Why energy level transitions only absorb or emit photons？

[arielleon]

The question occurred to me when I was thinking about black radiation. It is critically depend on the fact that the photon number is variable. Therefore I wonder why energy level transitions of atoms can only emit out the massless photons, but not any other kind of particles, even some with mass? It may be a stupid question, forgive me...

 

[mathman]

In most situations (electron transitions) there isn't enough energy to create a particle. In the case of nuclear transitions, particles (alpha, beta, neutrinos) are often emitted.

 

[Zarqon]

And even the energy from electron transitions can be used to emit particles with mass, as is the case with http://en.wikipedia.org/wiki/Auger_effect" for exampe, where the large energy from an electron falling into an inner core shell is enough to knock another electron out.

 

[Hawkwind]

The question occurred to me when I was thinking about black radiation. It is critically depend on the fact that the photon number is variable. Therefore I wonder why energy level transitions of atoms can only emit out the massless photons, but not any other kind of particles, even some with mass? It may be a stupid question, forgive me...

A transition from one energy level to another one must prevail the nature of the atom. Emission of an electron for instance would create a charged object instead of an atom; this process is called ionization.

 

[arielleon]

I see that the electron transition energy is too small to create particles with mass. Then I think of another question. If there're two atoms, one of which has an electron in excited state. Two kinds of processes are possible under the condition of the conservations of momentum and energy: the transition energy all becomes the kinetic energy, or emit a photon. Then what determines the actual case, or the possibility of each case? Thx.