Orientation-dependent final-state effects in photoelectron spectra of physisorbed molecules

We report high-resolution core- and valence-level photoemission and core-to-valence x-ray absorption spectra of ${\mathrm{O}}_2$ physisorbed on graphite. The submonolayer δ-ζ2 orientational phase transition is manifest in the core-level spectrum as an extra broadening of each component of the paramegnetically split O 1s doublet. This phase transition is not visible in the 1${\mathrm{\Pi }}_{\mathit{g}}$ valence spectrum or the 1s→1${\mathrm{\Pi }}_{\mathit{g}}$ absorption line. The results reflect the detailed shape of the charge distribution in the final state of the molecule. We suggest that the population of low-frequency librational modes makes an important contribution to the core-level linewidths.