Minima in Generalized Oscillator Strengths: C2H4

A characteristic of electron impact excitation of low-lying Rydberg states is a minimum in the generalized oscillator strength as a function of K, the momentum transfer. This characteristic is used as a probe of the Rydberg character of four transitions in C2H4. Three, the Ag1 → 1B3u(π → 3s + 3s), 1Ag → 1B3u(π → 3pz − 3pz), and Ag1 → 1B3u(π → 4s) transitions, have long been identified as Rydberg and are found to exhibit the characteristic minimum. A fourth transition Ag1 → 1B1u(π → π*) is normally termed a valence excitation, and a theoretical calculation using Hartree–Fock molecular orbitals had predicted no minimum. For an energy loss of 8.0 eV which is identified with the valence transition a definite minimum is observed. Speculation on the source of this anomaly centers on a type of valence–Rydberg configurational mixing which can occur in many molecules. It is suggested that the presence or absence of minima in generalized oscillator strength curves be used to probe this aspect of the character of the excited state.