Angle-resolved photoelectron spectroscopy of CCl4: The Cooper minimum in molecules

Abstract

Angle-resolved photoelectron spectra have been taken for the first five orbitals in CCl4 (2t1, 7t2, 2e, 6t2, and 6a1) using a variable photon source extracted from synchrotron radiation. From these data, the partial cross sections and angular distribution parameters β have been determined for photon energies from 16 to 80 eV. In the case of the lone pair orbitals (2t1, 7t2, and 2e) minima in both the cross sections and β values were noted experimentally in the photon energy range of 40 to 50 eV. The behavior is similar in appearance to that observed with the 3p subshell of argon and is believed to be the molecular counterpart of the Cooper minimum in atoms. Calculations of the cross sections and β values have been made for each of the different orbitals of CCl4 measured experimentally using the multiple scattering Xα theory. Agreement between theory and experiment for the lone-pair orbitals is qualitatively good and in the case of photoionization in the 2t1 orbital it has been possible to identify which continuum channels correspond to the atomic 3s and 3d counterparts. It was also observed that the nonlone-pair orbitals (6t2 and 6a1) show both experimentally and theoretically shallow minima in the vicinity of photon energies where the Cooper minimum is found. Finally, experimentally determined β values for the first three valence orbitals in CH3Br were obtained from 20 to 50 eV. The behavior of the lone-pair orbital of CH3 Br could be correlated with the Cooper minimum observed for the 4p orbital of Kr. The general nature of the Cooper minimum in molecules is discussed.