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Journal of Physics B Atomic Molecular and Optical Physics

Phase control of a two-channel ionization system

Emmanuel Paspalakis, Aabid Patel, M. Protopapas, P. L. Knight

Published Feb 28, 1998• 9 citations
Publisher

Abstract

A theoretical analysis of the phase control of a two-channel atomic ionization scheme is given. We report results on phase-selective excitation, both with square and smooth laser pulses. We show that the phenomenon of population trapping, which is known to occur in this system, can be greatly influenced by the phase difference of the lasers involved in the excitation. As a result, the time dependences of the ionization probability, the populations in the bound states and the form of the photoelectron energy spectra can all be controlled by varying the phase difference of the lasers. We also show that for specific atomic and phase parameters population can be entirely trapped within a superposition state, totally immune to decay, with no ionization whatsoever.

DOI
10.1088/0953-4075/31/4/023

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A theoretical analysis of the phase control of a two-channel atomic ionization scheme is given. We report results on phase-selective excitation, both with square and smooth laser pulses. We show that the phenomenon of population trapping, which is known to occur in this system, can be greatly influenced by the phase difference of the lasers involved in the excitation. As a result, the time dependences of the ionization probability, the populations in the bound states and the form of the photoelectron energy spectra can all be controlled by varying the phase difference of the lasers. We also show that for specific atomic and phase parameters population can be entirely trapped within a superposition state, totally immune to decay, with no ionization whatsoever.
A theoretical analysis of the phase control of a two-channel atomic ionization scheme is given. We report results on phase-selective excitation, both with square and smooth laser pulses. We show that the phenomenon of population trapping, which is known to occur in this system, can be greatly influenced by the phase difference of the lasers involved in the excitation. As a result, the time dependences of the ionization probability, the populations in the bound states and the form of the photoelectron energy spectra can all be controlled by varying the phase difference of the lasers. We also show that for specific atomic and phase parameters population can be entirely trapped within a superposition state, totally immune to decay, with no ionization whatsoever.
It is shown that the rate and degree of ionization of an atom can be controlled by preparing a specific superposition of Rydberg states. This effect is due to the Raman interaction of Rydberg levels via resonant lower-lying levels of the atom. Depending on the initial population of an atom interacting with a strong laser field, the atom either ionizes completely or exhibits an extremely high degree of stability (in this case the probability of the atom remaining in a bound state when the laser pulse is switched off is close to 1). It is shown that states which are stable against ionization decay or, conversely, decay rapidly into the continuum can be prepared using double laser pulses with a controllable time delay.
We study the nonlinear and resonant process of two-photon ionization of atoms (He and H) in a pump-probe scheme. The pump pulse prepares the quantum system in a superposition of the ground state and an excited bound state. By varying the phase difference between the pulses, we show how it is possible to coherently control the dressed-state population during the probe pulse. Our main result is that for certain laser parameters, the control over the dressed-state population leads to strong control of the ionization probability during the probe pulse. The effect arises due to one of the dressed states becoming stabilized against ionization. Contrasting effects from circular and linear polarized pulses demonstrate how such “bound states in the continuum” are sensitive to the degeneracy of the coupled continuum.
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.
Coherent control provides a quantum interference based method for controlling atomic and molecular dynamics. We report two experimental observations of different ways to achieve such a control in atomic photoionization continuum. One is the photoionization of sodium atoms, initially prepared in the 3P1/2 excited state, by a non resonant three-photon interaction with 1064-nm laser light and by a one-photon interaction with the third-harmonic field, at 355 nm. Interference between the two transition paths allows the control of the total ionization rate by varying the relative phase of the laser fields. The second experiment is the observation of the modification of the ionization continua of xenon through laser induced continuum structure (LICS). The multiphoton radiative decay of the xenon ground state into the two electronic continua, that correspond to the two fine structure levels of the ground state of the xenon ion, is varied by electromagnetically embedding a bound state of the atom into the two continua. Both experiments are performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analyzing the emitted photoelectrons.