Precise measurement of the hyperfine splittings within the 6p$_{3/2}$ level of atomic indium using two-color diode laser spectroscopy.

The hyperfine splittings of the 6P$_{3/2 }$state of indium(I=9/2) have been measured for the first time using a two-step, two-color excitation scheme. These results provide a precise experimental test of new \textit{ab initio} wavefunction calculations of three-valence-electron atomic systems such as indium and thallium. We first excite ground-state atoms in a heated quartz indium cell to the intermediate 6S$_{1/2}$ state using a blue (GaN) diode laser at 410 nm. By measuring the differential atomic vapor cell absorption of double-passed, second-order-diffracted beams from an acousto-optic modulator, we are able to stabilize the blue laser frequency to the sub-MHz level [M. Gunawardena et al., Rev. Sci. Instrum. 79. 103110 (2008)]. A second laser beam at 1291 nm overlaps the first in the vapor cell, exciting Doppler-narrowed hyperfine transitions to the 6P$_{3/2}$ excited state. By modulating the blue laser beam and using lock-in detection, we obtain background-free and Dopper-free IR spectra. By locking first to one then the other intermediate hyperfine level, we determine all three hyperfine splittings, as well as the A, B, and C hyperfine constants, for the 6P$_{3/2}$ excited state. Our experimental value for the `A' hyperfine constant agrees with the published theoretical value to within 2{\%}. Details of the measurement and analysis will be presented.