Why is the positronium spectrum hard to find?

[Dadface]

The energy levels of positronium in eV are given by:

En = - 6.8/n²

From this the positronium spectrum must contain some frequencies smaller than those of gamma rays including some lines in the visible spectrum. My problem is that I have been trying to find photographs and other details of the positronium spectrum but have found nothing. Positronium has been known of for a long time so my guess is that there are problems in displaying the spectrum. Either that or I have been searching in the wrong places. Can anyone enlighten me please?

Thank you.

 

[DrClaude]

You can't simply prepare positronium and take a spectrum as you would a normal atom. It's hard to get it to radiate before it annihilates. The Lyman-α line, which is in the UV, was observed for the first time in 1975: Canter et al., Phys. Rev. Lett. 34, 177 (1975). Things have progressed since then, see Mills, J. Phys.: Conf. Ser. 488, 012001 (2014), which is open access.

 

[Dadface]

Thank you so much DrClaude. That's clarified everything.