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Applied Optics

Laser excitation and ionization of dense atomic vapors

Thomas B. Lucatorto, Thomas J. Mcilrath

Published Dec 1, 1980• 110 citations
Publisher

Abstract

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.

DOI
10.1364/ao.19.003948

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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.
We report on the time behavior of ionization signals produced by laser excitation of Ca and Ba atomic vapor to high-Rydberg states. A space-charge-limited thermionic diode detector shows a long-lived (>I-msec) ionization signal. However, optical detection of atomic ions (Ca+, Ba+) shows that these species live for much shorter times (<100 microsec). These results, in conjunction with published results on mass-spectrometric studies of high-density atomic beams, suggest that our ionization signal is primarily due to molecular species (Ca2+, Ba2+). We also observed optically pumped amplified spontaneous emission and stimulated electronic Raman scattering in Ca+ and Ba+.
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A model of multiple resonances between groups of discrete and autoionizing levels is proposed for the description of the process of multiphoton ionization. The phenomena of the extremely rapid broadening and growth of amplitudes of the multiphoton resonances, observed experimentally in the yield of Sr+ and Ba+ ions [ J. Phys. B.14, 2253– 2359 ( 1981);Elementary Processes in Laser Radiation Field ( 1988), pp. 122– 153)] are explained on the basis of this model. The strong-coupling method was used in numerical calculations for Sr and Ba atoms in a strong laser field.
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.
We have observed ion formation (both Na+{\mathrm{Na}}^{+}Na+ and Na2+{\mathrm{Na}}_{2}^{+}Na2+​) and radiation from high-lying Na states when low-density vapor (\ensuremath{\sim}1013{10}^{13}1013 ${\mathrm{cm}}^{\ensuremath{-}3}$) is irradiated with a cw dye laser tuned to one of the DDD lines. The Na+{\mathrm{Na}}^{+}Na+ ions probably result from photodissociation of Na2+{\mathrm{Na}}_{2}^{+}Na2+​ formed by $\mathrm{Na}(3p)\ensuremath{-}\mathrm{Na}(3p)$ associative ionization, while the high-lying radiating states are formed in energy-pooling reactions.