Spectroscopic study of the cycling transition $4s[3/2]{}_2$–$4p[5/2]{}_3$ at 811.8 nm in ${}^{39}\mathrm{Ar}$: Hyperfine structure and isotope shift

Doppler-free saturated absorption spectroscopy is performed on an enriched radioactive ${}^{39}\mathrm{Ar}$ sample. The spectrum of the $3s^{2}3p^{5}4s[3/2]{}_2$–$3s^{2}3p^{5}4p[5/2]{}_3$ cycling transition at 811.8 nm is recorded, and its isotope shift between ${}^{39}\mathrm{Ar}$ and ${}^{40}\mathrm{Ar}$ is derived. The hyperfine coupling constants $A$ and $B$ for both the $4s[3/2]{}_2$ and $4p[5/2]{}_3$ energy levels in ${}^{39}\mathrm{Ar}$ are also determined. The results partially disagree with a recently published measurement of the same transition. Based on earlier measurements as well as the current work, the isotope shift and hyperfine structure of the corresponding transition in ${}^{37}\mathrm{Ar}$ are also calculated. These spectroscopic data are essential for the realization of laser trapping and cooling of ${}^{37,39}\mathrm{Ar}$.

Figure 1

Simplified energy level diagram of argon including the hyperfine levels for the spin 7/2 isotope ${}^{39}\mathrm{Ar}$. $4s-4p$ ($F=11/2\to F^{\prime }=13/2$) is the “cycling transition” to be used for laser trapping and cooling of metastable argon atoms.

Figure 2

A schematic of the saturated absorption spectroscopy setup. The 1-m-long spectroscopy quartz cell is filled with a mixture of an enriched ${}^{39}\mathrm{Ar}$ sample (${}^{39}\mathrm{Ar}{/}^{40}\mathrm{Ar}~1.5\times {10}^{-3}$, $~0.5$ Torr partial pressure) and natural krypton ($~2$ mTorr partial pressure).

Figure 3

Frequency-modulated saturated absorption spectroscopy data of the $4s[3/2]{}_2$–$4p[5/2]{}_3$ transition in ${}^{39}\mathrm{Ar}$ (solid line). The frequency range from $f_{\mathrm{Beat}}=-300\mathrm{\hspace{0.5em}}\mathrm{to}\mathrm{\hspace{0.5em}}200\mathrm{\hspace{0.5em}}\mathrm{MHz}$ is excluded from the scan because of the large amount of noise introduced by the absorption signal of the abundant ${}^{40}\mathrm{Ar}$. The absorption peaks of the two minor stable isotopes ${}^{36}\mathrm{Ar}$ and ${}^{38}\mathrm{Ar}$ and of the cycling transition in the radioactive ${}^{39}\mathrm{Ar}$ exhibit the expected dispersion-type signal shape. Additional hyperfine transitions and cross-over signals of ${}^{39}\mathrm{Ar}$ identified in the data are indicated by the vertical solid and dashed markers, respectively. The dotted line shows a global fit to the data.