Absolute-frequency measurements of theD2line and fine-structure interval inK39

Abstract

We report a value for the ${D}_{2}$-line frequency of $^{39}\mathrm{K}$ with $0.28\phantom{\rule{0.3em}{0ex}}\mathrm{ppb}$ uncertainty. The frequency is measured using an evacuated ring-cavity resonator whose length is calibrated against a reference laser. The ${D}_{2}$ line presents a problem in identifying the line center because the closely spaced energy levels of the excited state are not resolved. We use computer modeling of the measured spectrum to extract the line center and obtain a value of $391\phantom{\rule{0.2em}{0ex}}015\phantom{\rule{0.2em}{0ex}}578.040(110)\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. In conjunction with our previous measurement of the ${D}_{1}$ line, we determine the fine-structure interval in the $4P$ state to be $1\phantom{\rule{0.2em}{0ex}}729\phantom{\rule{0.2em}{0ex}}997.132(120)\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. The results represent significant improvement over previous values.