We have carried out a precise measurement of the wavelength dependence of the twostep photoionization cross section of triatomic hydrogen via the 3s^{2}
A_{1}' Rydberg state at energies beyond the ionization limits that belong to the first symmetricstretch excited vibrational state \{1,00\}
of the ion core. In this energy region, rotational and vibrational channel couplings due to the interaction between the outer p electron and the
H_{3}^{+} core are expected. We find strong coupling between the two p series with total angular
momentum N=2 which converge to the rotational levels N^{+}=1 and 3 of the underlying H_{3}
^{+} \{1,00\} core. Surprisingly, regular perturbations are also detected in the symmetricstretchexcited N=1 p series.
Rydberg levels of the ndE' \{1,00\} series are identified as the perturbing channel.
This implies a change in the orbital angular momentum of the Rydberg electron during the shortrange interaction. The experimental spectra are analyzed
in twochannel quantumdefect models. Eigenchannel quantum defects, coupling constants, and transition moment ratios are determined and compared to the
results of recent ab initio calculations. ©1999 The American Physical Society
