Sensitivity of the isotopologues of hydronium to variation of the   electron-to-proton mass ratio
  We study the sensitivity of the microwave and submillimeter transitions of the isotopologues of hydronium to the variation of the electron-to-proton mass ratio mu. These sensitivities are enhanced for the low frequency mixed inversion-rotational transitions. The lowest frequency transition (6.6 GHz) takes place for isotopologue H2DO+ and respective sensitivity to mu-variation is close to 200. This is about two orders of magnitude larger than the sensitivity of the inversion transition in ammonia, which is currently used for the search of mu-variation in astrophysics. 
M. G. Kozlov, S. G. Porsev, and D. Reimers
physics.atom-ph astro-ph.CO physics.space-ph