Knowledge Bank

Improved measurements on the rotational spectrum of $PH3$ have recently been reported by Stroup, Oetjen, and $Bell.1$ In addition to the usual rotation lines certain much weaker lines are also found. These may be classified into two series, one with a $B(xx)$ value ($B(xx)$) being one of the two equal reciprocals of inertia) greater than that for the normal vibration state and another with a $B(xx)$ value smaller than that for the normal vibration state. These lines may be interpreted as arising from rotation of the $PH3$ molecules in the first excited vibration states of $v2$ and $v4$. These levels perturb each other through Coriolis interaction. The magnitude of the perturbation is proportional to a constant $2(\zeta 2,4(x)B(xx))2/\delta$, where $\zeta 2,4(x)$ is the Coriolis coupling factor and $\delta$ is the frequency interval between $v2$ and $v4$. This constant may be estimated from the work of McConaghie and $Nielsen2$ to be about $0.1cm-1$. This constant may also be estimated from the differences between the $B(xx)$ values for the molecule in these two states, as revealed by the two sets of lines, and the $B(xx)$ value of the molecule in the normal vibration state. It is found that the value of $2(\zeta 2,4(x)B(xx))2/\delta$ estimated from the rotation spectra of $PH3$ agrees well with the value estimated from the vibration-rotation spectrum.