Which elements can be diatomic?
This leaves us with diatomic Selenium which I could not find anything on.


Selenium forms a dimer with bond energy of 79 kcal/mol, according to table 8.6 (page 451) of Comprehensive Handbook of Chemical Bond Energies, which provides 5 references.  

Tellenium dimer is 61 kcal/mol

Polonium dimer 45 kcal/mol