Rotational channel interactions of vibrationally excited np Rydberg statesof the triatomic hydrogen molecule

We have carried out a precise measurement of the wavelength dependence of t he two-step photoionization cross section of triatomic hydrogen via the 3s( 2)A(l)' Rydberg state at energies beyond the ionization limits that belong to the first symmetric-stretch excited vibrational state {1,0(0)} of the io n core. In this energy region, rotational and vibrational channel couplings due to the interaction between the outer p electron and the H-3(+) core ar e expected. We find strong coupling between the two p series with total ang ular momentum N = 2 which converge to the rotational levels N+ = 1 and 3 of the underlying H-3(+) {1,0(0)} core. Surprisingly, regular perturbations a re also detected in the symmetric-stretch-excited N = I p series. Rydberg l evels of the ndE' {1,0(0)} series are identified as the perturbing channel. This implies a change in the orbital angular momentum of the Rydberg elect ron during the short-range interaction. The experimental spectra are analyz ed in two-channel quantum-defect models. Eigenchannel quantum defects, coup ling constants, and transition moment ratios are determined and compared to the results of recent nb initio calculations. [S1050-2947(99)06611-1].