摘要
We present precise frequency measurement of the 2S01-3D21 two-photon transition in He4 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 2S01-3D21 transition is experimentally determined to be 594 414 291.803(13) MHz with a relative uncertainty of 1.6×10-11, which is more precise than previous determinations by a factor of 25. In combination with the theoretical ionization energy of the 3D21 state, the ionization energy of the 2S01 state is determined to be 960 332 040.823(24) MHz. In addition, the deduced 2S01 and 2S13 Lamb shifts are 2806.864(24) MHz and 4058.130(24) MHz, respectively, which are 1.6 times better than previous determinations, and the fine structure 3D13-3D21 is determined to be 101 143.889(29) MHz, improving the precedent determination by a factor of 11.