Experiment to Observe a Two-Photon Transition in Stored Hg +

The purpose of our experiment is to observe the two-photon transition between the fundamental state 5d106s2S1/2 and the metastable state 5d96s22D5/2 in stored Hg+. This transition is attractive as an optical frequency standard for two reasons. First, the radiative lifetime of the 2D5/2 state is about 0.1s and second, the first-order Doppler effect cancels for a two-photon absorption. Thus the transition line consists of a very narrow carrier and we can study the second-order Doppler effect (red shift and broadening, which is estimated to be a few kHz).