title PHOTOIONIZATION OF THE TRIATOMIC HYDROGEN MOLECULE authors U. M\"uller, R. Reichle, I. Mistr\'{\i}k, J. Stephens$^\ast$, M. Jungen$^\sharp$, and H. Helm affiliations Fakult\"at f\"ur Physik, Universit\"at Freiburg, Hermann-Herder-Str. 3, D-79104 Freiburg \\ $^\ast$ International Atomic Energy Agency, Wagramer Strasse 5, Vienna, Austria\\ $^\sharp$ Institute of Physical Chemistry, University of Basel, Klingelbergstrasse 80, 4056 Basel, Switzerland summary In a two-step photoionization experiment, we investigate np-Rydberg series of H3 built on symmetric-stretch excited H3+ core states. The perturbations in the quasi-discrete spectrum and the Beutler-Fano resonances are evaluated by a two-channel quantum defect model. To analyze resonances from rovibrationally excited interlopers, we apply a multichannel quantum-defect model using geometry- dependent quantum defects from highly accurate {\it ab-initio} calculations.