Level-Crossing Spectroscopy in $7{}_{}{}^2{}_{}{}^{}S$ Thallium. I. Studies of Coherence-Narrowing, Collision-Broadening, and Buffer-Gas Effects

The lifetime, coherence narrowing, and collision broadening of the $7{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ excited state of atomic thallium has been studied by the technique of zero-field level-crossing spectroscopy. The lifetime of the $7{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ state is determined to be 7.55 (0.08) nsec. The full coherence-narrowing effect gave a branching ratio, from $F=0\mathrm{}$ of $7{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ to the ground state, of 43%. The cross section for thallium-thallium resonance-broadening collisions is ${\sigma }_{\mathrm{T}\mathrm{l}-\mathrm{T}\mathrm{l}}=\frac{5.38\left(0.49\right)\mathrm{}\times {10}^{-8\mathrm{}}}{\overline{v}}$ ${\mathrm{cm}}^2$. The effect of 426-torr helium on the thallium lifetime, coherence narrowing, and resonance broadening was also investigated. From the low-thallium-density region the cross section for helium depolarization of the thallium $7{}_{}{}^2{}_{}{}^{}S$ state is ${\sigma }_{\mathrm{T}\mathrm{l}-\mathrm{H}\mathrm{e}}\cong \frac{7\times {10}^{-13\mathrm{}}}{\overline{v}}$ ${\mathrm{cm}}^2$. There is also indication of a buildup of population in the metastable $6{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ thallium state. From the data, the cross section for the process $\mathrm{Tl}\left(6\mathrm{}{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\right)\to \mathrm{Tl}\left(6\mathrm{}{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}\right)$ by helium collisions is $\sigma \simeq 1\times {10}^{-21\mathrm{}}$ ${\mathrm{cm}}^2$. The Tl branching ratio in the presence of helium is 39%. It was also found that the thallium-thallium resonance-broadening cross section is ${\sigma }_{\mathrm{T}\mathrm{l}-\mathrm{T}\mathrm{l}}=\frac{4.04\mathrm{}\left(0.32\right)\mathrm{}\times {10}^{-8\mathrm{}}}{\overline{v}}$ ${\mathrm{cm}}^2$ in the presence of 426 torr of helium. In the presence of 113 torr of He, the resonance-broadening cross section was found to be ${\sigma }_{\mathrm{T}\mathrm{l}-\mathrm{T}\mathrm{l}}=\frac{5.22\mathrm{}\left(0.13\right)\mathrm{}\times {10}^{-8\mathrm{}}}{\overline{v}}$ ${\mathrm{cm}}^2$. In the case of Ar as the buffer gas, ${\sigma }_{\mathrm{T}\mathrm{l}-\mathrm{A}\mathrm{r}}=\frac{8\times {10}^{-13\mathrm{}}}{\overline{v}}$ ${\mathrm{cm}}^2$.