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Science

Probing Two-Electron Dynamics of an Atom

S. N. Pisharody, R. R. Jones

Published Feb 5, 2004• 68 citations
Publisher

Abstract

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.

DOI
10.1126/science.1092220

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•Jan 1, 2012•0 citations
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.
Orbital dynamics in two-electron alkaline earth atoms in a static electric field are studied using scaled-energy recurrence spectroscopy. Photoabsorption spectra for Ba and Ca Rydberg atoms are measured as continuous functions of static electric field and energy. In both Ba and Ca the laser excitation proceeds through the singly excited ndndnd series which is strongly perturbed by doubly excited valence configurations. Clear signatures of the two-electron dynamics associated with these perturbing configurations are identified.
Atoms in doubly excited states above the first ionization limit can decay via autoionization in which an electron is emitted leaving an ion, or by photoemission which leaves the atom in a singly excited state. In this paper, it is demonstrated that interaction between the atoms and a laser pulse that is short compared to the autoionization lifetime can lead to large enhancement of the photoemission process by stimulating the atoms to emit a photon. Since the resultant singly excited atoms do not autoionize, this process can be viewed as an enhancement of the stabilization of the doubly excited atoms against autoionization. A simple theoretical model is outlined that shows good agreement with the experimental results.
The past thirty years witnessed rapid developing of strong field physics that leads to the first generation of attosecond (10−18 s) pulses. The duration of available pulses has recently been reduced to less than 100 as, approaching the Kepler period of a classical electron revolving around the nucleus. For the first time, electron motions inside atoms, molecules and solids can be traced, probed and even steered in real time. It pushed the study on ultrafast dynamic processes into a new frontier, giving birth to attosecond physics. The development of attosecond pulses provides unprecedented coherent ultrashort XUV (extreme ultra-violet) of soft X-ray sources that enable imaging the atomic structure and resolving the related quantum dynamics for matter science. In particular, it allows the probing and monitoring of electron dynamics in the natural time scale of attoseconds with atomic spatial resolution. The generation and applications of attosecond pulses are directly related to the control and manipulation of the sub-cycle electron dynamics. Recent studies on the correlated multielectron dynamics have provided insight into the optical and dynamical properties of matter in the perspectives of time, phase and entanglement. It deepens our understanding on some of the fundamental questions, e.g., how the single or multiple photons are absorbed and how short are the quantum processes such as photoionization, tunneling ionization and charge migration. Although currently attosecond pulses are limited by the flux and pulse duration, the combination of attosecond pulses with intense infrared laser pulses paves new ways of controlling and probing electron dynamics in the domains of energy, time and space that lays the foundation for petahertz optoelectronics and attosecond pulse based transient spectroscopy. In this review, we will first briefly summarize the advancement of strong field and ultrafast physics including the relevant experimental findings, the theoretical explorations, the related technologies and the possible applications. Then we focus on the sub-cycle electron or multielectron dynamics and their manifestation on the ionization and coherent radiation properties. We start with the time behavior of Fano resonance and how it can be probed in time domain. In section 2, the correlated electron dynamics is discussed in term of anti-screening effect and Pauli effect that influence strong field ionization and high harmonic generation processes. In section 3, the coherent emission with frequency from terahertz to soft XUV and their joint measurement are discussed by emphasizing the generation mechanism of terahertz wave in two-color laser pulses. The developed high-harmonic and terahertz wave spectroscopy (HATS) is shown capable of imaging the molecular structure and dynamics. In section 4, the time-resolving of electron and hole dynamics and their coherence is proposed based on attosecond transient absorption technique. The coherence of ionization and the correlated electron dynamics is hinted crucial for electron-hole interaction. The topics are very limited and mainly selected from the perspectives of our group.
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.
Within this work, electron dynamics in atoms are experimentally and numerically investigated on their natural attosecond time scales. To access these dynamics, ultrashort and moderately-intense few-cycle laser pulses are superposed with attosecond pulsed radiation in the extreme-ultraviolet spectral region. Both these sources are combined in a new experimental vacuum setup which was designed and built up from scratch, also in order to generate the attosecond pulses. Novel experimental schemes are developed which involve the temporal and spectral interferometric utilization of both ultrashort light sources and include the multidimensional spectroscopy employing different dynamical parameters. These are the intensity and the carrier-envelope phase of the laser pulses, as well as their temporal delay with respect to the attosecond pulses. Scientifically, the observation and control of the quantum-motion of bound electrons, as well as the laser-driven quasi-classical motion of free electrons was a key goal of this work. In particular, a bound two-electron wave packet in helium on attosecond time scales was experimentally observed for the first time. This was realized by combining unprecedented high temporal and spectral resolution. In addition, a new coupling mechanism of several doubly-excited states to an effective single-electron continuum is identified and analyzed. Both these two-electron states as well as single-electron states are interferometrically investigated with access to the phase of their quantum-mechanical wave function. This allows the observation and control of the quantum dynamics of two correlated electrons in atoms on the attosecond time scale which was demonstrated within the scope of this work.