Radiation

[PPonte]

O2 + 8,3x10^-19 J -> O + O

My textbooks says:

If an incident UV radiation has less energy then 8,3 * 10^-19 J nothing happens. The radiation is not absorbed.

My question is:
Shouldn't that radiation be absorbed increasing O2 kinetic energy? If not, why?

Thank you.:approve:

[Hootenanny]

I am assuming that upto know you have only ever learnt that electron orbital energy is quantised, however this is not the case. A molecule has energy associated with a number of different aspects these include;


Translational energy (molecular movement as a whole)
Rotational energy (the molecule rotating as whole)
Vibrational energy (of the bonds)
Electronic energy (associated with electronic orbitals)

Now, all of the above have quantised energy levels. However, as you go from bottom to top of the above list the spacing between the energy levels becomes smaller, so the quantised energy levels become less apparent. Infact, the translational energy levels are so close together that for most purposes they can be considered continuous.

As I said above, the seperation between these energy levels are different, and each quanta seperation corresponds to a different frequency (energy) of EM radiation. The spacing between vibrational energy levels corresponds to the energy of the infrared section of the spectrum. Therefore, if a molecules absorbs IR radiation the energy will be used to promote the molecule to a higher vibrational energy level. The rotational energy levels correspond to the energy of the microwave section. Now, as the spacings between the electronic energy levels are greatest, it follows that they would absorb the most energetic radiation, which is visble and UV light.

So basically, the answer to your question is that all energies associated with a molecule are quantised and therefore will only obsorb a specific type of EM radiation.

[Borek]

Infact, the translational energy levels are so close together that for most purposes they can be considered continuous.

I am not sure (QM was never my favorite subject) but IIRC distance between transitional energy levels depends on the size of the space available. In "narrow" potential well transitional energy levels can be quite distant, while in the infinite space there is no such thing as translational energy quantisation.

Best,
Borek
--
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[Hootenanny]

I am not sure (QM was never my favorite subject) but IIRC distance between transitional energy levels depends on the size of the space available. In "narrow" potential well transitional energy levels can be quite distant, while in the infinite space there is no such thing as translational energy quantisation.

Best,
Borek

You are quite correct Borek, I should have clarified that I was refering to a particle in a box. However, even if the dimensions of the box are in the same order as the molecular size; the energy levels are so compact that they behave as classical physics predicts, i.e. no quantised energy levels. As you also corectly stated that in a infite space no translational energy quanta exist.

[PPonte]

Thank you both for your help, although I haven't learnt much of that.

[GCT]

O2 + 8,3x10^-19 J -> O + O

My textbooks says:



My question is:
Shouldn't that radiation be absorbed increasing O2 kinetic energy? If not, why?

Thank you.:approve:

I think that they were referring to the absorption by the electrons, so I'm not quite sure why you would think that it would increase the kinetic energy of O2.