title: Precision frequency metrology of helium 2(1)S(0)→2(1)P(1) transition.
abstract: We report a precision frequency measurement of the (4)He 2(1)S(0)→2(1)P(1) transition at 2058 nm. The saturated absorption spectroscopy is performed in a rf discharge sealed-off cell with a volume Bragg grating-based Tm:Ho:YLF laser. The absolute transition frequency measured using a fiber optical frequency comb is 145 622 892 822 (183) kHz with a relative uncertainty of 1.3×10(-9). Our result is ten times more precise than current best theoretical calculations and is in reasonable agreement with the calculated values. However, the ionization energy of the 2(1)P(1) state, derived from our result and other precisely measured transitions, shows a discrepancy of approximately 3.5σ with the most precise atomic theory. We have also determined the isotope shift between (3)He and (4)He to be 4248.7 (5.3) MHz, which is more precise than the previous measurement by one order of magnitude.
authors.affiliation: Institute of Photonics Technologies, National Tsing Hua University, Hsinchu 30013, Taiwan.
authors.forename: Pei-Ling
authors.initials: PL
authors.lastname: Luo
authors.forename: Jin-Long
authors.initials: JL
authors.lastname: Peng
authors.forename: Jow-Tsong
authors.initials: JT
authors.lastname: Shy
authors.forename: Li-Bang
authors.initials: LB
authors.lastname: Wang