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Physical Review Letters

Rydberg State Ionization by Half-Cycle-Pulse Excitation: Strong Kicks Create Slow Electrons

A. Wetzels, Andreas Gürtler, Lambertus D. Noordam, F. Robicheaux, C. Dinu, Harm Geert Muller, Marc J. J. Vrakking, Wim J. van der Zande

Published Dec 19, 2002• 53 citations
Publisher

Abstract

The asymptotic velocity distribution of electrons ionized in half-cycle-pulse excitation of high Rydberg states (n=34), placed in a static electric field, is studied using electron velocity-map imaging. At weak half-cycle pulse strengths, the electrons escape over the saddle point in the potential. For strong half-cycle pulses, the electrons are emitted in the direction of the field kick. The much slower and less intense half cycle of opposite polarity, which necessarily follows the main half-cycle pulse, strongly affects the momentum distribution and reduces the excess energy of the electrons significantly.

DOI
10.1103/physrevlett.89.273003

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Physica Scripta•Jan 1, 2004•1 citation
The asymptotic velocity distribution of electrons ionized in half-cycle-pulse excitation of high Rydberg states (n=34), placed in a static electric field, is studied using electron velocity-map imaging. At weak half-cycle pulse strengths, the electrons escape over the saddle point in the potential. For strong half-cycle pulses, the electrons are emitted in the direction of the field kick. The much slower and less intense half cycle of opposite polarity, which necessarily follows the main half-cycle pulse, strongly affects the momentum distribution and reduces the excess energy of the electrons significantly.
The influence of the half-cycle pulse (HCP) kick on the asymptotic velocity of the ejected electron has been studied for excited xenon atoms (n∗=34){(n}^{*}=34)(n∗=34) in the presence of a static electric field $(220\mathrm{V}{\mathrm{cm}}^{\ensuremath{-}1}).$ We find that the HCP does not change the momentum distribution perpendicular to the kick direction. Therefore half-cycle pulse ionization and electron velocity map imaging can be used to obtain two-dimensional momentum distributions of atomic Rydberg states. Semiclassical and quantum mechanical calculations complement the experimental results.
We study the ionization of Rydberg atoms in a single-cycle-pulse electric field based on both classical and quantum calculations. The ionization-probability curve exhibits a ``ripple'' structure as a function of the pulse duration and the field amplitude. These ripple structures are found to be dependent on the angular distribution of the initial state. A large electron-emission asymmetry is observed, and the ionized electron is almost completely emitted to one side of the atom except when the pulse length is roughly one Rydberg period. In both the long-pulse and the short-pulse regimes, larger electron energy can be expected from the ionization of lower-lying Rydberg states, matching the observation in a recent experiment [S. Li and R. R. Jones, Phys. Rev. Lett. 112, 143006 (2014)]. This trend is closely related to the electron-emission asymmetry associated with the field-direction change in a single-cycle pulse. The possible implications of the different energy transfer in a single-cycle pulse from that in a multicycle pulse are also discussed briefly for the strong-field ionization of the ground atomic state.
Approximate momentum distributions of Rydberg electrons in static electric fields have been obtained using an improved impulsive momentum retrieval (IMR) technique. An imaging detector enables the measurement of half-cycle pulse (HCP) ionization probability across the spatial profile of a focused half-cycle pulse beam. By modulating the HCP amplitude we directly measure the derivative of the ionization vs HCP impulse curve, enabling the recovery of momentum distributions with better resolution than previously demonstrated with IMR. For example, for Stark states with small dipole moments, we observe predicted fine-structure in the projection of the momentum distribution along the Stark field axis. We use a semiclassical model to simulate the effect that the nonzero HCP duration has on our measurements. Good agreement between simulated and measured momentum distributions is obtained.
Ionization of Rydberg atoms by short, half-cycle electrical pulses is investigated experimentally in the intermediate-pulse duration regime where the ionization threshold fields are determined both by the slew rate of the ionizing pulse and its duration. Electrical pulses with durations ranging from 140 ps to 9 ns and amplitudes up to are applied to rubidium and lithium atoms prepared in high Rydberg states by laser excitation (n ranges from 42 to 82). The amplitude of the 140 ps pulse required for ionization of the Rb and Li Rydberg population is found to be a factor of two higher than the expected amplitude based on diabatic field ionization and recent theoretical calculations by Reinhold et al (1993 26 L659). Using the longer pulses we studied the transition from the diabatic/adabatic ionization towards ionization in the impulse-kick regime.
The characteristics of strongly-polarized quasi-one-dimensional (quasi-1D) Rydberg atoms produced by photoexcitation of selected Stark states in the n = 350 Stark manifold are investigated by studying their ionization when subject to sub-nanosecond unidirectional electric field pulses, termed half-cycle pulses (HCPs), directed both parallel and perpendicular to the atomic axis. The observed differences in the ionization characteristics provide a measure of the anisotropy of the momentum distribution and thus of the quasi-1D nature of the states. The response of such states to a train of HCPs directed parallel and anti-parallel to the axis of polarization of the initial state is also investigated. Pronounced differences in the survival probability are observed confirming theoretical predictions that the dynamical response of the system will be very different for these two conditions. The presence of noise is also observed to play a crucial role in the dynamics. Reasonable agreement between experiment and theory is achieved.
We report velocity map imaging experiments on the behaviour of electrons that are strongly driven by electric fields that vary over a wide range of timescales (from DC electric fields to laser fields at optical frequencies). We show how in slow photoelectron imaging using DC electric fields direct and indirect photoionization events can be distinguished and that a quantummechanical interference between several competing pathways that take the electron from the atom to the detector can be observed. We show that with a half-cycle THz pulse Rydberg states can be ionized, that the electrons can be accelerated (though the final kinetic energy of the electrons is small) and that the initial momentum distribution of the prepared Rydberg state can be observed. Finally, we show that—at optical frequencies—high harmonic generation can lead to the formation of trains of attosecond laser pulses, with a striking dependence of the appearance time of the attosecond pulses on the harmonic generation gas.