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Journal of Physics B Atomic Molecular and Optical Physics

The role of spontaneous relaxation in the three-photon ionization of Ba through the two-photon resonant 5d7s3D2state

І. І. Бондар, Svetlana Kotochigova, В. В. Суран

Published Jul 14, 1991• 3 citations
Publisher

Abstract

The influence of spontaneous relaxation on the multiphoton ionization process is considered. Experimental and theoretical investigations have shown that with a comparable weak field there is an interval of low stability of the ionization process, when the probabilities of the competing processes of spontaneous relaxation and ionization are nearly equal. It leads to the unusually rapid growth of the amplitude of the discrete two-photon 5d7s 3 D 2 resonance in the yield of Ba + , when the field strength was increased very little.

DOI
10.1088/0953-4075/24/13/013

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The influence of spontaneous relaxation on the multiphoton ionization process is considered. Experimental and theoretical investigations have shown that with a comparable weak field there is an interval of low stability of the ionization process, when the probabilities of the competing processes of spontaneous relaxation and ionization are nearly equal. It leads to the unusually rapid growth of the amplitude of the discrete two-photon 5d7s 3 D 2 resonance in the yield of Ba + , when the field strength was increased very little.
A model of multiple resonances between groups of discrete and autoionizing levels is proposed for the description of the process of multiphoton ionization. The phenomena of the extremely rapid broadening and growth of amplitudes of the multiphoton resonances, observed experimentally in the yield of Sr+ and Ba+ ions [ J. Phys. B.14, 2253– 2359 ( 1981);Elementary Processes in Laser Radiation Field ( 1988), pp. 122– 153)] are explained on the basis of this model. The strong-coupling method was used in numerical calculations for Sr and Ba atoms in a strong laser field.
The manifestation of spontaneous Raman scattering upon the three-photon ionization of a samarium atom has been investigated. The dependence of the Sm+ yield on the laser frequency, along with the peaks due to the two-photon excitation of bound states from different levels of the 4f 66s 2 7 F ground term, exhibits a strong peak due to single-photon excitation of the 4f 55d6s 2 7 D°3 state from the 4f 65d6s 9 H 2 excited state. The 4f 65d6s 9 H 2 state was populated as a result of spontaneous Raman scattering. The large amplitude of the observed peak indicates a high efficiency of this process. The estimations performed indicate that the probability of resonant ionization through an excited state populated due to spontaneous Raman scattering can be comparable with the probability of three-photon ionization through the two-photon intermediate resonance. The necessary conditions for implementing this channel of resonant three-photon ionization are formulated.
The manifestation of double resonances upon the three-photon ionization of the samarium atom has been studied in a frequency range of 17200–18400 cm −1 . Thirty peaks caused by double resonances have been found in the dependence of the yield of Sm + ions on the laser radiation frequency. The majority of these peaks greatly exceed in amplitude the peaks associated with ordinary two-photon resonances. The influence of double resonances on the three-photon ionization has been theoretically analyzed using the density matrix approximation. The calculations show that the probability of the three-photon resonance ionization can increase by more than two orders of magnitude due to the additional one-photon population of an intermediate level. The double resonance slows down the rate of increase in the probability of ionization with an increase in the laser radiation intensity compared to ordinary two-photon resonance.
The effect of photon correlations on two- and three-photon ionization via two near-resonant levels is discussed in terms of the resolvent-operator formalism. It is shown that under certain circumstances the ionization probability for a chaotic light can be enhanced by a factor larger than N!N!N! compared to the ionization probability for a coherent light. The dependence of this enhancement factor on the interaction time, the average intensity, the detunings of the photon frequency from the near-resonant levels, and the separation between the two near-resonant levels is discussed.
Despite inherently complex multiphoton dynamics, our observations show that Ba double ionization with an infrared laser (8800--8920 cm${}^{\ensuremath{-}1}$) resembles a single-photon process; namely, its probability is proportional to the laser intensity. In this regime, single-electron ionization is due to a four-photon resonant transition through the highly perturbed state 6p21D26{p}^{2}\phantom{\rule{0.16em}{0ex}}{}^{1}{D}_{2}6p21D2​, whereas double ionization is realized by the two-electron mechanism. Furthermore, we argue that these conclusions are valid for other alkaline-earth-metal atoms and other parameters of laser radiation.