Argue that the selection rule n(final)-n(initial)=1 implies that to the extent that a diatomic molecule..contd?

Argue that the selection rule n(final)-n(initial)=1 implies that to the extent that a diatomic molecule can be treated as a harmonic oscillator, it would only emit photons having a single wavelength, no matter what n(initial) might be. (if transitions between rotational states were not also a factor).