So, I am currently doing a course in Molecular Spectroscopy and I am finding some concepts very non-intuitive and very difficult to understand(maybe due to lack of my knowledge or the lack of explanations in the book).

So my problem is as follows- Suppose I have a box full of H-Cl gas molecules and since HCl is a molecule with a non-zero dipole moment, according to the book I am following, the dipole moment component of HCl along some axis changes sinusoidally, just like the time varying electric field of the EM wave which can cause photons to be emitted or absorbed when an EM wave interacts with the molecule. My first question is, how do molecules, which are rotating (or vibrating) really interact with the incoming EM wave and lead to change in energy levels of rotations (or vibrations.)

My second question is that considering HCl as a polar rotating molecule, the charges in the molecule undergo acceleration ( due to rotation) which should lead to charges emitting photons of fixed energies(not releasing a continuum spectrum but emitting photons of discrete energies) and thus molecules should fall to the ground state energy after some time. But this doesn't happen. I didn't understand the answers which my friends gave as they involved some population statistics and quantum particles not radiating energy but they didn't have any backing explanations for them.

I only propose a simple experiment. Suppose I have some HCl gas in an box which is raised to a temperature of 300K. This box is adiabatic so no heat transfers to the surroundings. Now, if the kinetic energy of the molecules is utilised in rotations and vibrations, they should drop down to ground state energy after some time as a result of continuous emission of photons and so all motion should cease after some time. Does this actually happen or am I wrong somewhere?

I am very sorry if the question feels too long. If there is something which needs elaboration, I would surely do that.

  • $\begingroup$ Those molecules will also absorb thermal radiation. The temperature of the gas stays in equilibrium with the container. $\endgroup$
    – user137289
    Aug 24, 2019 at 10:00

1 Answer 1


Well, let's consider the following three fields of spectroscopy:

  • atom spectroscopy; in the range of visible light and up into ultraviolet
  • the range of infrared
  • the range of microwaves

Historically it was spectrocopy of atoms that was among the first glimpses of the quantum world. Niels Bohr recounted that when he learned about the Rydberg formula it was immediately clear to him that the spectral lines described the Rydberg formula had to be energy levels that somehow the atom was restricted to.

Over time this led to the quantum mechanics of Schrödinger, Heisenberg, Dirac and the many others who contributed.

Once it is established that the spectral lines of atom spectroscopy are due to restriction to specific energy levels you can generalize that and infer that whenever you find spectral lines there must be restriction to specific energy levels.

The task is then to identify which spectroscopy arises from which motion.

Over time it was elucidated that infrared emission/absorbtion is due to molecular vibration states, and that microwave emission/absorbtion is due to rotational energy states.

This shows that the kind of restriction to specific energy states that is described by quantum physics is not unique to sub-atomic particles such as electrons. The realm of quantum physics extends to molecules.

  • $\begingroup$ Well. I understand which sort of radiation is emitted due to which type of motion, but my question was how it really works? I don't understand how the interaction of external EM radiation with rotating molecules causes them to jump or drop down in energy levels. What I don't clearly understand is the mechanism behind it. $\endgroup$
    – Tachyon209
    Aug 24, 2019 at 16:15
  • $\begingroup$ @Tachyon209 Well, quantum physics does not in any form attempt to describe the mechanism of the energy transfer. This is a 'choose your battles' kind of situation. If the physicists would have insisted on first finding a description of 'how it really works' they would have bogged down themselves completely. The succes of quantum mechanics is strong evidence that the supposed energy transfer does indeed occur. Another way of saying this: there is no attempt to describe the 'mechanism behind it'; in order to be able to formulate the theory at all this energy transfer must be accepted as is. $\endgroup$
    – Cleonis
    Aug 24, 2019 at 16:48

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