Carbon1sphotoelectron spectrum of methane: Vibrational excitation and core-hole lifetime

The carbon $1s$ photoelectron spectrum has been measured for ${\mathrm{CH}}_4$ at photon energies of 302, 320, and 330 eV and for ${\mathrm{CD}}_4$ at 330 eV with an instrumental resolution about half the natural linewidth. These spectra have been analyzed to obtain vibrational spacings, vibrational intensities, and the lifetime of the carbon $1s$ core-hole state. The vibrational intensities vary with photon energy, in agreement with earlier results. At 330 eV, the observed Franck-Condon factors for both ${\mathrm{CH}}_4$ and ${\mathrm{CD}}_4$ can be understood only if anharmonic effects (consistent with the predictions of theory) are included. On the other hand, the vibrational spacings in ${\mathrm{CH}}_4$ show no evidence for anharmonicity (in contrast with theoretical predictions). In ${\mathrm{CD}}_4$ the observed anharmonicity in the vibrational energy spacings is about half of the predicted value, but the experimental and theoretical values differ only by an amount comparable to the experimental uncertainty. The measured values of the lifetime show a dependence on photon energy; this is attributed to failure of the theory of post-collision interaction to predict correctly the observed electron spectrum near threshold. At 330 eV, the measured Lorentzian lifetime, 93–95 meV, agrees with predictions of simple theory, but not with the prediction of more complete theory. It is also observed that there are systematic discrepancies between the observed line shapes and those predicted by the theory of postcollision interaction.