|MadSci Network: Physics|
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
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