High Resolution Studies of Inner-Shell Excited States of Atoms and Molecules by Electron Impact Excitation

Abstract

Inner-shell excited states of atoms and molecules are formed when an inner-shell electron is promoted to an unoccupied valence or Rydberg orbital, for example the excitation of a 2p electron in Ar or a Is atomic electron in N₂. Traditionally these states have been studied by photoabsorption measurements using energetic photons (for example [1,2]). These inner-shell transitions can also, however, be studied by electron energy-loss spectroscopy and in fact this technique can have important advantages over photoabsorption measurements. Van der Wiel et al [3] employed the electron energy-loss technique and observed inner-shell transitions in N₂ and CO. This work was followed by a systematic and comprehensive study of inner-shell transitions in molecules by Brion and collaborators using the electron energy-loss technique at an energy resolution of approximately 0.5eV (for a recent review see [4]). Subsequently a significant improvement in resolution was obtained by the Manchester group (for example [5,6]) who obtained a resolution of 0.07eV in electron energy-loss measurements in a number of atoms and molecules. These latter studies illustrated one of the important advantages of the technique, namely its superior resolution to photo-absorption measurements for state excitation energies of above about 200eV. This allowed a full investigation of the parameters of inner-shell states including their energies, natural widths, and in the case of molecules, their vibrational spacings and equilibrium internuclear separations.