Spectroscopic observation of the highest vibrational level of the HD+(2Σ+) molecule

Spectroscopic observation of the highest vibrational level of HD+(v=21) in its ground electronic state is reported. Calculations of constants from the resolved hyperfine structure allow unambiguous determination of the Fermi contact parameters for the levels 17,1 and 21,0, showing that in the level 21,0 the electron distribution is asymmetric toward the lower dissociation limit H++D, with an average electron density of 0.094 at the proton and 0.902 at the deuteron.