Re: How many variations of atom or molecule excitations by a photon are there?

Hi,
Atom or molecule excitations by photons fall into two main categories,
resonant and non resonant excitations. Resonant excitations are those where
the energy of the incoming photon matches that of the energy difference
between two electronic shells in the atomic electron cloud. The absorption
of the photon must obey quantum mechanical selection rules and the full
derivation is quite involved but can be found in a graduate text such as:

Quantum Electronics by Yariv

In an atom there are only a certain number of energies which can be
resonantly absorbed due to the quantisation of the photon and the atom. It
has a defined energy and so does the transition between the elecronic
states, but a complex molecule on the other hand has many ways that it can
move as well as these quantised elecronic transitions. The molecule can
rotate and vibrate, which although these movements are also quantised,
smear out into a continuum of states and we use this fact to produce
tuneable lasers. If there are many states which all merge into one, then a
molecule with these so called rovibrational states will absorb a wide
number of photons, which allows for spontaneous emission and then
stimulated emission on a large scale. The electron, once in the upper state
having absorbed a resonant photon will decay over a timescale which depends
on how well its wavefunction is matched to that of the lower state that it
wants to decay into. When it decays, it can be cause emission of another
photon and that photon will take on the same properties of the first
photon. This duplication process goes on and on until you have a laser beam.

Non resonant absorption does occur always and this is due to the small but
non zero probability of an electron in an atom absorbing the incoming
photon energy. Mathematically, as we represent the photon lineshape as a
Lorentzian, it has a probability of being at the energy required for
absorption, no matter how small it is. 

There is another possibility that can happen, that being Compton
scattering. When a photon comes into contact with matter, it scatters off
the matter and leaves some of its energy behind and then continues
travelling through at a lower energy. This is the non elastic scattering
you spoke about in your question and this can be a big problem when you
want to know the energy of a photon exactly. When gamma rays are emitted
from the nucleus, nuclear physicists often want to know what energy they
are exactly so they can put together a picture of how the nucleus decays.
It is important that all the energy of the gamma ray (which is exactly the
same as a photon, but with higher energy) is left in the detector. If not
then, the decay puzzle doesn't add up. To try and stop this happening (or
to be more correct, ignore it when it does happen) more detectors are
placed around the main detector and they are there to try and detect the
scattered, lower energy gamma ray. If there is a detection in the main
detector and in the surrounding detector, then the signal is ignored
because it means that this Compton scattering has occurred. This is called
Compton suppression and is a powerful tool in gamma ray spectroscopy. 

To summarise, all these mechanisms can happen and resonant and non resonant
absorption are mathematically described by the same equations, which can be
found in the book I referenced. There are many other non radiative ways to
excite atoms, but I think that resonant and non resonant is a good way to
categorise the radiative ones,

Ben