Gas Phase UV Photoelectron Spectroscopy as a Tool for the Investigation of Electronic Structures of Coordination Compounds

Abstract

Photoelectron spectroscopy is nothing but the application of the photoelectron effect to the study of the photoionization processes occurring in matter interacting with electromagnetic radiation. More precisely, photoelectronic spectroscopy deals with the measurement of the kinetic energy of the electrons emitted by matter owing to this interaction. When the matter is in the gas phase, and the light is of short wavelength, i.e. in the far ultraviolet, the entities involved in the photoionization are free molecules and the photoelectrons are ejected from the occupied molecular orbitals. The kinetic energy (KE) of a photoelectron ejected from the jth MO is given by the difference between the energy of the monochromatic radiation and the ionization energy corresponding to the removal of an electron from the j^th MO: KE=hv − IE, (1.1) provided we disregard, as a first approximation, additional possibilities of vibrational or rotational excitation on ionization. Gas-phase UV photoelectron spectroscopy (UPS) is the study of the energies, abundances and angular distributions of photoelectrons. Knowledge of the ionization energies for the various molecular orbitals in a molecule is of fundamental importance in the study of molecular electronic structure. In fact, following an approximation known as Koopmans' theorem,¹ each ionization energy IE, is equal in magnitude to the calculated SCF MO eigenvalue e_y IE,=− e_j, (1.2)