A new precise value of the absolute , , transition frequency in

Abstract:

The absolute frequency of the optical electric-dipole transition between the metastable 1 s 2 s ³ S1, F =5/2 and the short-lived 1 s 2 p ³ P2, F =7/2 hyperfine structure sublevels of the helium-like 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 -molecule at MHz. While crossing two counterpropagating beams of the other laser perpendicularly (lab angle ) with a low-velocity ion beam and tuning its frequency over the transition under study, the distance of the Lamb-dip center frequency from 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 was measured for different ion velocities (in units of the velocity of light) in the range of . The parameters and of the relativistic Doppler formula were extracted via a fit to the experimental data set , providing MHz and . This result of differs noticeably from MHz, measured by another group in a collinear-beam configuration at an ion beam propagating with .