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N*: 1 0 2 3 Diss. Diss. I I I I 1 0 Figure 5. Model invoked for the coupling of Rydberg states with different rotational angular momentum of ion core, leading to intensity gain for the A" = -1 and A" = -2 transitions. # 0 transitions that involve a change in angular momentum. An intensity gain for the AN+ < 0 transitions compared to the AN+ > 0 transitions is made possible by the contributions (of strong intensity) from Rydberg states converging to rotational states of the ion core for which N* = NA. These Rydberg states can coupleto Rydberg states of lower rotational quantum number (the equivalent of rotational autoionization) and hence lead to an intensity gain for the AN+ C 0 compared to the AN+ > 0 transitions. The intensity gain for the AN+ C 0 transitions depends on the relative rate of predissociation and "autoionization", where autoionization denotes the coupling between the different Rydberg states. Figure 5 shows a possible explanation of this coupling. It is apparent that the intensity gain coming from the high transition probability of the AN+ = 0 transition only applies to the AN+ < 0 transitions observed in the ZEKE spectrum. ConclusionThe rotationally resolved ZEKE spectra of nitric oxide for selected rotational quantum numbers NA in the A-state show intensity dependencies that can be explained by a coupling of Rydberg states of different ionic core rotational quantum numbers. The intensity gain for the "downward" ionizing transitions compared to the "upward" ionizing transitions can hence be explained. The observed alignment effects apparent when using different transitions to populate the same intermediate NA state are not so easily interpreted. A full calculation as presented in ref 13, using ab initio matrix elements, has to be carried out to explain this. Acknowledgment.We are grateful to E. W. Schlag (Garching) for his continuous encouragement and his support. We also thank the Commission of the European Communities for support of this research.We compare the ZEKE (zero kinetic energy) photoelectron spectrum of p-difluorobenzene, obtained by two-color 1 + 1' photon ionization via the SI 6' vibrational state as intermediate resonance, with a recent timeof-flight photoelectron (TOF-PE) spectrum by Sekreta et al. The ZEKE spectrum resolves a large number of fundamental and combination bands of the electronic ground state of the cation, which in the TOF-PE spectrum appear as congested and merged peaks. The assignment of the vibrations observed in the ZEKE spectrum agree well with the assignments obtained for the TOF-PE spectrum. Due to the enhanced resolution of ZEKE spectroscopy (2-3 orders of magnitude compared to photoelectron spectroscopy) improved vibrational frequencies in the cation are derived. The adiabatic ionization energy is determined to 73872 f 3 cm-l.

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