On the Jahn-Teller effect in the ²T₂ valence state of NH₄⁺⁺ as observed in the nitrogen K emission spectrum of NH₄Cl

Abstract

The nitrogen K emission spectrum of NH₄ ⁺ in NH₄Cl has been recorded with a high resolution concave grating grazing incidence spectrometer. The spectrum shows a very broad band-like structure covering about 10 eV with two pronounced maxima at 390 eV and 394·5 eV, enclosing a weak yet well defined third maximum at 392·7 eV. In the low energy part of the high energy maximum vibrational structure with an average spacing of 0·16 eV could be discerned. From the MO configuration of tetrahedral NH₄ ⁺ only one diagram line corresponding to the 1a ₁ ¹ 2a ₁ ² 1t ₂ ^{6 2} A ₁ — 1a ₁ ² 2a ₁ ² 1t ₂ ^{5 2} T ₂ electronic transition is expected to arise in the X-ray spectrum. The complexity of the spectrum obtained is therefore ascribed to very strong Jahn-Teller splitting which the NH₄ ⁺⁺ ion exhibits in the final ² T ₂ state.