THE VIBRATIONALLY RESOLVED C-1-S CORE PHOTOELECTRON-SPECTRA OF METHANE AND ETHANE

Recent progress in the development of high-resolution electron spectro meters in combination with highly monochromatized undulator radiation has allowed observation of the vibrationally resolved gas-phase C Is p hotoelectron spectra of methane and ethane. For both molecules, the C- H stretching modes are well resolved and for ethane the active C-C str etching mode has been observed for the first time. The spectra have be en measured at low kinetic energies and detailed fittings using post-c ollision interaction line profiles have been made both, using a free p arameter fit and a fit adhering to a linear coupling model. The free p arameter fit allows for any anharmonicity in the vibrational energies. The linear coupling model, on the other hand, assumes that the initia l and final state potential curves are harmonic and differ only in the normal coordinates. This simple model is used to reduce the number of free parameters in the fit, which greatly simplifies the analysis. An intensity model based on the linear coupling predicts that the intens ities of the C-H stretching modes are directly related to the number o f C-H bonds around the core ionized atom. The result is verified for e thane and shows a potential for further reduction of free parameters f or large molecules and polymers. Ab initio calculations of molecular g eometry and vibrational frequencies have also been carried out using t he equivalent core (Z+1) approximation. The values predicted for the d ecrease in bond length have then been compared to those determined emp irically by the linear coupling approach. The calculation of ethane in dicates that symmetric C-H and C-C stretching modes are important upon core ionization. The corresponding vibrational frequencies have been calculated and agree well with observed values. (C) 1997 American Inst itute of Physics.