On the photoelectron spectrum of the NO dimer, and the ground state of (NO)(2)(+)

The vibrational structure of the photoelectron spectrum of the NO dimer is calculated by ab initio methods, and compared with the observed ZEKE (zero electron kinetic energy! spectra published by I. Fischer et al. [J. Chem. P hys. 96, 7171 (1992)] and A. Strobel et al. [J. Phys. Chem. 99, 872 (1995)] . The ground state potential of (NO)(2)(+) is calculated to have almost iso -energetic minima at planar cis (C-2v) and trans (C-2h) geometries, but the Franck-Condon factors from the ground state of (NO)(2) strongly favor tran sitions to the cis isomer. The good agreement for the vibrational frequenci es and intensities shows that the (NO)(2)(+) isomer reached in the photoele ctron spectrum has a cis-planar C-2v structure, rather than the twisted C-2 structure proposed by Fischer et al., or the trans-planar C-2h structure p roposed by A. Strobel et al. (C) 1999 American Institute of Physics. [S0021 -9606(99)01713-4].