Are there any differences between photons emited /absorbed by antimatter atoms to photon in usual atoms? (Theoretically)
Are there any differences between photons emited /absorbed by antimatter atoms to photon in usual atoms?
for example, is it impossible to tell the difference between a photon emmited by an atom and it's anti matter counter part? Assuming the same orbital transitions for their electorn and positrons?
Are the absorptions lines the same as well? or are there absoption lines for anti hydrogen are expected to be different than the hydrogen?
What is convention for anti elements? e.g is anti matter counter part of He is just denoted as -He or $\bar{He}$?
 A: Yes, photons are the same if produced by matter or antimatter. A simple way to see this is that electromagnetism is CP, C and P invariant. Then, changing particles by antiparticles in any process one should obtain exactly the same "amplitude" (i.e. energy levels, probability of absortion emmission, etc...).
Weak interactions are neitehr CP nor C or P invariant,and there you can find processes where the probability is different if you change all particles by antiparticles. Take a look to CP violation in wikipedia.
About antielemtns, as far as I know, it is difficult to produce them experimentally, and I seriously doubt that something like anti-Helium have ever been measured. Only the nuclei of antihelium have been produced, but the full atom with positrons orbiting around is by far other history. Take a look at wikipedia again
A: According to the theory, there is no difference between matter and antimatter radiation of photons. Experimentally, of course, there is a big difference - antielements are much harder to obtain and thus make an antimatter radiation source ;-). 
