= Rotational line spacing and diatomic bond length
{title2=$r=\sqrt{h/(4\pi^2\mu\Delta\nu)}$}
For a rigid heteronuclear diatomic rotor, $E_J=\hbar^2J(J+1)/(2I)$ and electric-dipole absorption has $\Delta J=+1$. Therefore the $J'\to J'-1$ line <frequency> is $hJ'/(4\pi^2I)$ and adjacent absorption lines are spaced by $h/(4\pi^2I)$. With <reduced mass> $\mu$, the bond-length estimate is $r=\sqrt{I/\mu}$. The conversion assumes <frequency> rather than <angular frequency>; <centrifugal distortion> makes real high-J spacings imperfectly uniform.
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