I am having a bit of a conceptual issue with excitation and change of state.

I understand that when a light hits a gas, some of the light may be absorbed by the gas to excite the atoms in the gas; the idea of absorption spectra. But where is this energy that excites the atom going?

If we have water at close to boiling point and put light with sufficient energy incident on it, could it lead to a change of state of that water to a gas?

I don't know if my question is clear and I believe I have a misunderstanding somewhere. Any help would be appreciated.


2 Answers 2


When some light is incident on water close to boiling point,

  • If it is absorbed by an electron, the electron will get excited to a higher state.

  • If the light is used up in breaking inter-molecular interactions (H-bonding in water), the molecule may break free from the surface and vaporise.

But how does light break bonds?

It turns out, the atoms of surface undergoes a transition from electronically excited to vibrationally excited. If the vibrations are sufficient to break free from interactions, the molecule leaves from the surface.

A nice answer can be found at this site: Scientific American

Light from the sun excites electrons in the atoms which constitute the brick wall. How does that electronic energy get converted to heat, you ask. The key is 'radiationless transitions.' Here's how it works: the atoms of the brick are perpetually vibrating. Some of those atoms vibrate sufficiently vigorously that their vibrational energy is roughly equal to the electronic energy (photons) absorbed from the sun--in essence, they are in resonance with the solar energy. Those atoms then make a quantum transition from 'electronically excited' to 'vibrationally excited,' meaning that the energy causes the whole atom to move. We feel that motion as "heat." The atoms which make the jump to vibrational excitation soon collide into neighboring atoms, dissipating their vibrational energy throughout the entire brick, making the brick hot throughout.

  • $\begingroup$ How does it undergo such a transition? Is there a 'threshold' in that the photons will cause vibrations rather than just excitation? $\endgroup$ Commented May 16, 2017 at 9:20
  • 1
    $\begingroup$ @PhysicsMathsLove Presently I am unaware of the underlying mechanism. For more information I recommend reading about Vibronic Spectroscopy. $\endgroup$
    – jonsno
    Commented May 16, 2017 at 12:32

Absorption spectra are a result of the energy from the light moving the atoms' electrons into higher energy levels. That's why elements have specific absorption and emission spectra; the electrons experience specific jumps in energy level that occur as photons with compatible energies are absorbed then emitted.

As for the second part of your question, this question may be useful: Does visible light heat things up?


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