Past exam of the physics course of the year 4 of the natural sciences course of the University of Cambridge / 2014 / Paper 1 / A8 / Solution

In the rigid-rotor approximation, $E_J={\hslash }^{2}J(J+1)/(2I)$ and electric-dipole absorption has $\Delta J=+1$. A line labelled $J^{\prime }$ therefore hasThe original graph gives adjacent spacing about $0.115\mathrm{THz}$; for example the $J^{\prime }=5$ line is near $0.575\mathrm{THz}$. Thus $I\simeq 1.46\times 10^{-46}\mathrm{kg}{\mathrm{m}}^2$. Using the reduced mass,Centrifugal distortion of a nonrigid molecule explains small departures from equal spacing at higher $J$.