Rotational channel interactions of vibrationally excited $\mathrm{np}$ Rydberg states of the triatomic hydrogen molecule

We have carried out a precise measurement of the wavelength dependence of the two-step photoionization cross section of triatomic hydrogen via the $3s{}^2{A}_1^{\prime }$ Rydberg state at energies beyond the ionization limits that belong to the first symmetric-stretch excited vibrational state $\{{1,0}^0\}$ of the ion core. In this energy region, rotational and vibrational channel couplings due to the interaction between the outer p electron and the ${\mathrm{H}}_3^{+}$ core are expected. We find strong coupling between the two p series with total angular momentum $N=2\mathrm{}$ which converge to the rotational levels $N^{+}=1\mathrm{}$ and 3 of the underlying ${\mathrm{H}}_3^{+}\{{1,0}^0\}$ core. Surprisingly, regular perturbations are also detected in the symmetric-stretch-excited $N=1\mathrm{}$ p series. Rydberg levels of the ${\mathrm{ndE}}^{\prime }$ $\{{1,0}^0\}$ series are identified as the perturbing channel. This implies a change in the orbital angular momentum of the Rydberg electron during the short-range interaction. The experimental spectra are analyzed in two-channel quantum-defect models. Eigenchannel quantum defects, coupling constants, and transition moment ratios are determined and compared to the results of recent ab initio calculations.