Spectroscopy and multichannel quantum-defect theory analysis of thenpRydberg series of H3

The lowest np Rydberg series of triatomic hydrogen has been studied using optical-optical double-resonance excitation from the metastable 2p2 A2{''} level. Bound states, detected by field ionization as well as autoionizing states of the p series are characterized using rotational multichannel quantum-defect theory. Deviations from the calculations appear when vibrational interactions give rise to predissociation below threshold or autoionizing interlopers above threshold. The np Rydberg manifold of H3 provides an almost complete panorama of channel interactions in a polyatomic molecule: rotational and vibrational interactions and autoionization with rich Fano profile structure as well as predissociation.