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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Two-photon resonant three-photon ionization and atomic energy-pooling-induced ionization in potassium vapor

Changyong Li, Liantuan Xiao, Qian Li, Suotang Jia, Guosheng Zhou

Published Sep 8, 1999• 0 citations
Publisher

Abstract

The decay curves of potassium ion produced by Two-Photon Resonant Three-photon Ionization were measured. From the relation between ion current and multi-photon dynamic parameter, photo-ionization cross-section of 6S state of atomic potassium was determined. Potassium atom was ionized by Energy Pooling Effect when excited wavelength was 766.4 nm or 769.8 nm corresponding to the first excited state of potassium atom.

DOI
10.1117/12.361142

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The decay curves of potassium ion produced by Two-Photon Resonant Three-photon Ionization were measured. From the relation between ion current and multi-photon dynamic parameter, photo-ionization cross-section of 6S state of atomic potassium was determined. Potassium atom was ionized by Energy Pooling Effect when excited wavelength was 766.4 nm or 769.8 nm corresponding to the first excited state of potassium atom.
Atomic potassium is ionized by three channels when it is excited by a pulse dye laser. They are energy pooling induced ionization, 4S→6S and 4S→4D two photon resonant three photon ionization. The ionization current intensity at various vapor temperature and laser intensity were investigated for the former two ionization channels. The atomic radiation trapping and nonlinear ionization were found in ionization current spectra. The qualitative explanations for above phenomena are given.
The intensity and polarization dependence of the ion production due to three quantum ionization was investigated in potassium. The energy of two of the photons was tuned into resonance with n 2 D levels for 4 < n < 22. The ratio of the ion signals produced with circular polarization to that found with linear polarization was significantly less than that predicted by a single rate approximation. The discrepancy is explained by the increased linewidth of the resonant 2 D level due to the ionization process itself. Single quantum photoionization cross sections for the excited levels calculated by means of quantum defect theory are shown to be consistent with the experimental results.
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We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.