Photoelectron spectra of conjugated molecules
The various approaches to the assignment of molecular ultraviolet photoelectron spectra are described. An interpretation of ir-electron structure of selected classes of cyclic, heterocyclic and substituted conjugated molecules is given in terms of common constituent parts. The results are supported by molecular orbital calculations, photoelctron spectra of fluorinated compounds and comparison of spectra recorded under Hel and Hell excitation, as well as by symmetry considerations. Molecular ultraviolet photoelectron (PE) spectroscopy (Ref. 1,2) measures ionization energies of valence electrons of free molecules. Interaction of radiation of sufficient energy and a free molecule in the gas phase releases electrons from the latter and produces positive ions in various electronic states. The process can be described by 1 +2 M( 'Y0) + hv + M ( + e (Tk) where M(1V0) stands for a neutral molecule in its singlet ground state described by the electronic wave function hv the incident photon, M(2'11k) the ion in the k-th doublet state described by 'k and e(Tk) the released photoelectron with a kinetic energy Tk. Since mM>>me, the energy difference hv — Tk = — c(1V0) Ei,k is an important experimental quantity of a molecule and is called the k-th ionization energy. The ionization energy can be determined by employing constant (monochromatic) radiation and by analysing and counting the electrons of given kinetic energy. A plot of thus counted electrons vs. their energy (or the difference hvTk) is called a PE spectrum. The quantity E k depends on both the ground and the ionic state. The energy of the ground state e(1'Y0) is relatively easy to determine e.g. describing the wave function of the 2N electrons in a single determinantal form 1 i — — — 'V0 = 'Vi'Vi... . k'Vk .'VN'YN and minimizing the energy by a Hartree-Fock procedure yielding F'V0 = where are the orbital energies of the canonical orbitals 'V1. The same set of canonical orbitals 'V is usually used as an approximation for the spacial part of 2'k as