Laser spectroscopy of low-lying states in atomic helium
Pei-Ling Luo1, Jin-Long Peng2, Li-Bang Wang1*, Jow-Tsong Shy1
1Physics Department, National Tsing Hua University, Hsinchu City, Taiwan
2Center for Measurement Standards, Industrial Technology Research Institute, Hsinchu City, Taiwan
* presenting author:Li-Bang Wang,
We report on our recent measurements of helium 2³P-3³D and 2³P-3¹D transitions using Doppler-free intermodulated fluorescence spectroscopy. Laser power of more than 1 watt at 588 nm was produced by frequency doubling of a Raman fiber amplifier using a MgO:PPLN crystal.
The absolute frequency of the 2³P-3³D transition is determined to a precision that is better than previous measurement by two orders of magnitude and the Lamb Shift of the 2³S state can then be deduced and compared to atomic calculations.
In addition, the spin-forbidden 2³P-3¹D transitions is measured for the first time. Combined with our former measurements on 2¹S-2¹P-3¹D states, this enables us to complete a self-consistent experimental determination of the helium low-lying levels connecting both the singlet and triplet states.

Keywords: Laser spectroscoy, Helium spectroscopy, Precision measurement