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Acta Physica Sinica

STRONG-FIELD AUTOIONIZATION BY SQUARE LASER PULSES

CHEN RONG-QING, Zhizhan Xu, SUN LAN, Guanhua Yao, ZHANG WEN-QI, Ping Li, 中国科学院上海光学精密机械研究所,上海,201800

Published Jan 1, 1991• 1 citation
Publisher

Abstract

Previous studies on strong-field autoionization involving high-order ionization processes are mainly investigated for the cases of CW laser irradiations. This paper extends those studies to the cases of square laser pulses with arbitrary widths. Populations of the ground state and the line shapes of the autoionizing resonance are discussed. It is shown that strong-field autoionization and the high-order ionization processes are drastically affected not only by the intensities but also by the time of interaction between atoms and laser field.

DOI
10.7498/aps.40.1584

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Previous studies on strong-field autoionization involving high-order ionization processes are mainly investigated for the cases of CW laser irradiations. This paper extends those studies to the cases of square laser pulses with arbitrary widths. Populations of the ground state and the line shapes of the autoionizing resonance are discussed. It is shown that strong-field autoionization and the high-order ionization processes are drastically affected not only by the intensities but also by the time of interaction between atoms and laser field.
Strong-field autoionization induced by smooth laser pulses is studied to take into account the high-order ionization processes. A second-order differential equation of the amplitude of the ground state is derived. For some special laser pulses, which include all the exponential ones, analytical results may be obtained. As an example, the explicit results of ground-state population and the photoelectron spectrum are given for an exponentially switched-on cw pulse. The multipeak structure in the photoelectron spectrum is more obviously manifested in the high spectrum resulted from the high-order ionization processes.
In the last decade, laser-induced autoionization has been an active subject of Strong laser-atom interaction. The main interest in the subject arises from many interesting high-field effects in the problem, notably the stabilization of the autoionizing state (AIS) and population trapping, which leads to possible applications in lasers without inversion.1 However, many decay mechanisms may destroy this interference effect to various extents. Many authors have addressed this problem, but few 2,3 have investigated the effects of high-order processes in strong-field autoionization.
We have performed an experimental study of the atomic system ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$, for which the ground state 5s2{\mathit{s}}^{2}s2 is coupled to the autoionizing state 5${\mathit{p}}_{1/2}$6s by means of a strong resonant laser field. The investigations are based on irradiation of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ with intense (I\ensuremath{\le}1.5\ifmmode\times\else\texttimes\fi{}1010{10}^{10}1010 W/cm2{\mathrm{cm}}^{2}cm2) laser pulses of duration \ensuremath{\sim}1 ns. Single-photon detachment of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ is suppressed due to the Fano interference between transitions directly to the continuum and transitions via the autoionizing state. In contrast, two-photon absorption is at the same time resonantly enhanced since no such destructive interference is present in this process. These observations constitute experimental evidence of general features predicted from theoretical considerations of autoionizing states in strong laser fields. Furthermore, we discuss the role of autoionizing states in multiphoton double-ionization processes. Electron spectroscopy has been used to show that double ionization of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ primarily is the result of a sequential removal of the two electrons via an excited Rb state. The perspectives for direct two-electron ionization in negative ions are briefly discussed.
A theory of two- and three-photon autoionization is presented for intermediate resonant and non-resonant cases, stressing the laser-intensity effect on the configuration mixing of the final state. Sample calculations are performed for the 5p6s1P5p6s^{1}P5p6s1P autoionizing state. The involvement of the 5s5d5s5d5s5d intermediate state includes the effects of ac Stark shifts on the ionization rate, the line shape, and the photoelectron angular distributions.