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The Journal of Chemical Physics

Cross sections for production of atomic thallium states in photodissociation of thallium halides

D. A. Scott, James A. Piper

Published Apr 15, 1986• 4 citations
Publisher

Abstract

The cross sections for production of 62P1/2 and 62P3/2 ground and metastable atomic thallium states via UV dissociation of the thallium halides TlCl, TlBr, and TlI have been measured at wavelengths 193, 222, 249, 308, and 353 nm using time-resolved atomic resonance absorption spectroscopy. The measured cross sections, which were independent of halide densities up to densities ∼1017 cm−3, were found to represent only 10% to 36% of the total UV absorption cross section at the appropriate wavelength.

DOI
10.1063/1.450052

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The cross sections for production of 62P1/2 and 62P3/2 ground and metastable atomic thallium states via UV dissociation of the thallium halides TlCl, TlBr, and TlI have been measured at wavelengths 193, 222, 249, 308, and 353 nm using time-resolved atomic resonance absorption spectroscopy. The measured cross sections, which were independent of halide densities up to densities ∼1017 cm−3, were found to represent only 10% to 36% of the total UV absorption cross section at the appropriate wavelength.
Decay rates of the 6 2P3/2 metastable and 6 2P1/2 ground state atomic thallium densities produced in UV photodissociation (at λ193, 222, 249, 308, and 353 nm) of TlI, TlBr, and TlCl molecular vapors have been measured for vapor densities from 1013 to 1017 cm−3. Dominant relaxation processes have been identified as (i) for the metastable state, collisional deexcitation by thallium halide molecules at high halide densities (≥1015 cm−3) and by free halogens and trace impurities at lower halide densities and (ii) for the ground state, diffusion in the ambient (argon/thallium halide) gas at high halide densities and volume reassociation under certain conditions at low halide densities. Cross sections and diffusion coefficients are evaluated.
The UV absorption spectra of thallium halide vapors consist of a series of peaks corresponding to various photodissociation channels leading to the formation of different states of the product atoms.1 There has been renewed interest in determining the yields of different states of the metal atom produced by photodissociation of the thallium halides, particularly with a view to establishing conditions for selective production of metastable excited states for anti-Stokes Raman shitting2 and other purposes.
Results of experiments investigating the influence of resonance radiation trapping on the power and efficiency of the atomic thallium photodissociation laser are reported. Transverse pumping of a thallium iodide cell with added free metallic thallium has enabled conversion efficiencies from the ArF laser pump beam of greater than 7% to be achieved on a single Tl laser transition at λ 535.0 nm. Collisional deexcitation of the 72S1/2 upper laser level imposes a limit on the TlI densities which can be employed.
Photodissociation of TlBr and CsBr by the 1850-Å mercury resonance line could lead to laser transitions at 5350 Å and 2.93 μ in Tl and Cs, respectively. The transition probabilities of Cs are such that radiative decay will maintain a population inversion between the 72P3/2 and 72S½ levels. Calculations indicate that a gain of 4% per m is expected at a temperature of 890° for photodissociation into the 72P3/2, ½ Cs levels. In Tl, however, the 62P3/2 level is metastable. A suitable quenching gas must be found before oscillation is possible at 5350 Å from photodissociation into the 72S½ level. Nitrogen and CO2 have been tried thus far without success.