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JOURNAL OF ZHEJIANG NORMAL ONIVERSITY

Influences of polarization of lasers on efficiency of multi-step resonance excitation of complex atoms

Fang Da

Published Jan 1, 2000• 0 citations
OpenAlex

Abstract

Based on the method of expanding the wave function with eigenstates, the production of ions in the procedure of three step resonance excitation with polarized lasers were calculated theoretically under the condition that the final states of transitions were autoionizing states. The influences of polarization of lasers on the efficiency of multi step resonance excitation of complex atoms were discussed also.

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Based on the method of expanding the wave function with eigenstates, the production of ions in the procedure of three step resonance excitation with polarized lasers were calculated theoretically under the condition that the final states of transitions were autoionizing states. The influences of polarization of lasers on the efficiency of multi step resonance excitation of complex atoms were discussed also.
An improved method for the angular momentum measurement of autoionizing levels of complex atoms based on photoionization polarization spectroscopy is reported. In this method, a variation of the ion yield is measured as a function of the combination of the polarizations of the lasers used to excite the autoionizing level through a ladder of intermediate resonant states. Here, eight combinations of different polarizations in three lasers are used, which include not only circular but also linear polarizations. It has been confirmed that the angular momentum of the autoionizing level can be determined uniquely by relative ion-yield measurements for any combination of angular momenta of the levels of the excitation ladders by choosing appropriate intensities of the lasers.
Based on rate equation a theoretical study on laser power balance in a resonance multi-step ionization process of atoms has been presented in this paper.The calculation results made for a typical three-level atom show that there is an optimization laser power assignment when the total pumping laser power is limited,and under the optimization laser power assignment the ionization probability of a three-level atom can reach its maximum value.This conclusion is very different from that obtained under saturation excitation conditions.The method used here can be applied to more complicated multi-step ionization process of atoms.
A theoretical model of two-step resonant ionization which includes an autoionizing state is developed. The effects examined include interactions with a multimode laser and Zeeman and hyperfine multiplets. We approach this problem starting from first principles by treating the atom-radiation system as a unit. Theoretical analysis and numerical calculations of ionization probabilities are given. The intermode scattering is investigated in detail, and a simple physical interpretation of the dynamical effect is given. Comparisons of ionization probabilities for two-step processes with first-step excitations by single-mode or two-mode lasers are made. It is found that, if the mode spacing in a multimode-laser beam is less than the Doppler width of the atom, the multimode laser can more effectively produce ions than a single-mode laser does. This leads to the conclusion that the resonance effect is more important than the power-broadening effect. This is shown to be also true when Zeeman and hyperfine multiplets are included.
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.
The induction of higher polarization moments-the octupole orientation and the hexadecapole alignment—in the case of two-step excitation of atoms through dipole transitions and their manifestation in the polarization of dipole emission under conditions of anisotropic collisions are studied. For all channels of the two-step excitation process J 0→J 1 J, the efficiencies of production of higher polarization moments with integer and half-integer angular momenta from J=2 to J=7 through the absorption of linearly or circularly polarized light are calculated. The rates of collisional relaxation and interconversion of higher polarization moments, for both orientation and alignment, as functions of frequency detuning of the laser line from the center of the Doppler profile are calculated. The theory is illustrated by an example of two channels of excitation of the atomic state J=2 by circularly polarized light: J 0=0→J 1=1J=2 and J 0=3→J 1=2 (or 3)→J=2. In the first case, the octupole orientation enhances or attenuates the signal of circular polarization depending on the laser frequency detuning. In the second case, it represents the only source producing orientation and circular polarization of the emitted light.