Branching ratios for autoionisation of ν=3 Rydberg states in nitric oxide

An investigation is reported of the autoionising decay of the n=18-22 members of the nu =3 Rydberg series, converging to the X1 Sigma +( nu 2+3) state of NO+. Photoelectron spectra for two-colour multiphoton ionisation processes via the A2 Sigma +( nu '=3) intermediate state are recorded. Photoelectron analysis of the decaying 22p( nu =3) Rydberg state leads to branching ratios of 1.00:0.06:0.25 for autoionisation into the nu +=2, 1, 0 channels respectively. These branching ratios deviate from the Delta nu =-1 propensity rule for pure vibrational autoionisation. The effect of electronic coupling with a dissociative (valence) continuum is discussed. Experimental branching ratios are compared with results of two-step multichannel quantum-defect calculations by Guisti-Suzor and Jungen (1984) in which vibrational autoionisation and (pre)-dissociation are treated simultaneously.