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Physical Review A

Population trapping in short-pulse laser ionization

Jonathan S. Parker, Carlos R. Stroud

Published Feb 1, 1990• 88 citations
Publisher

Abstract

The finite bandwidth of a short laser pulse often overlaps several initially unpopulated low-lying excited states of an atom. We show that when such a pulse is used to photoionize an atom, the low-lying levels may be pumped into a coherent superposition that inhibits or prevents excitation of the atom, effectively trapping population in the initial state of the atom. The existence of the effect was first noted in a numerical study of short-pulse excitation of hydrogen. Here we explore the effect through a series of simple models that demonstrate the physics of the effect in the simplest possible situations.

DOI
10.1103/physreva.41.1602

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The finite bandwidth of a short laser pulse often overlaps several initially unpopulated low-lying excited states of an atom. We show that when such a pulse is used to photoionize an atom, the low-lying levels may be pumped into a coherent superposition that inhibits or prevents excitation of the atom, effectively trapping population in the initial state of the atom. The existence of the effect was first noted in a numerical study of short-pulse excitation of hydrogen. Here we explore the effect through a series of simple models that demonstrate the physics of the effect in the simplest possible situations.
We have studied population trapping in a one-dimensional model atom interacting with a short-pulse high-intensity laser, using Floquet analysis and direct numerical integration of the time-dependent Schrödinger equation for a range of peak pulse intensities. We find that photoelectrons are efficiently produced only by pulses whose peak intensities lie in a narrow range around the resonant intensity for a given intermediate state. Excited-state populations, however, are generated with comparable efficiencies for all pulses whose peak intensities exceed the resonant intensity. This implies that, for a realistic laser pulse with an inhomogeneous spatial distribution of intensity, excited atoms are generated throughout a larger volume than are photoelectrons. Interpretations of recent experiments are reexamined in light of this result.
It is often desirable in laser spectroscopy and isotope separation to extract as much as possible of an atomic or molecular population that is distributed among a number of ground-state sublevels and low-lying metastable levels. We describe a form of coherent trapping that occurs when multiple resonant laser beams are used to couple the various ground states to a common upper level. This effect prevents the extraction of the entire population. We then study the effect with two dye lasers and an atomic beam and suggest possible ways to maximize the pumping efficiency.
Two laser pulses that overlap in time have been successfully used for coherent excitation of a molecule or atom to a desired state. An intermediate state that is at or near resonance with the laser frequencies has been used in past experiments and calculations. Here we replace this intermediate state by a continuum of intermediate states. An analytic solution of a simple model suggests that the continuum can be used in such an excitation scheme, provided that the laser pulses are arranged in the so-called counterintuitive order. We use a special case of this solution to relate this work to ionization suppression and coherent population trapping.
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.
It has been widely believed that short-pulse multiphoton ionization is dominated by a resonance process: excited states of the atom are shifted by the radiation field into multiphoton resonance with the ground state, are populated, and then rapidly ionized. A recent experiment, shows significant population remaining in excited states at the end of the laser pulse, and this result has been interpreted as indicating that ionization proceeds slowly out of these states. We investigate this issue in the framework of Floquet theory, supplemented by direct numerical solution of the time-dependent Schrodinger equation. In this framework, the standard picture is portrayed as an adiabatic transition between Floquet eigenstates, which occurs as the amplitude of the laser field is ramped through the resonance value (i.e. through an avoided crossing of Floquet quasienergies). Thus it follows that the same type of transition must occur as the field is turned off. This mechanism can yield significant populations of excited states that do not suffer appreciable ionization. We exhibit this effect by numerical studies of a model atom with two bound states and two continua.