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Hello all,

 

Basic but (1) when an electron is excited by a photon with a frequency between two allowable energies, does it just promote the electron to the orbital it was just able to pass, or is it just as precise as frequencies emitted when falling to lower levels in spectroscopy, requiring EXACTLY the necessary frequency?

 

(2) Will the excess kinetic energy be enough to ionize the electron with any other higher orbital electrons falling to repopulate the missing areas? 

 

(3) Is there a tolerance in populating an orbital? I believe it should be more fuzzy as you approach with higher n, becoming classical. 

 

Thanks!

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Hello all,

 

Basic but (1) when an electron is excited by a photon with a frequency between two allowable energies, does it just promote the electron to the orbital it was just able to pass, or is it just as precise as frequencies emitted when falling to lower levels in spectroscopy, requiring EXACTLY the necessary frequency?

 

(2) Will the excess kinetic energy be enough to ionize the electron with any other higher orbital electrons falling to repopulate the missing areas? 

 

(3) Is there a tolerance in populating an orbital? I believe it should be more fuzzy as you approach with higher n, becoming classical. 

 

Thanks!

What a nice subject, reminding me of my first year at uni!

 

You are quite right, there is a tolerance in practice - what is known as "broadening" of spectral lines.

 

I can recall two forms of this, offhand:-

 

One is Doppler broadening, whereby the fact that the absorbing or emitting atoms are in motion leads to absorption and emission frequencies that vary slightly, according to whether the atom is advanced towards, or retreating from, the radiation source or detector.

 

The other is uncertainty broadening, which is a consequence of the uncertainty principle, in its form relating energy to lifetime. Short-lived excited states have energy levels that are relatively poorly defined (i.e. there is a large uncertainty in what the energy actually is), whereas long-lived ones are well-defined. In a system with frequent collisions between atoms, the lifetime of the state may be determined by the interval between collisions (collisions can cause states to change by non-radiative processes - which is a topic in itself, but that's by the way). So as pressure goes up, the lines get broader - what is sometimes called "pressure broadening".

 

There is a short summary here: http://hyperphysics.phy-astr.gsu.edu/hbase/Atomic/broaden.html

 

However, if the energy difference between a photon and the energy level gap is outside the tolerances set by processes such as these, no absorption will generally take place.

 

The exception to this is if the photon has enough energy to ionise the atoms, i.e. to eject the electron completely. The energy gap between successive atomic orbitals reduces at higher energy until a limit is reached. This is the ionisation energy. A photon with enough energy to ionise the electron can be absorbed at any energy above this, with the surplus going into kinetic energy of the emitted electron, as you suggest.

 

I do not recall much about what happens when you ionise an electron from an orbital below the highest populated level, but I am sure you would expect emission from electrons falling from the higher levels to fill the gap. I rather think some X-ray generation may rely on this but I am not sure.

Edited by exchemist
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