100 MHz frequency comb for low-intensity multi-photon studies: intra-cavity velocity-map imaging of xenon.

We raise the power from a commercial 10 W frequency comb inside an enhancement cavity and perform multi-photon ionization of gas-phase atoms at 100 MHz for the first time, to the best of our knowledge. An intra-cavity velocity-map-imaging setup collects electron-energy spectra of xenon at rates several orders of magnitude higher than those of conventional laser systems. Consequently, we can use much lower intensities ${\sim}{{10}^{12}} \;{\rm W}/{{\rm cm}^2} $∼10