[disillusioned19]

I'm studying vibrational-rotational spectra at the moment, and have a question about HCl. The question that I've been given is to:

Sketch the energy levels and allowed transitions in the gas phase vibration-rotation spectrum of
HCl (this is a case of a parallel transition)

I'm then told that since this is a parallel transition, the Q branch is non-existent. That leads to my question, what is a 'parallel' transition, and why doesn't the Q branch exist?

Thanks in advance.

[dipesh747]

This website will be useful too you http://web.nmsu.edu/~snsm/classes/chem435/Lab9/