So let's say you do some reflection/transmission spectroscopy of a material. It's clear that it's absorbing in some range.

What would be your first step in identifying the source of the absorption? The two main clues I would look at would be

  1. The range it occurs in (if your incident light is in the x-ray range, it's clearly not phonon absorption, for example)
  2. The shape of the absorption (is it a peak? Or an absorption 'edge', where it isn't absorbing below some energy, and absorbs everything above, for example)

But I'd like to know if there are other ways as well.

Additionally, does anyone know of a 'map' of the ranges you'll typically find different types of absorption in? I saw one on wikipedia once, but it only had three types, and now I can't find it of course. For example, a few types of absorption I can think of off the top of my head are

  • Interband electron transitions
  • Intersubband electron transitions
  • Impurities
  • Phonons
  • Vibrational modes
  • Atomic transitions

I understand that a lot of these will be material-dependent, but often the same mechanism in different materials will yield absorption in the same order of magnitude range. What ranges would one typically see these (and other) absorption mechanisms in?

Thank you!


This is a really broad question. There are so many different spectroscopic techniques. I'll try to give a broad overview.

Rotational Spectroscopy

Rotation states of molecules are quantized. Transitions between rotational states are generally in the microwave range.

Vibrational Spectroscopy

Vibration states of molecules are quantized. Transitions between vibrational states are generally in the infra-red range.

Spectroscopy of Electronic Transitions

Electronic States of atoms and molecules are quantized. Transitions could be across a vast range of the spectrum. Inner shell electon transitions would be in the X-ray range. Valence electron transitions are in the UV, visible and IR ranges.

Nuclear Spectroscopy

Transition between states of nuclei are in the gamma-ray range.

Nuclear Magnetic Resonance (NMR) Spectroscopy

Transitions between spin states of nuclei in a magnetic field. NMR is usually in the 30MHz to 1GHz range, but depends upon the magnetic field you use.

Electron Spin Resonance (ESR)

Transitions between spin states of electrons in a magnetic field.
Usually in the GHz range, again depending upon the magnetic field you use.


A very simple diagram shown below is from the wikipedia article on Permittivity. "Electronic" encompasses level/band transitions, atomic represents such phenomena as vibrations (as shown) and rotations (which is how microwave ovens work).

Simple spectrum

Here is a more complicated spectrum for semiconductors/solid state matter taken from here.

Solid state spectrum


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