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Journal of Anhui Normal University

TIME CHARACTERISTIC OF ATOM AUTOIONIZATION INDUCED BY LASER PULSES

Zhong Wang

Published Jan 1, 1999• 0 citations
OpenAlex

Abstract

Time charactristic of autoionization of two level atoms induced by laser pulses was investigated. Schrodinger's motion equations of two level atoms were solved numerically for Guassian laser pulses. There was a particular value of laser intensity, in which the ionization number in per nuit time becomes biggest. An analytic formula about autoionizing probability was derived for exponential laser pulses.

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Time charactristic of autoionization of two level atoms induced by laser pulses was investigated. Schrodinger's motion equations of two level atoms were solved numerically for Guassian laser pulses. There was a particular value of laser intensity, in which the ionization number in per nuit time becomes biggest. An analytic formula about autoionizing probability was derived for exponential laser pulses.
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.
Traditional schemes for multistep resonance photoionization of atoms let every employed laser beam interact with the atoms simultaneously. In such a situation, analyses via time-dependent Schr\"odinger equation show that high ionization probability requires all the laser beams must be intense enough. In order to decrease laser intensity, we proposed a scheme that the laser beam used to pump the excited atoms (in a higher bound state) into an autoionization state does not interact with the atoms until all the population is transferred by the other lasers from a ground state to the bound state. As an interesting example, we examined three-step photoionization of 235U{}^{235}\mathrm{U}235U with our scheme, showing that the intensity of two laser beams can be lowered by two orders of magnitude without losing high ionization probability.
A theoretical study of laser-induced autoionization of an atomic system in interaction with a time-dependent excitation is presented. Interference effects between the two ionization channels of the atom are found to be responsible for coherent phenomena, such as the enhancement of the photon yield, for a pulsed excitation. An analysis of the pulse-shape effects shows that pumping mechanisms of the proper dressed state, occurring during the rise time of flat-top pulses, can result in an enhancement of the photon yield. An adiabatic approximation based on the use of time-dependent complex dressed states is shown to be adequate to describe the interaction with smooth pulses.
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.
Laser excitation of a two-level atom is analytically studied for arbitrary pulse shape, area, and energy. The ionization mechanism is modeled as a homogeneous width $\ensuremath{\gamma}$ of the atomic excited state. A transformation of the optical Bloch equations allows the dynamics to be completely described by a single equation for the area of the pulse at time ttt, $\ensuremath{\Phi}(t)$. Analytic solutions for the total bound population for a wide variety of pulse shapes can be found in two regions: $\ensuremath{\Phi}(t)\ensuremath{\approx}sin\ensuremath{\Phi}(t)$ and $\ensuremath{\Omega}(t)>\ensuremath{\gamma}$, where $\ensuremath{\Omega}(t)$ is the resonance Rabi frequency. The generalization of the two-level atom results to the NNN-level atom is discussed.
We theoretically study intensity-saturation effects in resonant harmonic generation and multiphoton ionization. A first laser excites a two-photon resonance which is coupled by a second laser to an autoionizing state. Starting from an effective Hamiltonian for the three resonant atomic states, we derive a set of equations for the density matrix of the gaseous medium and the electromagnetic field within a semiclassical framework. We present and discuss analytical and numerical solutions of these equations which show a variety of line profiles depending critically on the intensities of the incident laser pulses.