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When looking at absorption or reflectance spectra, say in the range of 400nm to 2500nm, you can see peaks (or dips) at certain wavelengths, that are characteristic for the material absorbing and reflecting the light. What property of the material determines the wavelength(s) where it absorbes light?

I am familiar with energy levels for electrons, as with atomic emission spectroscopy. I also know about different vibrational and rotation states for molecules, which have a way higher "resolution" than the electron states.

When a photon of a certain wavelength is absorbed, which state does the molecule absorbing it reach? Can it jump to any combination of electron, rotation and vibration state? If so, why do materials seem to only absorb certain wavelenghts? In my understanding materials generally reflect all wavelengths, unless there is an absorption band very nearby. Is this correct?

Note: I'm not asking why the absorbed light is not immediately radiated at the same wavelength. My understanding is that a molecule in an excited state can emit parts of its energy to reach lower electron, vibrational, and rotational states, drifting down in small steps.

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  • $\begingroup$ Your understanding is good. 2 bodies of the same temperature will exchange energy/photons at an equilibrium rate. A "hotter" body will send more to the cooler body like the sun to the earth. $\endgroup$ Feb 27 at 15:40

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What property of the material determines the wavelength(s) where it absorbes light? ... Can it jump to any combination of electron, rotation and vibration state?

The absorption can change electronic, rotational, or vibrational states, but not every transition is allowed.

There are selection rules that determine what transitions are allowed and disallowed.

If so, why do materials seem to only absorb certain wavelenghts?

Even if every transition were allowed, the number of available states in a confined system is still finite, leading to a finite set of strongly absorbed wavelengths.

In my understanding materials generally reflect all wavelengths, unless there is an absorption band very nearby. Is this correct?

It's also possible for a material to transmit the light, rather than reflect or absorb it. Consider ordinary silica glass as an example.

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  • $\begingroup$ Thank you for your answer. Yes I forgot about transmittance. But if there was none, would my statement be correct? Or is there always at least a tiny amount of transmittance? $\endgroup$
    – YPOC
    Feb 27 at 16:22
  • $\begingroup$ With the selection rules I understand the vibrational-level can change by at most 1 per transition, the R-level only by a few. This means they may be indistinguishable from purely electronic transitions, if my resolution is maybe 1nm. Does this mean I can calculate where a known material will absorb by referencing something like the NIST Atomic Spectra Database? Or are these atomic emissions entirely different to the molecular absorption? $\endgroup$
    – YPOC
    Feb 27 at 16:34

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