Partial cross sections and angular distributions of resonant and nonresonant valence photoemission of ${\mathrm{C}}_{60}$

We have performed high-resolution measurements of photoelectrons emitted from the valence shell of ${\mathrm{C}}_{60}$, for both gas phase and solid state, in order to obtain branching ratios, partial cross sections, and the angular distribution anisotropy parameters of the two highest occupied molecular orbitals. The analysis is based on the Fourier transform of the cross-section oscillations and the results are corroborated by different theoretical models. In contrast to this good overall agreement between theory and experiment there is a striking disagreement with respect to predicted discrete resonance structures in the partial cross sections. Possible reasons for this behavior are discussed.

Figure 1

Scheme of the experimental setup consisting of the rotatable electron time-of-flight spectrometers and an evaporative molecular beam source.

Figure 2

The electron yield at the magic angle (54.7°), which is proportioned to the partial cross section, as a function of the binding energy for three different photon energies.

Figure 3

Partial cross-section ratio HOMO/HOMO-1 for gas and solid ${\mathrm{C}}_{60}$ for different photon energies. The lines show the results of different theoretical models: theoretical curves with a jellium model (-----) and with LDA calculations (-∙-∙-); semiempirical curves with a shell model (⋯) and with a $\text{shell}+\text{cage}$ model (—). The experimental data of Ref. 17 are shown by open rhombs, whereas the data of the present work are given by open circles for the gas-phase data and crosses for the solid-state measurements.

Figure 4

Fourier transform of the cross-section ratio in Fig. 3 for the experimental data for the solid state (filled circles) and the LDA calculation using a spherical jellium potential (dashed line).

Figure 5

Absolute cross sections for the two outermost orbitals of ${\mathrm{C}}_{60}$ (a) HOMO and (b) HOMO-1. The experimental points are shown by open circles. The theoretical curves are from Ref. 21 (dashed line) and Ref. 22 (solid line), respectively.

Figure 6

The angular distribution asymmetry parameter $\beta$ for the two outermost orbitals of ${\mathrm{C}}_{60}$ (a) HOMO and (b) HOMO-1 as a function of the photon energy. The figure shows experimental results (open circles) and theoretical curves from Ref. 22 (solid line) and Ref. 23 (dashed line), respectively.