Carbon 1s photoelectron spectrum of methane: Vibrational excitation and core-hole lifetime

The carbon Is photoelectron spectrum has been measured for CH4 at photon en ergies of 302, 320, and 330 eV and for CD4 at 330 eV with an instrumental r esolution about half the natural linewidth. These spectra have been analyze d to obtain vibrational spacings, vibrational intensities, and the lifetime of the carbon Is core-hole state. The vibrational intensities vary with ph oton energy, in agreement with earlier results. At 330 eV, the observed Fra nck-Condon factors for both CH4 and CD4 can be understood only if anharmoni c effects (consistent with the predictions of theory) are included. On the other hand, the vibrational spacings in CH4 show no evidence for anharmonic ity (in contrast with theoretical predictions). In CD, the observed anharmo nicity in the vibrational energy spacings is about half of the predicted va lue, but the experimental and theoretical values differ only by an amount c omparable to the experimental uncertainty. The measured values of the lifet ime show a dependence on photon energy; this is attributed to failure of th e theory of post-collision interaction to predict correctly the observed el ectron spectrum near threshold. At 330 eV, the measured Lorentzian lifetime , 93-95 meV, agrees with predictions of simple theory, but not with the pre diction of more complete theory. It is also observed that there are systema tic discrepancies between the observed line shapes and those predicted by t he theory of postcollision interaction. [S1050-2947(99)02305-7].