Fine Structure in the X-ray Emission Spectrum of N₂, Compared with Electron Spectroscopy

Abstract

X-RAY emission Spectroscopy has considerable potential in studying the valence electron structure of matter, and has been used to examine the electronic band structure of solids. To a much lesser extent it has been applied to the study of molecular orbitals in free molecules, but owing to lack of resolution in the experimental spectra it is difficult to discern very well the different valence orbitals in a molecule. These orbitals have instead been observed and studied in great detail by other techniques, among which electron Spectroscopy (ESCA) has been successful in elucidating both valence electron orbitals and core electron orbitals^1,2. If X-ray transitions involving molecular valence electrons could conveniently be examined with a resolution of 1 eV or better there should be many applications for these types of study. We have constructed an apparatus for this purpose in which the X-ray emission spectra from gaseous samples are excited by electron impact and analysed in a grazing incidence grating spectrometer at a resolving power of 3,000^3,4. This corresponds to ≈0.1 eV for the characteristic X-radiation of the second period elements. We show here that such high resolution in the X-ray spectrum reveals the molecular orbital structure and molecular states in sufficient detail to allow direct comparisons with molecular ESCA spectra and lets us observe fine structure due to molecular vibration in an X-ray line. Dipole selection rules control the X-ray transitions and serve as a guide in the assignment of the different valence orbitals of the molecule.