A new precise value of the absolute 2

The absolute frequency $\nu_0$ of the optical electric-dipole transition between the metastable 1s2s 3S_1, F=5/2 and the short-lived 1s2p 3P_2, F=7/2 hyperfine structure sublevels of the helium-like $\rm ^7Li^+$ ion in its rest frame was determined by combining Doppler-free saturation spectroscopy and laser heterodyning. One of two cw single-mode dye lasers was locked to the calibrated w-component of the [R(85) 26-0]-line in the (B-X)-system of the $^{127}{\rm I}_2$-molecule at $\nu_R=546 462 926.790(71)$ MHz. While crossing two counterpropagating beams of the other laser perpendicularly (lab angle $\alpha \approx90^\circ$) with a low-velocity ion beam and tuning its frequency over the $\rm Li^+$ transition under study, the distance of the Lamb-dip center frequency $\nu_L$ from $\nu_R$ was obtained by mixing both laser frequencies with an avalanche diode and registering their beat frequency at about 4 GHz with a quartz-stabilized high-frequency counter. The Lamb-dip position $\nu_L$ was measured for different ion velocities β (in units of the velocity of light) in the range of $(3{-}4)\times 10^{-4}$. The parameters $\nu_0$ and $\cos \alpha$ of the relativistic Doppler formula $\nu_0=\nu_L\gamma(1-\beta \cos \alpha)$ were extracted via a fit to the experimental data set $(\beta, \nu_L)$, providing $\nu_0 = 546 466 916.49(87)$ MHz and $\cos \alpha =4.580(0.045)\times 10^{-4}$. This result of $\nu_0$ differs noticeably from $\nu_0=546 466 918.79(40)$ MHz, measured by another group in a collinear-beam configuration at an ion beam propagating with $\beta=6\times 10^{-3}$