Observation of forbidden transitions of ammonium ion (NH+4) ν3 band and determination of ground state rotational constants. Observation of ν3 band allowed transitions of ND+4

The high resolution infrared spectrum of forbidden transitions of the ν3 band of ammonium ion, NH+4, has been recorded using a difference frequency spectrometer. These are the first forbidden NH+4 lines to be observed. For 5≤R≤11, 24 forbidden transitions have been assigned. From combination difference relations involving the same excited state level, 13 independent energy differences between ground state levels with 4≤J≤11 have been determined with uncertainties of ≲0.004 cm−1. From this data, the following values (in cm−1) for ground state constants of NH+4 have been determined (±3σ):B=5.929 32 ±0.000 20, D=(−1.282±0.020)×10−4, and Dt=(4.80±0.35)×10−6. The value of r0 is calculated to be 1.028 73±0.000 02 Å, while re is estimated to be 1.0208±0.0020 Å. More than 100 transitions of the ν3 band of ND+4 were also observed (J≲10) by the same spectrometer. Nearly all of these have been assigned and fit together using a linear model. A determination of B0 for ND+4, while unsuccessful at this time, will allow a more accurate determination of re.