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 ${\text{5S}}_{1\mathrm{/}2}\to {\text{5D}}_{3\mathrm{/}2,5\mathrm{/}2}$ two-photon transitions excited in a rubidium atomic vapor cell. The transitions occur via step-wise excitation through the ${\text{5P}}_{1\mathrm{/}2,3\mathrm{/}2}$ 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 $\approx 925$ MHz and one with a repetition rate of $\approx 250$ MHz. The experimental spectra are compared to each other and to a theoretical model.