Photoelectron trapping in N 2 O 7σ→kσ resonant ionization

Vibrationally resolved photoelectron spectroscopy of the N(2)O(+)(A (2)Sigma(+)) state is used to compare the dependence of the photoelectron dynamics on molecular geometry for two shape resonances in the same ionization channel. Spectra are acquired over the photon energy range of 18 <= h nu <= 55 eV. There are three single-channel resonances in this range, two in the 7 sigma -> k sigma channel and one in the 7 sigma -> k pi channel. Vibrational branching ratio curves are determined by measuring vibrationally resolved photoelectron spectra as a function of photon energy, and theoretical branching ratio curves are generated via Schwinger variational scattering calculations. In the region 30 <= h nu <= 40 eV, there are two shape resonances (k sigma and k pi). The k sigma ionization resonance is clearly visible in vibrationally resolved measurements at h nu=35 eV, even though the total cross section in this channel is dwarfed by the cross section in the degenerate, more slowly varying 7 sigma -> k pi channel. This k sigma resonance is manifested in non-Franck-Condon behavior in the approximately antisymmetric nu(3) stretching mode, but it is not visible in the branching ratio curve for the approximately symmetric nu(1) stretch. The behavior of the 35-eV k sigma resonance is compared to a previously studied N(2)O 7 sigma -> k sigma shape resonance at lower energy. The mode sensitivity of the 35-eV k sigma resonance is the opposite of what was observed for the lower-energy resonance. The contrasting mode-specific behavior observed for the high- and low-energy 7 sigma -> k sigma resonances can be explained on the basis of the "approximate" symmetry of the quasibound photoelectron resonant wave function, and the contrasting behavior reflects differences in the continuum electron trapping. An examination of the geometry dependence of the photoelectron dipole matrix elements shows that the k sigma resonances have qualitatively different dependences on the individual bond lengths. The low-energy resonance is influenced only by changes in the end-to-end length of the molecule, whereas the higher-energy resonance depends on the individual N-N and N-O bond lengths. Branching ratios are determined for several vibrational levels, including the symmetry-forbidden bending mode, and all of the observed behavior is explained in the context of an independent particle, Born-Oppenheimer framework.