The two-photon transitions 4S(1/2) --> 6S(1/2) and 4S(1/2) --> 4D(5/2) in a
tomic potassium have been observed free of the first-order Doppler effect.
The rate for the S-D transition, which has been detected for the first time
, is two orders of magnitude lower than that for the S-S transition; the ob
served signal to noise ratios for the S-S and S-D transitions were respecti
vely >200 and similar to3, The laser excitation frequencies are located in
the near vicinity of 1/6 of the 1S-2S transition interval of hydrogen, deut
erium and the hydrogen-like atom, muonium. These potassium transitions may
therefore provide a suitable frequency transfer standard for all these syst
ems and a useful link between them.