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Measurement of 7p$_{1/2}$-state hyperfine structure and 7s$_{1/2}$-7p$_{1/2}$ transition isotope shift in $^{203}$Tl and $^{205}$Tl

A two-step, two-color laser spectroscopy technique has been used to measure the hyperfine splitting of the 7p$_{1/2}$ excited state in $^{203}$Tl and $^{205}$Tl, as well as the isotope shift within the 7s$_{1/2}$ - 7p$_{1/2}$ transition. Our measured values for the hyperfine splittings, 2153.2(7) MHz (in $^{203}$Tl) and 2173.3(8) MHz (in $^{205}$Tl), each differ by 20 MHz from previously published values which quoted comparable precision. The transition isotope shift of $^{203}$Tl relative to $^{205}$Tl was measured to be 534.4(9) MHz. In our experiment, one laser was locked to the thallium ground-state 6p$_{1/2}$ - 7s$_{1/2}$ 378 nm transition, while the second, spatially overlapping laser was scanned across the 7s$_{1/2}$(F=1) - 7p$_{1/2}$(F=0,1) hyperfine transitions. To facilitate accurate frequency calibration, radio-frequency modulation of the laser was used to create sidebands in the absorption spectrum.