Excited states and photochemistry of saturated molecules. VII. Potential energy surfaces in excited singlet states of methane

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Abstract
Potential energy surfaces are investigated for a number of low‐lying excited singlet states of methane, using a split valence plus Rydberg basis set and singly excited CI. Of the six minima found, the lowest two are valence states. The lowest minimum corresponds to the products 1 1B1 CH2+1 1Σg+ H2, in agreement with the observed threshold photochemistry, and is accessible by five separate routes from the lowest vertical state. The second valence minimum is a square planar structure. Of the four Rydberg minima detected, one is a local minimum on the otherwise dissociative B1 surface and two correspond closely to CH4+ structures previously determined. None of the minima have a structure from which the radical products CH3⋅+H⋅ are likely to be obtained.