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Journal of the Optical Society of America B

Multiphoton ionization of atoms in the presence of multiple resonances

Svetlana Kotochigova

Published Aug 1, 1992• 6 citations
Publisher

Abstract

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.

DOI
10.1364/josab.9.001215

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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.
In an effort to gain more insight into the process of multiphoton multiple ionization, several experiments have recently been carried out to determine the wavelength dependence of the double ionization of Sr and Ba. 1-3 These studies were not as conclusive as one might hope, in that the observed resonances in the production of Sr++ and Ba++ did not quite match the energy levels of Sr+ and Ba+. Here we report experiments in which we have compared the spectra for the production of Ba by an intense laser with and without the population of specific states of Ba by modest secondary lasers. The results of these experiments show clearly that Ba++ is produced by first producing real intermediate states of Ba+ which are multiphoton ionized through intermediate resonances in Ba+.
The most important results of study of multiphoton ionization of atoms are reviewed. Discussions are presented of specific features of the process, theoretical methods of description of ionization of atoms in a strong light field, the basic principles of design of experiments in which intense laser radiation is used, results of investigation of direct and resonance processes of multiphoton ionization, and the mutual relation between multiphoton processes (multiphoton excitation and ionization) and various phenomena leading to perturbation of the spectrum of bound states of an electron in an atom under the influence of a strong light field.
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.
The process of resonant multiphoton ionization of a hydrogen atom in the ground 1 s state is studied by direct numerical integration of the nonstationary Schrödinger equation for a quantum system in an electromagnetic field. The dependence of photoionization probability on the radiation intensity is found to be nonmonotonic. It is established that the minima of ionization correspond to multiphoton resonances between the ground state and one of the excited (Rydberg) atomic states perturbed by the laser field. It is shown that ionization is suppressed due to rearrangement of Rydberg states in a strong electromagnetic field and is accompanied by efficient Raman Λ transitions, which connect a set of closely lying Rydberg states via the continuum.