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Journal of Physics B Atomic and Molecular Physics

Comment on 'The effect of radiation trapping of high-intensity scattered radiation on multiphoton ionisation rates and resonance fluorescence'

Thomas J. Mcilrath, Thomas B. Lucatorto

Published Nov 14, 1980• 9 citations
Publisher

Abstract

Recent claims (Salter, ibid., vol.12, p.L763) have been made that the trapping of resonance radiation explains both the efficient ionisation of dense metal vapours irradiated by intense resonant laser radiation and previous experiments concerning anomalous behaviour of resonance fluorescence. The authors examine these claims and show that they are incorrectly developed and that basic physical arguments show that resonance trapping cannot explain the phenomena mentioned above.

DOI
10.1088/0022-3700/13/21/004

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Time-resolved fluorescence and population measurements in laser-pumped barium vapour

J. LEONARD BOWEN, Anne P. Thorne

Time-resolved measurements of fluorescence and level populations have been made on barium vapour illuminated by an intense pulse of laser radiation (1 MW cm -2 , 50 ns FWHM) tuned to the resonance transition of Ba I, 6s 2 1 S 0 -6s6p 1 P 1 , at 5535 AA. Barium number densities between 10 13 -10 15 cm -3 were investigated, using a buffer gas of helium at 100 Torr. Fluorescence was measured with two different geometrical arrangements, one allowing partial spatial resolution. In neither case did the observed fluorescence saturate with increasing laser power, although the maximum flux was more than sufficient to burn through the vapour column, as confirmed by transmission measurements. The power loss from the laser beam was found to be compatible with the scattered radiation from the pumped column in the absence of radiation trapping. In contrast to other similar experiments, no significant ionisation was detected (<5%). Time-resolved population measurements by anomalous dispersion on the 6s6p 3 P 2 and 6s5d 1 D 2 metastable levels yielded quenching cross sections for both Ba-Ba and Ba-He collisions, but gave anomalous result for the peak populations. A qualitative explanation of the fluorescence anomaly is proposed on the basis of the AC Stark effect, and it is shown that the same effect can inhibit the onset of stimulated emission to the metastable D levels.

Journal of Physics B Atomic and Molecular Physics•Jan 14, 1985•31 citations
Recent claims (Salter, ibid., vol.12, p.L763) have been made that the trapping of resonance radiation explains both the efficient ionisation of dense metal vapours irradiated by intense resonant laser radiation and previous experiments concerning anomalous behaviour of resonance fluorescence. The authors examine these claims and show that they are incorrectly developed and that basic physical arguments show that resonance trapping cannot explain the phenomena mentioned above.
The intensity spectrum of the scattered radiation in two-atom resonance fluorescence is investigated and compared with the one-atom spectrum. For intense fields the spectra are found to be proportional to one another. A conjecture is made for the NNN-atom spectrum.
The non linear interaction between an intense laser radiation and atoms leads to ionization through the absorption of N photons from the laser radiation via laser-induced virtual states. The multiphoton ionization rate varies as a function of the laser intensity I as IN. We discuss the two most important effects which govern multiphoton ionization processes : resonance effects and laser-coherence effects. In a moderate laser intensity range (107 - 109 W cm-2) corresponding to the two, three or four-photon ionization of atoms, resonance effects play a dominant role while the photon statistics of the laser radiation are relatively unimportant. On the contrary, in the very high intensity range (1012 - 1013 W cm-2) required to observe large Nth - order ionization processes, coherence effects play a dominant role through N! enhancement in the N-photon ionization rate, while resonance effects are completely damped due to the high laser intensity. We conclude by reviewing the possible applications of multiphoton absorption processes in other fields.
Time-resolved measurements of fluorescence and level populations have been made on barium vapour illuminated by an intense pulse of laser radiation (1 MW cm -2 , 50 ns FWHM) tuned to the resonance transition of Ba I, 6s 2 1 S 0 -6s6p 1 P 1 , at 5535 AA. Barium number densities between 10 13 -10 15 cm -3 were investigated, using a buffer gas of helium at 100 Torr. Fluorescence was measured with two different geometrical arrangements, one allowing partial spatial resolution. In neither case did the observed fluorescence saturate with increasing laser power, although the maximum flux was more than sufficient to burn through the vapour column, as confirmed by transmission measurements. The power loss from the laser beam was found to be compatible with the scattered radiation from the pumped column in the absence of radiation trapping. In contrast to other similar experiments, no significant ionisation was detected (<5%). Time-resolved population measurements by anomalous dispersion on the 6s6p 3 P 2 and 6s5d 1 D 2 metastable levels yielded quenching cross sections for both Ba-Ba and Ba-He collisions, but gave anomalous result for the peak populations. A qualitative explanation of the fluorescence anomaly is proposed on the basis of the AC Stark effect, and it is shown that the same effect can inhibit the onset of stimulated emission to the metastable D levels.