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The weak mystery band found in the mono-olefins at 5–6 eV is here reassigned as a symmetry-allowed π→CH*(1Ag→1B2u) transition in which the CH* sigma upper orbital has considerable ns Rydberg character. Evidence supporting this assignment and militating against the recent CH→π*(1Ag→1B3g) assignment of Berry [J. Chem. Phys. 38, 1934 (1963)], the only other plausible assignment, is drawn from (a) ab initio calculations of the electronic spectrum and orbital energies of ethylene in large Gaussian bases, (b) a study of the intensities and vibronic band shapes of the mystery bands of cis—trans pairs of dialkylethylenes, (c) the observed effect of ring strain on the mystery-band intensity, (d) a measurement of the Cotton effect and rotational strength of the mystery-band transition, and (e) the spectra of olefins in condensed phases which demonstrate the Rydberg nature of the mystery-band upper state. Evidence is also presented that shows that the first electron affinity of ethylene places an electron in the CH* orbital, rather than in the π* orbital, as previously assumed. The implication of this low-lying π→σ* transition for the common assumption of π—σ separability is mentioned, and the importance of the almost certain presence of relatively low-lying π→σ* transitions with out-of-plane polarization in hydrocarbons of all sizes is stressed.

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