We have measured the hyperfine splitting and level isotope shifts (LIS) of the ${(6s6p}^2)$ ${}^4{P}_{1/2}$ level of ${}^{205}\mathrm{Tl}$ and ${}^{203}\mathrm{Tl}$ using laser absorption spectroscopy in a thallium oven near 1373 K. We determine that the hyperfine splitting between the $F=1\mathrm{}$ and 0 levels of the ${}^4{P}_{1/2}$ state in ${}^{205}\mathrm{Tl}$ is $75.179\pm 0.087\mathrm{GHz}.$ The $F=0\mathrm{}$ LIS is $-7.333\pm 0.025\mathrm{GHz}$ and the $F=1\mathrm{}$ LIS is $-6.500\pm 0.030\mathrm{GHz},$ with the ${}^{205}\mathrm{Tl}$ level below the ${}^{203}\mathrm{Tl}$ level in both cases. We use our results together with other data to analyze the configuration mixing of the ${}^4{P}_{1/2}$ state with the nearby ${(6s}^{2}10s)$ ${}^2{S}_{1/2}$ state. The mixing inferred from the hyperfine results is consistent with that inferred from the isotope shift results.

• Received 12 November 2002

DOI:https://doi.org/10.1103/PhysRevA.68.032504