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Soviet Journal of Quantum Electronics

Selective ionization of atoms in optical and electric fields

Л. Н. Иванов, Vladilen S Letokhov

Published Mar 31, 1975• 9 citations
Publisher

Abstract

A theoretical analysis is made of the method of selective ionization of atoms based on selective resonance excitation by laser radiation to quantum states near the ionization threshold and subsequent autoionization decay of the excited states under the influence of an external electric field. The autoionization of atoms in static and pulsed electric fields is considered. Calculation formulas are obtained for the probability of autoionization of a hydrogenlike atom and corrections are computed for the influence of the electron shells. Numerical estimates are obtained for the autoionization of the n = 7–12 states of the Rb atom in an electric field.

DOI
10.1070/qe1975v005n03abeh011006

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A theoretical analysis is made of the method of selective ionization of atoms based on selective resonance excitation by laser radiation to quantum states near the ionization threshold and subsequent autoionization decay of the excited states under the influence of an external electric field. The autoionization of atoms in static and pulsed electric fields is considered. Calculation formulas are obtained for the probability of autoionization of a hydrogenlike atom and corrections are computed for the influence of the electron shells. Numerical estimates are obtained for the autoionization of the n = 7–12 states of the Rb atom in an electric field.
Processes of ionization at the interaction of attosecond pulses of an electromagnetic field with atoms in nonstationary states have been considered. The probabilities and ionization cross section at the radiative relaxation of an excited state of a single-electron atom and at the Auger decay of the autoionization state of a two-electron atom have been calculated. The developed method allows the expansion to the case of more complex targets, including those in the collision state, and to various chemical reactions.
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An experimental investigation was made of the influence of a laser radiation field on the spectrum of barium atoms. The investigation was carried out by the method of three-photon ionization spectroscopy using dye laser radiation (ω = 14 800–18 700 cm − 1 ). The electric field intensity of the laser radiation was 10 3 –10 6 V/cm. This laser radiation field had a strong influence on a number of bound and autoionizing states. The nature of this influence depended on the ratio of the excitation frequencies of bound and autoionizing states.
We report experiments in which positronium (Ps) atoms, optically excited to Rydberg states with principal quantum numbers nnn in the range 18--25, were selectively ionized by both static and pulsed electric fields. The experiments were modeled using Monte Carlo simulations that include tunnel ionization rates calculated for hydrogen and scaled by the Ps reduced mass. Our measurements exhibit a small disagreement with the calculated tunnel ionization rates. Despite this we show that the electric fields in which different Ps states are ionized are sufficiently separated to allow selective field-ionization methods to be used in typical experimental conditions.
In the work, the main results on excitation and decay processes of autoionizing states in alkali atoms are considered. As follows from the data on excitation dynamics of subvalence shells in an impact energy region from the lowest excitation threshold up to 600 eV, at threshold energies their electron excitation results exclusively from the formation of negative-ion resonances. At mean impact energies, the cascade population processes due to resonance excitation and subsequent radiative decay of the upper states play an essential role for the low-lying autoionizing states. By comparing with theoretical data, the absolute excitation cross sections for autoionizing states and negative-ion resonances were determined. The absolute autoionization cross sections as well as the autoionization contribution to the total single ionization cross section were also determined. Keywords: atom, ion, autoionization, resonances, ionization, effective cross section.
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