The Photoelectron Spectrum of the NO_3 Radical Revisited: A Theoretical Investigation of Potential Energy Surfaces and Conical Intersections

摘要

The vertical ionization spectrum of the nitrogen trioxide radical (NO_3) has been examined by the DIP-STEOM-CCSD and DIP-EOM-CCSD methods (double ionization potential [similarity transformed] equation-of-motion coupled-cluster singles and doubles). The DIP-STEOM and DIP-EOM approaches avoid artifactual symmetry breaking of the reference wavefunction by starting from the symmetry-correct nitrate anion (NO_3~-) orbitals and provide a balanced treatment of dynamical and non-dynamical correlation effects. The five lowest singlet and five lowest triplet states of the cation (NO_3~+) are considered. The calculated vertical ionization potentials match well with the band maxima in the photoelectron spectrum obtained by Wang. D. X.; et al. (J. Chem. Phys. 1997, 706, 3003). A broad feature in the spectrum, which has previously been assigned to the ~1E″state, is assigned by us to the nominally forbidden ~3A_2′ state. The ~3A_2′ state exhibits a conical intersection with the nearby, allowed ~3E′ state and is thus expected to gain intensity through a vibronic coupling mechanism; the ~1E″ state is instead found to nearly coincide with the lower-energy ~3E″ state. Another conical intersection may account for the broad feature associated with the ~3E′state.