Photoelectron angular distribution of benzene: Can the asymmetry parameter be considered a benchmark?

Abstract

The calculated asymmetry parameter ($\beta$) and photoionisation cross sections for eight outermost orbitals ($1e_{1g}$, $3e_{2g}$, $1a_{2u}$, $3e_{1u}$, $1b_{2u}$, $2b_{1u}$, $3a_{1g}$, and $2e_{2g}$) of benzene, in the gas phase, are presented in the range from threshold to 35 eV. The Schwinger Variational Method with Padé approximants was employed to generate the continuum wavefunction of photoelectrons and the dipole matrix transition was computed in the velocity ($V$) and length ($L$) approach of the dipole operator. This operator in $V$ or $L$ form does not affect significantly $\beta$ values in the energy range studied. The cross sections in $V$ form produce lower magnitudes than $L$ and are closer to the measurements. The resonance peaks were observed in the cross sections for some orbitals and their positions were not affected by the dipole approach. Finally, symmetry analysis is done on the partial cross sections to assign the main contributing final state to the resonance feature and also link this to the minimum in the asymmetry parameter. The comparison of our results with other theories has, in general, good qualitative and some quantitative agreement. For some orbitals ($1a_{2u}$, $2b_{1u}$), severe disagreements between theory and experiment are highlighted. Some possible causes for these differences are pointed out. Because of this, we conclude that some asymmetry parameters and PICS for benzene, in ionisation studies, cannot yet be considered as a benchmark and will be necessary further investigation to clarify such discrepancies.