Measurement of hyperfine structure and isotope shifts in the $8p$ excited states of thallium and the $7p$ excited states of indium using two-step laser spectroscopy*

Using a two-color, two-step vapor cell spectroscopy technique we have completed measurements of hyperfine splittings of 8$p$ states in $^{205}$Tl and $^{203}$Tl, as well as the $7s-8p$ transition isotope shifts. The same experimental scheme has been used to measure the hyperfine splitting of the $^{115}$In 7$p_{1/2}$ state and the hyperfine $a, b, c$ constants within the 7$p_{3/2}$ state. An external-cavity diode laser locked to the 1st step transition excites atoms to an intermediate state and a second, red laser diode overlaps the first within a heated atomic vapor cell in both a co-propagating and counter-propagating configuration. Analysis of subsequent Doppler-free absorption spectra of the second-step transitions (7$s_{1/2}$ $\rightarrow$ 8$p_{1/2,3/2}$ in thallium and 6$s_{1/2}$ $\rightarrow$ 7$p_{1/2,3/2}$ in indium) allows us to extract both hyperfine and isotope shift information with uncertainties well below 1 MHz. Frequency modulation of the red laser provides convenient \textit{in situ} frequency calibration. For the case of thallium $^{205}$Tl 8$p_{3/2}$ state hyperfine splitting, our results disagree with older measurements and show a well-resolved hyperfine anomaly not previously observed for this state.