Lifetime Studies of Na(32P) and Tl(72S) Produced by Photodissociation and Quenched by Halogens

The time dependence of $\mathrm{Na}{\text{(3}}^2\mathrm{P)}$ fluorescence produced by pulsed, far‐ultraviolet photodissociation of NaI vapor at 600°C has been directly observed via the method of single‐photon counting. The experimental apparatus and procedure are described. Sodium iodide molecules photodissociate in a time much less than 1 nsec, and the fluorescence decay is exponential with $\text{τ\hspace{0.17em}=\hspace{0.17em}16.0\hspace{0.17em}±\hspace{0.17em}0.2}\mathrm{nsec}$ . No evidence of collision‐induced fluorescence is seen, and quenching of the fluorescence by NaI vapor itself is not observed. Quenching by iodine vapor is studied as a function of pressure, and I₂ molecules are shown to be at least 30 times more effective quenchers than I atoms. The velocity dependence of the I₂ cross section in the range ∼ 12 − 25 × 10⁴ cm/sec is studied by varying the photodissociation pulse wavelength. The fluorescence of $\mathrm{Na}{\text{(3}}^2\mathrm{P)}$ following photodissociation of NaBr and of $\mathrm{Tl}{\text{(7}}^2\mathrm{S)}$ following photodissociation of TlI are also observed. An average cross section for the quenching of $\mathrm{Na}{\text{(3}}^2\mathrm{P)}$ by Br₂ is reported. Both TlI and I₂ are observed to quench $\mathrm{Tl}{\text{(7}}^2\mathrm{S)}$ quite strongly, with I₂. having a cross section of ∼ 490 Ų at ∼ 4 × 10⁴ cm/sec. The quenching process is discussed, principally in terms of the “harpooning” mechanism. Comparison is made with analogous molecular beam experiments.