High-Precision Optical Measurement of the $2S$ Hyperfine Interval in Atomic Hydrogen

We have applied an optical method to the measurement of the $2S$ hyperfine interval in atomic hydrogen. The interval has been measured by means of two-photon spectroscopy of the $1S-2S$ transition on a hydrogen atomic beam shielded from external magnetic fields. The measured value of the $2S$ hyperfine interval is equal to 177 556 860(16) Hz and represents the most precise measurement of this interval to date. The theoretical evaluation of the specific combination of $1S$ and $2S$ hyperfine intervals $D_{21}$ is in fair agreement (within $1.4\sigma$) with the value for $D_{21}$ deduced from our measurement.

Figure 1

AOM frequencies corresponding to the singlet (circles) and triplet (squares) transitions. The frequency dependence of the data is due to the cavity drift. The linear fit with the same slope of four data runs is shown.

Figure 2

Averaged results of measurements of the $2S$ hyperfine interval at different background gas pressures (logarithmic pressure scale, error bars give statistical error). The cross represents a measurement with doubled hydrogen flow. a, nine days of measurement; b, four days; c, two days; d, one day.

Figure 3

$D_{21}$ values corresponding to the several $2S$ hyperfine interval measurements. The dashed lines represent the error bar of the theoretical value.