Towards optical frequency metrology of the electron-to-proton mass ratio

We are setting up a vibrational two-photon spectroscopy experiment on trapped H₂ ions. It has been shown that such an experiment is feasible on transitions between the v=0 and v=1 levels in the 9-10 mum range, using a low-power laser source matched in a high-finesse Fabry-Perot cavity and interaction times of a few tens of milliseconds. One difficulty is the detection of the two-photon transition, because the excited level is metastable; it can be solved by using photo-dissociation by a KrF laser at 248 nm, which will selectively photo-dissociate the vges1 vibrational levels. This laser can also be used to optimize the preparation of the H₂ ⁺ ion cloud in v=0 level. The first observation of a two-photon line of metrological interest is expected in the near future.