# Rotational Spectra of Diatomic Molecules

1. Jul 7, 2015

### Vaibhav DixiT

I was wondering how Rotational Spectra of diatomic molecule can be related to Heisenberg Uncertainty principle (Qualitatively). Being a QM model where rotational energies are quantized, there should be a qualitative reasoning on lines of the uncertainty principle, right?

Anyone can direct me to a good source? or maybe enlighten me if you know t

2. Jul 7, 2015

### BvU

Uncertainty principle is immediately related to spectral line broadening due to finite lifetime of the excited state. A simple $$\Delta E \Delta t \ge {\hbar\over 2} \ ,$$so quite quantitative, actually.
Rotational energy levels are very close together ($\approx$ 10-3 eV), compared to vibrational ($\approx$ 10-1 eV) and electronic (several eV). So you need very high resolution spectroscopy and provisions to eliminate other sources of line broadening (e.g. Doppler).

The energy differences show up in Raman spectroscopy (and here -- a bit hefty), or in
rotation-vibration spectroscopy in the infrared range
Purely rotational spectroscopy is in the microwave range; I don't know much about that.