Localization effects in the Auger spectra of ring nitrogen systems: Pyridine, poly(2-vinyl)pyridine, borazine, and boron nitride

The N(KVV) Auger spectra of gas phase pyridine (C₅H ₅N) and borazine (B₃N₃H₆), and of solid phase poly (2-vinyl) pyridine (PVP) and hexagonal boron nitride [(BN) _x] are reported and analyzed. The data indicate two Auger "fingerprint" types of nitrogen. Ammonia (NH₃) is the prototype for the first, where three of the five valence electrons are σ bonding and the other two are the lone pair. This localized electronic structure gives rise to relatively sharp features in the N(KVV) spectrum. Typical of the second fingerprint type is pyridine, where there are two σ bonding electrons, a lone pair of electrons, and one electron contributing to the delocalized π system. Theoretical nitrogen Auger transition energies and intensities are calculated for pyridine to demonstrate the general origin of the overlapping features in the relatively broad N(KVV) spectrum of this molecule. PVP fits into the second fingerprint type while borazine and boron nitride give nitrogen Auger spectra more like ammonia. Approximate calculations using the equivalent core concept are used to clarify the relationship between the ammonia, borazine, and boron nitride spectra. It is shown that in these systems the initial Auger state (core-hole) largely localizes the bonds and lone pair on the nitrogen. The Auger spectra show that it is the σ, π and nonbonding orbital characters that provide the Auger fingerprint.