A He(I) photoelectron spectroscopic study of the X̃2A″2 state of NH3+ and ND3+ .: A reanalysis and evidence for the coriolis coupling between the bending v2 and v4 modes Original Research Article

In the high-resolution He(I) photoelectron spectrum of NH3 and its isotopomer ND3, the complete vibrational structure of the X̃2A″2 state of NH3+ and ND3+ is examined in detail. Three vibrational progressions are identified. The well-known strongest progression, already unambiguously assigned to the v2 out-of-plane bending mode, is observed from v=0–17 in NH3+ and from v=0–20 in ND3+. For NH3+ this vibration could be characterized not only by its energy hcω2=0.109±0.001 eV (or ω2=878±7 cm−1), but also by its first anharmonicity constant hcω22x22=−(16.2±1.2)×10−4 eV (or ω22x22=−13.0±1.0 cm−1). The best fit of the experimental data required the introduction of a second anharmonicity constant, i.e. hcω22y22=−(30.7±4.2)×10−6 eV (or ω22y22=−0.248±0.034 cm−1). The earlier reported weak progression, assigned to the vibrational combination v1+nv2 transitions has been reexamined. Suitable handling of the data leads to two possible energies for the v1 degenerate NH stretching vibrational normal mode, i.e. hcω1=0.306±0.006 or 0.422±0.005 eV. Several arguments are brought to favour the value of hcω1=0.422 eV (or ω1=3404 cm−1). Finally a third weak progression, reported for the first time, is assigned to v4+nv2 transitions where the v4 in-plane bending mode is optically forbidden. This vibrational normal mode is characterized by an energy hcω4=0.186±0.010 eV (ω4=1500±80 cm−1). In agreement with theoretical predictions, this transition becomes allowed through a strong Coriolis vibro-rotational coupling between the v4 and the v2 vibrational normal modes. The same measurements and the isotope effect on the molecular constants are investigated in ND3+ too.