The Jahn-Teller effect in the presence of partial isotopic substitution: the B1E" state of NH2D and NHD2

Rotationally resolved resonance enhanced multiphoton ionisation spectra of the 1E′′ state of NH2D are presented and analysed. The analysis indicates a small (34.9 cm−1) lifting of the vibronic degeneracy of the zero point level, approximately equal in sign but opposite in magnitude to the splitting observed in NHD2 in previous work. This observation is consistent with previous measurements on systems with partial isotopic substitution subject to a mild Jahn–Teller effect. A model is developed to calculate the splitting induced by asymmetric isotopic substitution of a degenerate electronic state, based on a harmonic force field with linear and quadratic Jahn–Teller terms added. The force field is developed in internal co-ordinates to allow the same parameters to be used to calculate the pattern of vibronic levels for all four isotopologues. The lifting of the degeneracy of the zero point level on asymmetric substitution comes from the quadratic Jahn–Teller effect; the linear term does not lift the degeneracy.