Dependence of Generalized Oscillator Strengths of Ethylene on Momentum Transfer

Generalized oscillator strengths are calculated as a function of scattering angle or K, the momentum transfer, for seven electronic transitions in ethylene in order to characterize each type of transition. The valence transitions Ag1 → 1B1u[π → π*] and Ag1 → 1B1g[σ → π*], and the Rydberg transitions Ag1 → 1B1g[π → σ*], 1Ag → 1B2g[π → σ*], 1Ag → 1B3u[π → σ*], 1Ag → 1B3g[σ → σ*] and Ag1 → 1B3u[π → σ*] are studied. Characteristic minima in f(K) for the Rydberg transitions appear. This behavior is attributed to the presence or absence of nodes in the molecular orbitals. It is suggested that this phenomenon be used as a probe of the type of molecular orbitals which are involved in a particular transition as well as the location of optically forbidden transitions. Qualitative agreement is obtained with experimental results for all but the π → π* transition, which does display a minimum. This is probably due to configurational mixing with Hartree–Fock Rydberg states.