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Physica Scripta

Spectral Variation of Collisional Ionization of Excited Potassium Atoms

Leslie Pendrill, M Pettersson, Ulf L. Österberg

Published Apr 1, 1984• 2 citations
Publisher

Abstract

A comparison of the spectral variation of ions, produced in a thermal vapour of potassium irradiated with resonant laser light, with the absorption spectral profile of the vapour confirms earlier conclusions that ionization upon collision of two excited atoms in the 4P state is the dominant ionization process, at least in the spectral line wings. No evidence of contributions from supereleastically-heated electrons to the ionization is found. Strong spatial distortions of the laser beam transmitted by the vapour (self-focussing and anomalous conical emission) are observed and discussed in relation to the ion production.

DOI
10.1088/0031-8949/29/4/006

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A comparison of the spectral variation of ions, produced in a thermal vapour of potassium irradiated with resonant laser light, with the absorption spectral profile of the vapour confirms earlier conclusions that ionization upon collision of two excited atoms in the 4P state is the dominant ionization process, at least in the spectral line wings. No evidence of contributions from supereleastically-heated electrons to the ionization is found. Strong spatial distortions of the laser beam transmitted by the vapour (self-focussing and anomalous conical emission) are observed and discussed in relation to the ion production.
Femtosecond (fs) laser propagation and fluorescence of dense potassium vapor was studied, and the spectral region around the first and the second doublets of the principal series lines of potassium atoms was investigated. In our search we did not observe the conical emission in the far field, although it was previously observed in the case of rubidium. We discuss the possible reason of this unexpected result. The fluorescence spectrum revealed Rb impurity resonance lines in emission due to the collisional redistribution from the K(4p) levels into the Rb(5p) levels. In the forward propagation of 400 nm femtosecond light we observed the molecular band red shifted from potassium second doublet. However, no molecular spectrum was observed when the mode-locked fs laser light was discretely tuned within the wings of the first resonance lines, at 770 nm.
Two investigations are reported on the application of resonantly enhanced multiphoton ionization (RMPI) to gaseous mixtures of an atomic vapor and a few Torr of argon. Photon fluxes large enough to ionize atoms by RMPI were produced by focussing down the light from a tunable pulsed dye laser. The irradiances generated were of the order of 500MWcm⁻². Ionization was detected by a voltage biased wire electrode that simply collected the photoelectrons either directly or after some gas multiplication. One investigation was the measurement of the dependence of RMPI processes in mercury on the polarization of the incident light for comparisons with theoretical calculations. The processes were four-photon resonant absorptions to either a ¹S₀ or ¹’³D₂ level followed by single photon ionization. Complete photoionization of all atoms excited to the resonant levels is established and the measured polarization dependences are found to agree with the calculated polarization dependence for the resonant excitation step of the RMPI process. Unexplained observations of the distortions in the polarization dependence of the ion yield and the absorption linewidth of the 6d ¹D₂ resonance are discussed. The second investigation was a study of the density dependent electric dipole forbidden two-photon resonant transition ²S → ²P in three-photon RMPI spectra of the Rydberg states of potassium. Stark interactions are shown to be unlikely and too weak. From the characteristics of the spectra, the excitation process is identified as a laser-assisted collision interaction.
The production of Na+{\mathrm{Na}}^{+}Na+ and Na2{\mathrm{Na}}_{2}Na2​+{\mathrm{}}^{+}+ in sodium vapor irradiated by pulsed laser light in the D-line region of the spectrum has been studied by varying the laser power density and the wavelength. The data, which were acquired with an order-of-magnitude higher resolution than those in our previous experiments, support our earlier conclusion that ions formed at laser wavelengths close to, but not coincident with, the D lines result from three-photon resonance-enhanced ionization of Na2{\mathrm{Na}}_{2}Na2​ molecules present in the vapor.
We have observed the dominant wing spectroscopy of energy pooling collision near the boundary layer involving Cs atoms under the condition of moderate-to-high optical depths at line-centre. It appears from our experimental investigations that the energy-pooling fluorescence presents about 16 spectral lines, and all the lines can be assigned to the Cs atomic transitions. We find that all lines of the energy-pooling retrofluorescence from the heated Cs atomic vapour cell show two-peak profiles. In addition, its pumping power linear dependence in the energy pooling process has been measured and analysed.
It is shown that by proper use of the excitation spectra, the emission spectra of the transition and the far-field patterns it is possible to distinguish between competing processes involved in the two-photon excitation to the 6S state of potassium, namely: (a) resonant fluorescence emission associated with the cascading spontaneous emission (ASE) and (b) the four-wave mixing parametric process. The effect of the buffer gas on the intensities of the spectra of the radiations emitted is discussed. Conical emission and spatial instabilities of the excitation radiation and the emission were also observed for high excitation and high vapour pressure of potassium.
It has been shown that a dense (>10(14) -cm(-3)) atomic vapor, irradiated by a saturating pulse of resonance radiation, will ionize on a time scale of <10(-6) sec. The ionization can be 95% complete and has been observed in Li, Na, Ca, Sr, and Ba. A large number of physical processes contribute to the ionization with different processes dominating at different stages in the ionization. These processes are discussed, and two models for calculating the ionization are described. The results of the various experiments and the applications to the spectroscopy of ions and to atomic physics studies in general are reviewed.