A He(I) photoelectron spectroscopic study of the [formula omitted] state of NH 3+ and [formula omitted]: A reanalysis and evidence for the coriolis coupling between the bending v2 and v4 modes

Abstract

In the high-resolution He(I) photoelectron spectrum of NH 3 and its isotopomer ND 3, the complete vibrational structure of the X ̃ 2 A ″ 2 state of NH 3 + and ND 3 + is examined in detail. Three vibrational progressions are identified. The well-known strongest progression, already unambiguously assigned to the v 2 out-of-plane bending mode, is observed from v=0–17 in NH 3 + and from v=0–20 in ND 3 +. For NH 3 + this vibration could be characterized not only by its energy hcω 2=0.109±0.001 eV (or ω 2=878±7 cm −1), but also by its first anharmonicity constant hcω 22 x 22=−(16.2±1.2)×10 −4 eV (or ω 22 x 22=−13.0±1.0 cm −1). The best fit of the experimental data required the introduction of a second anharmonicity constant, i.e. hcω 22 y 22=−(30.7±4.2)×10 −6 eV (or ω 22 y 22=−0.248±0.034 cm −1). The earlier reported weak progression, assigned to the vibrational combination v 1+ nv 2 transitions has been reexamined. Suitable handling of the data leads to two possible energies for the v 1 degenerate NH stretching vibrational normal mode, i.e. hcω 1=0.306±0.006 or 0.422±0.005 eV. Several arguments are brought to favour the value of hcω 1=0.422 eV (or ω 1=3404 cm −1). Finally a third weak progression, reported for the first time, is assigned to v 4+ nv 2 transitions where the v 4 in-plane bending mode is optically forbidden. This vibrational normal mode is characterized by an energy hcω 4=0.186±0.010 eV ( ω 4=1500±80 cm −1). In agreement with theoretical predictions, this transition becomes allowed through a strong Coriolis vibro-rotational coupling between the v 4 and the v 2 vibrational normal modes. The same measurements and the isotope effect on the molecular constants are investigated in ND 3 + too.