing, the interference between coherent superpositions of atomic excited states produced by ultrafast (/spl sim/150 fs) pump and probe pulses provides a new and powerful tool with which fundamental processes, such as molecular dissociation and Rydberg-Rydberg atomic collisions, can be observed with the extraordinary sensitivity afforded by a coherent nonlinear optical process. Experiments are described in which the dissociation of an electronically excited molecule (Rb/sub 2/) and the distribution of atomic fragments into excited states spanning >10 000 cm/sup -1/ are observed. Also, resonant collisions between Rb atoms in the 7s and 5d states are detected by monitoring the shift in the frequency of an atomic wavepacket induced by the dipole-dipole interaction.
New experimental quantum simulation platforms have recently been implemented with divalent atoms trapped in optical tweezer arrays, with promising performance.The second valence electron also brings about new prospects through the so-called isolated core excitation (ICE).However, autoionization presents a strong limitation to this use.In this study, we propose and demonstrate a new approach to applying a sizable light shift to a Rydberg state with close-to-resonant ICE while avoiding autoionization.In particular, we investigate the ICE of ytterbium atoms in 1 S 0 Rydberg states.Spectroscopic studies of the induced autoionization and the light shift imparted to the Rydberg states are well accounted for with multichannel quantum defect theory.Such control over the inner electron without disturbing the Rydberg electron brings about a new tool for the targeted coherent manipulation of Rydberg states in quantum simulation or quantum computing experiments performed with alkaline-earth atoms.
Helium atoms in the presence of extreme ultraviolet light pulses can lose electrons through direct photoionization or through two-electron excitation followed by autoionization. Here we demonstrate that, by combining attosecond extreme ultraviolet pulses with near infrared femtosecond lasers, electron dynamics in helium autoionization can be not only monitored but also controlled. Furthermore, the interference between the two ionization channels was modified by the intense near infrared laser pulse. To the best of our knowledge, this is the first time that double excitation and autoionization were studied experimentally by using isolated attosecond pulses.
Strong-field ionization of Rydberg atoms is investigated in its dependence on phase features of the initial coherent population of Rydberg levels. In the case of a resonance between Rydberg levels and some lower-energy atomic level (V-type transitions), this dependence is shown to be very strong: by a proper choice of the initial population an atom can be made either completely or very little ionized by a strong laser pulse. It is shown that phase features of the initial coherent population of Rydberg levels and the ionization yield can be efficiently controlled in a scheme of ionization by two strong laser pulses with a varying delay time between them.
We have performed nonperturbative time-dependent calculations of single and double ionization of atomic calcium by short and intense laser pulses using a two-active electron model (TAE). It is shown that the significant enhancement of the Ca 2+ yield observed in a recent experiment [1] using feedback control techniques originates from the time asymmetry of the pulse shape. The initial part of the pulse prepares a coherent superposition of doubly excited states which is transferred into the double electron continuum at later times. An asymmetric shape, with a slowly decreasing tail, therefore favors the production of Ca 2+ .