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Physical Review A

Off-resonance production of ions in laser-excited sodium vapor

Charles E. Burkhardt, W. P. Garver, Jacob J. Leventhal

Published Jan 1, 1985• 13 citations
Publisher

Abstract

The production of ${\mathrm{Na}}^{+}$ and ${\mathrm{Na}}_{2}$${\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 ${\mathrm{Na}}_{2}$ molecules present in the vapor.

DOI
10.1103/physreva.31.505

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Open Physics•Apr 10, 2008•4 citations
The production of Na+{\mathrm{Na}}^{+}Na+ and {\mathrm{Na}}_{2}$${\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.
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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.
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.
The recently observed efficient ionization of sodium vapor by a laser tuned to the sodium resonance line can be explained in terms of the large population of resonance-state atoms created by laser-induced radiative equilibrium. This large population of excited atoms represents (a) a reservoir of energy that can rapidly be transferred to the free electrons via superelastic collisions and (b) a source of pseudo-ground- state atoms possessing a much reduced ionization energy. Three-photon ionization is proposed as the mechanism for creation of the primary pool of free electrons.
TPHE purpose of this investigation was to study the resonance spec--*-trum of sodium vapor when the resonance was excited by a single D line.In a paper on " Resonance Radiation of Sodium Vapor "* published by one of us in 1905, it was shown that if a bulb containing pure sodium vapor was illuminated by light from a sodium flame, the vapor emitted a yellow light which spectroscopic analysis showed to be identical with the exciting light, in other words, the two D lines.It was suggested at this time that it would be interesting to see if both D lines appeared when the vapor was illuminated by a single D line.This would determine whether the mechanisms that give rise to the D lines are separate or in some way connected.In 1914 this experiment was tried by Wood and Dunoyer. 2 This experiment was made possible by the discovery of a polarization method of separating close spectral doublets that eliminated the great loss of light involved in a high-power monochromator, but even with this method the resonance light is very faint.The spectrum of resonance excited by D 2 was photographed with exposures varying from 10 to 15 hours.The majority of the plates showed only the D 2 line, but owing to under exposure Di would not have been visible if it was less than one fourth of the intensity of D 2 .Some plates indeed showed both Di and D 2 , but the presence of Di in the exciting light was judged to be the cause of this.Imperfections in the optical system made it impossible to entirely remove Di from the exciting light and as it was sensitive to temperature changes the nature of the transmitted light sometimes changed during an exposure.It was concluded that sodium resonance excited by D 2 consisted of D 2 alone, but the results admit of two other possibilities.Di light may always be present though much fainter than D 2 , or it may only appear under certain conditions.Other resonance phenomena show that the radiation centers in sodium are not entirely independent.One of us 3 showed in 1905 that excitation 1
Abstract We have studied the formation of the molecular ion Rb2+ and the atomic ion Rb+. These are created in laser excited rubidium vapor at the first resonance, 5s–5p and 5p-nl transitions. A theoretical model is applied to this interaction to explain the time evolution and the laser-power dependence of the population density of Rb+ and Rb2+. A set of rate equations which describe: the temporal variation of the population density of the excited states; the atomic ion density; and the electron density, were solved numerically under the experimental conditions of Barbier and Cheret. In their experiment the Rb concentration was 1×1013cm−3 and the laser power was taken to be 50–500 mW at vapor temperature = 450 K. The results showed that the main processes for producing Rb2+ are associative ionization and Hornbeck-Molnar ionization. The calculations have also showed that, the atomic ions Rb+ are formed through the Penning Ionization (PI) and photoionization processes. Moreover, a reasonable agreement between the experimental results and our calculations for the ion currents of the Rb+ and Rb2+ is obtained.