PROBING THE TRANSITION-STATE OF THE HE-ASTERISK(2(1)S)-2 PENNING IONIZATION REACTION WITH ELECTRON-SPECTROSCOPY(N)

State-selected electron spectra characterizing the transition state in the Penning ionization of N-2 by He(2(1)S) have been measured in cro ssed supersonic beams at seven collision energies in the range E = 1.5 -4.2 kcal/ mel. The spectral peaks are assigned by analogy with the ph otoelectron spectrum of N to the three lowest electronic states X(2) S igma(g)(+), A(2) Pi(u), and B-2 Sigma(u)(+) of N-2(+), as in earlier w ork. Evidence is presented, however, based on vibrational spacings, th at the X-state vibrational distribution is bimodal. The spectra are le ast-squares analyzed to yield nascent electronic-state branching fract ions, vibrational populations, and spectral line shifts and widths, al l as a function of E. The X- and B-state intensities remain in a nearl y constant ratio, while the A state begins with very low intensity rel ative to X and B but increases by more than a factor of 2 over the ran ge of E. The relative vibrational populations within a given electroni c state are E-independent within error, with a slight apparent deviati on from Franck-Condon behavior only in the B state. The lines are all blue-shifted (to higher electron energy E) relative to the difference epsilon(0)(n',upsilon') between the excitation energy of He and the v ibronic energy of N-2(+) The blue shifts for X and B are small and slo wly increasing with E, while those for A are substantially larger and more rapidly increasing; the line widths behave similarly, although th eir actual magnitudes are obscured by the analyzer bandpass. All of th ese features, except the bimodal X-state distribution and the non-Fran ck-Condon B-state behavior, are reproduced semiquantitatively by model calculations with a strongly anisotropic complex potential energy sur face and an extension of a recently proposed vibrationally-adiabatic q uantum scattering theory for molecular Penning ionization (J. Chem. Ph ys. 1995, 102, 1934). The theory is also used to demonstrate the contr ol exerted by the real part of the potential surface over the electron ic branching fractions and the sensitivity of the branching to the sig n and magnitude of the N-2 electric quadrupole moment. The reaction is found to be dominated by transition states with large He-N-N bond ang les, i.e., by near-collinear intermediates. The bimodal X-state distri bution may arise from X-A coupling induced by He, while the slight dev iation from Franck-Condon behavior may be interpreted in terms of the N-2 bond-length dependence of the B-state discrete-continuum coupling. The B-state coupling is found to increase as the N-2 bond stretches.