Knowledge Bank

The photolysis of ethylene in the $V⟵N$ and Rydberg regions and its sensitization by excited triplet species is presented in terms of a molecular orbital theory similar to the one outlined by us for the photodecomposition of formaldehyde (J. Chem. Phys. in press). Primary processes that involve the formation of (1) molecular hydrogen from geminal atoms and (2) two hydrogen atoms derived from adjacent carbon atoms are pictured as arising from one electron population of sigma molecular orbital which are antibonding but respectively H-H bonding and antibonding. These orbitals appear to be higher in energy than the $xco$ in the $D2h$ planar geometry of ethylene but on population of the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}(\pi ,\pi \ast )$ state a non-planar geometry can be assumed in which both antibonding sigma orbital have lower energy arising from enhanced favorable overlap conditions. This permits population of the appropriate decomposition states via allowed radiationless transitions from the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}(\pi ,\pi \ast )$ state. Radiationless transitions are allowed from the first Rydberg state into the decomposition state forming molecular hydrogen, but require a vibronic perturbation to reach the one forming two hydrogen atoms.