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Probing the Transition State of the He*(2S) + N₂ Penning Ionization Reaction with Electron Spectroscopy

The Journal of Physical Chemistry1996Vol. 100(1), pp. 21–29

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Abstract

State-selected electron spectra characterizing the transition state in the Penning ionization of N2 by He*(21S) have been measured in crossed supersonic beams at seven collision energies in the range E = 1.5−4.2 kcal/mol. The spectral peaks are assigned by analogy with the photoelectron spectrum of N2 to the three lowest electronic states X2Σg+, A2Πu, and B2Σu+ of N2+, as in earlier work. Evidence is presented, however, based on vibrational spacings, that the X-state vibrational distribution is bimodal. The spectra are least-squares analyzed to yield nascent electronic-state branching fractions, vibrational populations, and spectral line shifts and widths, all as a function of E. The X- and B-state intensities remain in a nearly constant ratio, while the A state begins with very low intensity relative to X and B but increases by more than a factor of 2 over the range of E. The relative vibrational populations within a given electronic state are E-independent within error, with a slight apparent deviation from Franck−Condon behavior only in the B state. The lines are all blue-shifted (to higher electron energy ε) relative to the difference ε0(n‘,v‘) between the excitation energy of He* and the vibronic energy of N2+. The blue shifts for X and B are small and slowly increasing with E, while those for A are substantially larger and more rapidly increasing; the line widths behave similarly, although their actual magnitudes are obscured by the analyzer bandpass. All of these features, except the bimodal X-state distribution and the non-Franck−Condon B-state behavior, are reproduced semiquantitatively by model calculations with a strongly anisotropic complex potential energy surface and an extension of a recently proposed vibrationally-adiabatic quantum scattering theory for molecular Penning ionization (J. Chem. Phys. 1995, 102, 1934). The theory is also used to demonstrate the control exerted by the real part of the potential surface over the electronic branching fractions and the sensitivity of the branching to the sign and magnitude of the N2 electric quadrupole moment. The reaction is found to be dominated by transition states with large He−N−N bond angles, i.e., by near-collinear intermediates. The bimodal X-state distribution may arise from X−A coupling induced by He, while the slight deviation from Franck−Condon behavior may be interpreted in terms of the N2 bond-length dependence of the B-state discrete−continuum coupling. The B-state coupling is found to increase as the N2 bond stretches.

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Abstract

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