Dependence of the Molecular Near-threshlod-structure on the Bond Angle

Excited molecular structure in gas phase usually consists or two types or energy states:Rydberg states with diffused molecular orbitals and valencetype excited states with rather localized molecular orbitals. Such localized orbitals may be classilied as bonding, non-bonding and anti-bonding types. Some anti-bonding states which may lie above the threshold can be regarded as resonances, sometimes referred as shape resonances. Excitation from molecular inner-shell orbitals into the above-mentioned excited orbitals leads to near-threshold structures. Thus, the near-threshold structure of molecules in gas phase consists of Rydberg states and valence-type excited states mainly involving localized anti-bonding orbitals. Based on the multiple-scattering method, we present our theoretical study on the near-threshold structures of triatomic molecules NO₂ and SO₂. we also elucidate their dependence on the molecular bond-angles. In the bent triatomic molecules such as NO₂ and SO₂, the coordination number of the central atom (e.g., N or S) is two, namely two O atoms bonded directly to the central atom. The coordination pair corresponds to the relevant diatomic molecule (e.g. NO or SO). We illustrate the correlations between the anti-bonding states (b₁*, a₁*, b₂*) of the bent triatomic molecule and anti-bonding states (π*, π*) of the corresponding diatomic molecule (coordiation pair, As the molecules are laid in a condense environment, for example the molecules absorbed on the metal-surfaces, the near-threshold structures involving localized anti-bonding states still resemble the corresponding structures in gas phase. Thus, such near-threshold structures involving the anti-bonding states are most suitable to probe the orientations of the absorbed molecules as well as change of the absorbed molecular bond-angles.