High-energy non-Franck-Condon vibrational excitation of CH₄ by intramolecular photoelectron diffraction

Distinct oscillations in vibrationally resolved cross section ratios for the photoionization of CH₄ from the C 1s orbital at photon energies as high as 1keV are predicted. The oscillations are attributed to the different relative vibrational excitation due to the scattering of the photoelectron by the peripheral hydrogen atoms. The latter effect is also responsible for the well known EXAFS oscillations in the integrated photoelectron spectrum. The calculations are performed with an ab-initio DFT method [1], as well as with a single-particle semi-analytical model, which incorporate both the effect of the nuclear recoil and of the Coulomb corrections.