Filtering core excitation spectra:: vibrationally resolved constant ionic state studies of N 1s→2π core-excited NO

High-resolution electron spectroscopy studies of the NO molecule in the regions of the N 1s --> 2pi core excitations have been performed. By selecting electrons within certain binding energy ranges, either the Auger electron yield-a good approximation for the x-ray absorption spectrum-or the electrons emitted after decay to a particular ionic final state (constant ionic state (CIS)) were detected. By selecting the X(1)Sigma(+) (2pi(0)) final state, the superposition of several intermediate states can be disentangled by exploiting a selection rule which permits only two of the three dipole-allowed intermediate states to decay to this specific final state. This makes it possible to obtain more detailed information on the potential energy curves of the intermediate states than is available from regular absorption measurements. We have also obtained CIS spectra for individual vibrational sublevels within this state. The role of lifetime vibrational interference on the appearance of these spectra is discussed.