Evidence of Fermi resonance in core-ionized methane

A full quartic potential energy surface is determined for core-ionized meth ane and used to investigate coupling between vibrational modes. A strong Fe rmi resonance is found between the first excited state of the symmetric str etching mode nu(1)(') and a doubly excited bending mode, whereas the corres ponding interaction is less pronounced for v(1)(')=2. In terms of the carbo n 1s photoelectron spectrum of methane, the net effect of the mode coupling is to reduce the apparent contribution from anharmonicity to peak position s. The contribution from anharmonicity to the intensity of each peak is dom inated by cubic and quartic terms in the symmetric stretching coordinate, a nd remains significant. This resolves a paradox pointed out in a recent exp erimental work [Carroll , Phys. Rev. A 59, 3386 (1999)]. (C) 2000 American Institute of Physics. [S0021-9606(00)00718-2].