THE INVERSION SPLITTING OF $^{15}$NH$_2$D AND $^{15}$ND$_2$H AS OBTAINED FROM THEIR FIR SPECTRA

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The far infrared absorption spectra of the partially deuterated species $^{15}$NH$_2$D and $^{15}$ND$_2$H have been recorded in the 20 to 105 cm$^{-1}$ region using a Brucker IFS 120 Fourier transform spectrometer. A 25 cm long absorption cell with teflon windows was filled with different mixtures of $^{15}$NH$_3$ and $^{15}$ND$_3$ at total pressures ranging from 0.8 to 2 mbar. Several spectra with different $^{15}$NH$_2$D to $^{15}$ND$_2$H ratio were thus recorded. For all spectra the maximum path length was used resulting in a $2\times 10^{-3}$ cm$^{-1}$ resolution. Prior to the line assignment, rotational constants for $^{15}$NH$_2$D and $^{15}$ND$_2$H where calculated from those of $^{14}$NH$_2$D and $^{14}$ND$_2$H. The IAM-type theoretical approach developed by Cohen and Pickett,} {\bf 93,} 83 (1982).} which accounts for the large amplitude inversion motion and its coupling with the overall rotation, was afterwards used to obtain rovibrational energies and predicted spectra. In the case of $^{15}$NH$_2$D, observed line positions for low-{\em J} lines were sometimes within 0.050 cm$^{-1}$ from their predicted values. The line position analyzes are still in progress. For both species, they are carried out with the help of the IAM-type theoretical approach.$^a$ For $^{15}$NH$_2$D more than 300 $a$- and $c$-type lines have already been assigned and the observed wavenumbers are reproduced with an RMS deviation of $0.4\times 10^{-3}$ cm$^{-1}$. This will be discussed in the paper and the values obtained for the spectroscopic constants will be reported and compared to those obtained by Fusina {\em et al.}} {\bf 127,} 240 (1988).}\ for the $^{14}$N-species. The decrease of the tunneling inversion splitting which goes from 0.406 cm$^{-1}$ in $^{14}$NH$_2$D$^b$ to 0.383 cm$^{-1}$ in $^{15}$NH$_2$D will also be discussed.

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{E. A. Cohen and H. M. Pickett, {\em J. Molec. Spectrosc.{L. Fusina, G. Di Lonardo, J. W. C. Johns and L. Halonen, {\em J. Molec. Spectrosc.
Author Institution: Laboratoire d'Etudes Physico Chimiques, 20000 Sa\ida,; Algerie; LISA, CNRS/Universites Paris 12 et 7, 61 Avenue du General de Gaulle, 94010 Creteil, France; Laboratorio de Espectroscopia e Laser, Universidade Federal; Fluminense, Campus da Boa Viagem, Niteroi, RJ, 24210-340, Brazil; Institut d'Electronique, Universite Mokhtar Badji, 23000; Annaba, Algerie

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