High-resolution infrared study of the 2ν3 (A1, E) and ν1 + ν3 (E) bands of 14NF3

Abstract The 2 ν 3 overtone ( A 1 ,  E ) and the ν 1  +  ν 3 ( E ) combination bands of the oblate symmetric top 14 NF 3 were studied by FTIR spectroscopy with a resolution of 2.5 × 10 −3  cm −1 . Nearly 500 lines up to K max / J max  = 30/43 were observed for the weak A 1 component reaching the v 3  = 2 0 substate (1803.1302 cm −1 ), the majority of which corresponded to reinforced K  = 3 p -type transitions. For the strong E component reaching the v 3  = 2 ±2 substate (1810.4239 cm −1 ), about 3550 transitions were assigned up to K max / J max  = 65/69, favoring a clear observation of the l(4, −2) and l(4, 4) splittings within the k l = −2 and +4 sublevels, respectively. The two v 3  = 2 substates are linked by the l(2, 2)- and l(2, −1)-type interactions, providing severe crossings, respectively, at K ′ = 6 and near K ′ = 24 on the v 3  = 2 +2 side. A model working in the D -reduction and including all these l-type interactions could reproduce together 3695 nonzero weighted experimental data (NZW) through 33 free parameters with a standard deviation of σ  = 0.357 × 10 −3 cm −1 . As for the ν 1  +  ν 3 ( E ) combination band, about 3690 lines were assigned up to K max / J max  = 45/55. Its v 1  =  v 3  = 1 upper state (1931.577 5 cm −1 ) was treated using the same model recently applied to the v 3  = 1 ( E , 907.5413 cm −1 ) state. It yielded 21 free parameters through 3282 NZW experimental data, adjusted with σ  = 0.344 × 10 −3 cm −1 in the D -reduction. For the two excited states, the small and unobserved l(0, 6) interaction was tested as useless. To confirm the adequacy of the vibrationally isolated models used, some other reductions of the Hamiltonian were tried. For the v 3  = 2 state, the D -, L -, and LD -reductions led to similar σ ’s, while the Q one was not successful. For the v 1  =  v 3  = 1 state, the D - and Q -reductions gave comparable σ ’s, while the QD -reduction was not as good. The corresponding unitary equivalence relations are generally more nicely fulfilled for the v 3  = 2 state than for the v 1  =  v 3  = 1 state. The three derivable anharmonicity constants in cm −1 are x 33  = −4.1528, g 33  = +1.8235 and x 13  = −7.9652.