摘要
Phys. Rev. A 97, 032516 (2018) We present the first precise frequency measurement of the 2 ¹ S ₀ -3 ¹ D ₂two-photon transition in⁴ He at 1009 nm. The laser source at 1009 nm is stabilized on an optical frequency comb to perform the absolute frequency measurement. The absolute frequency of 2 ¹ S ₀ -3 ¹ D ₂transition is experimentally determined to be 594 414 291 803(13) kHz with a relative uncertainty of 1.6×10 ⁻¹¹which is more precise than previous determination by a factor of 25. Combined with the theoretical ionization energy of the 3 ¹ D ₂state, the ionization energy of the 2 ¹ S ₀state is determined to be 960 332 040 866(24) kHz. In addition, the deduced 2 ¹ S ₀and 2 ³ S ₁Lamb shifts are 2806.817(24) and 4058.8(24) MHz respectively which are 1.6 times better than previous determinations.