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Physical Review A

Dynamical stabilization of atoms in intense laser pulses accessible to experiment

Etienne Huens, Bernard Piraux

Published Feb 1, 1993• 20 citations
Publisher

Abstract

We analyze the excitation of atoms by intense pulsed laser fields and describe a mechanism leading to effective dynamical stabilization at a photon energy below the unperturbed binding energy. At low and intermediate intensities, the atom is left in a coherent superposition of the initial state and a set of Rydberg states which is stable against ionization; this effect subsists for pulse durations presently accessible to experiment. At higher intensities, population is transferred through degenerate Raman coupling to Rydberg states of higher angular momentum.

DOI
10.1103/physreva.47.1568

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We analyze the excitation of atoms by intense pulsed laser fields and describe a mechanism leading to effective dynamical stabilization at a photon energy below the unperturbed binding energy. At low and intermediate intensities, the atom is left in a coherent superposition of the initial state and a set of Rydberg states which is stable against ionization; this effect subsists for pulse durations presently accessible to experiment. At higher intensities, population is transferred through degenerate Raman coupling to Rydberg states of higher angular momentum.
An analytical study is presented of two models of intense-laser ionization from an isolated initial state via a band of high-Rydberg states in a system which bears many features of the hydrogen atom. The models differ in the way the population from the directly excited Rydberg states can migrate to the states of higher angular momenta. In one of them it migrates via nonresonant and in the other via resonant degenerate Raman coupling. Within these models we draw conclusions about the efficiency of migration, the initial-state lifetime, the redistribution of population, the laser intensities stabilizing the atom against ionization, the effect of the initial state on the threshold stabilizing intensity, as well as about stabilizing mechanisms versus the initial-state choice.
We have observed trapping of population in Rydberg states of atoms and ions exposed to intense laser radiation. The trapping occurs in nonstationary ``wave-packet'' superposition states, created, via ac Stark-shifted multiphoton resonances during the laser pulse. The formation of these highly excited bound states requires the nonresonant absorption of up to 20 or more photons; some of the ionic Rydberg states lie nearly 32 eV above the ionic ground state. All of the trapped states having binding energies much less than the photon energy but remain stable against ionization even when the multiphoton ionization probability of the atom is saturated.
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 show that, when an atom prepared in a high Rydberg state is one-photon ionized by a laser, a near resonance with some lower-lying state permits the formation of a nondecaying dark state of the coupled laser–atom system. This dark state traps population in an amount that increases to some upper limit as the laser radiation used becomes stronger. Thus the dark state formed causes a decrease in the Rydberg-atom ionization with increasing laser intensity, i.e., stabilization, which survives even for long times. This long-time stabilization is described by a simple zero-pole formula, and the physical meaning of the term long time is explained. We estimate, for model situations, the laser frequencies that ensure dark-state formation and maximum ionization suppression. The laser pulse intensity and duration that are required for the observation of the dark state are evaluated.
We have observed suppression of ionization in Rydberg atoms irradiated by an intense short-pulse laser field. We find that single-photon ionization of the outer electron in a Ba 6snk Stark state can be severely suppressed relative to high-order (\ensuremath{\ge}3 photon) nonresonant multiphoton ionization of the tightly bound 6s core electron. This population trapping occurs in intense short laser pulses, where the bandwidth of the ionizing laser is greater than the spacings between adjacent Stark states, and may be due to a substantial modification of the initial Stark state via stimulated Raman processes through the continuum. The Ba+∗{\mathrm{Ba}}^{+\mathrm{*}}Ba+∗ ion should be left in a higly excited superposition state, or wave packet.