Velocity-selective direct frequency-comb spectroscopy of atomic vapors

We present an experimental and theoretical investigation of two-photon direct frequency-comb spectroscopy performed through velocity-selective excitation. In particular, we explore the effect of repetition rate on the 5S(1/2) -> 5D(3/2,5/2) two-photon transitions excited in a rubidium atomic vapor cell. The transitions occur via stepwise excitation through the 5P(1/2,3/2) states by use of the direct output of an optical frequency comb. Experiments were performed with two different frequency combs, one with a repetition rate of approximate to 925 MHz and one with a repetition rate of approximate to 250 MHz. The experimental spectra are compared to each other and to a theoretical model.