Measurement of the 10$p$ fine structure interval of lithium

The fine structure interval of the 10$p$ atomic state of 7Li has been measured by laser spectroscopy. Our result of 74.97 (74) MHz for the 10$p$ interval has a precision five times higher than previous measurements of fine structure intervals of Rydberg lithium $p $states. It also provides an experimental value for the only $n $= 10 fine structure interval which is yet to be calculated theoretically and therefore provides a benchmark for such a calculation. In our experiment a beam of lithium atoms is excited by a total of four grating-stabilized diode lasers. The excitation to the 10$p$ state proceeds via the 2$p$ and 3$s$ intermediate states. Three of the lasers are frequency locked to their respective optical transitions and the fourth is scanned across the 10$p$ fine structure components. Optical sidebands imprinted on this fourth laser provide a calibration for the scan and allow a determination of the fine structure interval.