Direct measurement of a pure rotation transition in H2(+).

The N = 1<--0 pure rotation transition in the nu = 19 level of the ground electronic state of H2(+) was observed at 14,961.7+/-1.1 MHz. Recent theory predicts significant electric dipole intensity in forbidden rotation and rotation-vibration transitions involving levels near the dissociation limit; the relevant levels are bound by only 0.74 and 0.22 cm(-1). The transition was predicted to have a transition moment of 0.42 D; our measurement is consistent with this value.