Electronic band structure of solid CO2 as determined from the hv-dependence of photoelectron emission

Abstract

Abstract Photoelectron energy distribution curves from solid CO 2 have been determined for excitation energies from hv = 14 up to 40 eV using synchrotron radiation. A 1:1 correspondence to the gas-phase photoelectron spectrum is observed for the occupied molecular orbitals. The vertical binding energies E B v ( E VAC = 0) and widths (fwhm) of the valence bands of solid CO 2 are determined to be 13.0 and 0.95 eV (1π g ); 16.7 and 1.1 eV (1π u ); 17.6 and 0.85 eV (3σ u ) and 18.8 and 0.8 eV (4σ g ) for the individual bands respectively. The partial photoemission cross sections differ importantly from those of the gas phase in exhibiting pronounced maxima at 5.2 eV (1π g ), 4.4–5.3 eV (1π u + 3σ u ) and 4.2 eV (4σ g ) above the vacuum level, which is attributed to effects of high density of final (conduction-band) states. Further weaker maxima are observed at higher photon energies. Contrary to the case for the gas phase, the resonances are unperturbed in the solid by degenerate autoionizing molecular Rydberg states. The molecular origin of the resonances in the continuum is discussed and related to X-ray absorption spectra, electron-scattering data and to theoretical cross-section calculations. It is shown that the same set of resonances is observed in the different experiments. The resonances occur however at different energies due to different Coulomb interactions. The photoemission results presented provide also a key to the hitherto unexplained optical spectrum of solid CO 2 in the VUV range, making possible an assignment of the structures observed to Frenkel-type excitons ( hv ≤ 15 eV) and interband transitions ( hv ⪰ 15 eV).