# Help interpret quantum states of molecular rotation and torsion

• B
• syfry
syfry
TL;DR Summary
Could use help understanding how to interpret a description about quantum energy of microwaves that relate to quantum states of molecules.
Was exploring how light interacts with matter out of layperson curiosity, when a sentence suddenly tripped me up:

"The quantum energy of microwave photons is in the range of 0.00001 to 0.001 eV, which is in the range of energies that separate the quantum states of molecular rotation and torsion"

That's the first time I've heard of a 'quantum energy' for light. (photons)

Also first time hearing about rotation and twisting of molecules. (had to search the meaning of torsion too)

Main issue is understanding what's affecting what in the description, and, how:

What does it mean for the twisting and turning action of molecules to be quantum states?

Are the microwaves causing that? Or is their energy separating the states? Are the states together until then?

The sentence is so brief without any clarifying description that it's unclear, and a search didn't help.

The quoted sentence is from the link below:

First, molecules have a fairly complicated spectrum of states. Certain subsets of these states can be associated heuristically with what you might call normal modes (e.g., bending oscillations) in a classical multi-particle systems.

Second, molecules will be a in particular state or combination of states from their spectrum. For the molecule to move to a state of higher energy it needs to acquire that energy from somewhere.

Third, the cited article recognizes that the photon (light particle) energy for wavelengths corresponding to microwaves is close to the difference in energy between some of the states in the spectrum of molecules. So these microwave photons can be absorbed by the molecules to put them in a higher energy state. You can think of that higher energy states as being associated with modes of faster oscillation.

syfry and PeterDonis
syfry said:
That's the first time I've heard of a 'quantum energy' for light. (photons)
The energy of a photon is proportional to its frequency. See: Photon energy

syfry
It is easiest to start with atoms, in particular the hydrogen atom, see
http://hyperphysics.phy-astr.gsu.edu/hbase/Bohr.html#c4
http://hyperphysics.phy-astr.gsu.edu/hbase/hyde.html#c2

Once you understand this idea that the state of an atom (the relative motion of the electron and the nucleus) is quantized, i.e., only certain states of discrete energy are possible, it shouldn't be too hard to get that the same will apply to the state of a molecule, but not only for the relative motion of electrons and nuclei, but also for the relative motion of the nuclei themselves.

It turns out that the different motions are characterized by very different energy scales. In terms of the wavelength/frequency/energy of the photons implied, the highest energy states are those correspond to electronic motion (ultraviolet and visible photons), then it is the relative distance between the nuclei, known as vibrations (infrared photons), then relative position of the nuclei (torsion) and rotation of entire frame of the molecule, which as the text you cited mentioned, implied microwave photons.

syfry and Haborix
@DrClaude makes an important point about the types of motions associated with different energy scales. @syfry I know you asked for a layman level, but if you wanted to dig into this more, these kinds of things are really the meat of a subject called physical chemistry. If you've taken undergraduate level intro chemistry and have a decent handle on math, you could dig deeper, if you're interested. (And even if you don't have that background you could do it, but it would be a longer slog)

syfry

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