Abstract
We report a frequency measurement of the 21P 1 →31D 2 transition in 4He at 668 nm. Doppler-free saturation spectroscopy is performed in an rf discharged sealed-off cell using an external cavity diode laser stabilized by an optical frequency comb. The measured absolute frequency is 448 791 399.113 (268) MHz with a relative uncertainty of 6×10 - 10. Our result is more precise than the previous measurement by one order of magnitude. The ionization energy of the 21P 1 state using this result and the theoretical value of the 31D 2 state is derived to be 814 709 148.133 (269) MHz, in agreement with our previous determinations derived from other precisely measured transitions. We also derive the ionization energy of the 21S 0 state and the energy separation between 31D 2 and 33D 1 states using this result and other transitions. These determinations should be essential for comparison with the theoretical calculations. © 2013 American Physical Society.