Rotational-state-selective field ionization of triplet molecular Rydberg states of H$_{\mathrm{2}}$

We present rotational-state-selective field ionization spectra of highly excited triplet Rydberg states of molecular hydrogen. The experimental technique employs a fast 6 keV Rydberg molecular beam and a tunable static electric field of up to 40 kV/cm. The metastable c $^{\mathrm{3}}\prod _{\mathrm{u}}^{\mathrm{-\thinspace }}$ 2p$\pi $ state, prepared by electron capture of 6 keV H$_{\mathrm{2}}^{\mathrm{+}}$ ions in potassium vapor, is excited by a frequency doubled tunable dye laser to access the v$=$0 R(1)nd1 (n$=$21-27) Rydberg series. In each Rydberg state's field ionization spectra we observe multiple classical field ionization thresholds corresponding to the rotational states of the H$_{\mathrm{2}}^{\mathrm{+}}$ core. A model, based on diabatic traversal of the Stark map, is used to explain the data.