Non-Franck–Condon effects in photoionization of the 3 ^3Π Rydberg state of NH

Strong non‐Franck–Condon behavior is predicted to occur in the vibrationally resolved photoionization spectra of NH for (3+1) resonance enhanced multiphoton ionization processes via the 3 ^3Π Rydberg state. The non‐Franck–Condon effects are interpreted on the basis of rapid orbital evolution, Cooper minima, and internuclear distance dependence of the dipole transition moment and cross sections. A Cooper minimum occurs in the 5σ→kπ channel at small internuclear distances, where NH resembles its united atom, oxygen. The iterative Schwinger variational method and multiplet‐specific ion potentials are employed in the calculation of the photoelectron continuum wave functions. Cross sections and asymmetry parameters for photoionization of the NH ground state leading to the X  ^2Π, a  ^4Σ^−, and A  ^2Σ^− ions are also reported.