  We investigated the vibrational energies in the 2p^2A^"_2 and 3d^2E^" states
of the triatomic deuterium molecule D_3. The experiments were performed using a
fast neutral beam photoionization spectrometer recently developed at Freiburg.
A depletion type optical double-resonance scheme using two pulsed dye lasers
was applied. The measured vibrational frequencies of the 2p^2A^"_2 state of D_3
are compared to those of H_3 and to theoretical values calculated from an ab
initio potential energy surface. The data give insight into the importance of
the coupling between the valence electron and the ion core.

Chemical Physics