Abstract
The high resolution He I photoelectron spectrum of the (a12t25)2T2 < - (a12t26)1A1 transition in methane has been recorded and analyzed and a value has been obtained for the adiabataic ionization potential. The electron spectrum resulting from ionization of a t2 electron is a composite of at least three overlapping electronic bands which are separated by several tenths of an eV. These bands are the split components of the 2T2 electronic state of the CH4+ ion which is distorted by strong Jahn-Teller forces. The symmetry of the molecule in these three electronic states is believed to be, in order of increasing energy, D2d<C3e<D2h. The active vibration which causes the Jahn-Teller splitting is the r2(e) mode. The adiabatic ionization potential of methane is certainly below 12.62 eV. Our extrapolated value is approx- 12.51 eV. Four different vibrational progressions have been found, one of which shows vibronic splittings.
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