A He(I) photoelectron spectroscopic study of the (X-tilde)~2A'_2 state of NH_3~+ and ND_3~+. A reanalysis and evidence for the coriolis coupling between the bending v_2 and v_4 modes

摘要

In the high-resolution He(I) photoelectron spectrum of NH_3 and its isotopomer ND_3, the complete vibrational structure of the (X-tilde)~2A"_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 250L?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.