Knowledge Bank

The analysis of the $\nu 1(A1)$ and the $\nu 3(E)$ fundamental vibration-rotation bands of $PH3$ in the spectral interval $2440-2190cm-1$ has been carried out on spectra obtained with an effective spectral resolution of about $0.03cm-1$. The $\ell$-type and the K-type doubling affecting the $k\ell =1,-2$ levels of $\nu 3$ have been observed. A computer program therein the ordinary centrifugal distortion terms up to the sixth power terms of the angular momentum operator which explicitly treats the above cited resonances together with the $‘‘2,-1\prime \prime$ and the Coriolis resonance between the $\nu 1$ and $\nu 3$ was set up to simultaneously fit the data for these two bands which are separated by only $5.7cm-1$. However, these interactions seem to be insufficient in explaining the pattern of the $PH3$ spectrum even for low J quantum numbers. The band centers obtained for the transitions are: \begin{eqnarray*} \nu_{1}&=&2321.12 \pm 0.01;cm^{-1}\ \nu_{3}&=&2326.86 \pm 0.01 ;cm^{-1} \end{eqnarray*} Numerous `forbidden’ $\Delta |k-\ell |=\pm 3$ transitions due to the interaction between the $\nu 1$ and $\nu 3$ robibrational levels belonging to both these states are also presented. Among she set of the observed lines, those corresponding to $k\ell =-2(A+andA-),k\ell =-3$ are found to be of comparable intensity with the lines of the two fundamental bands.