# What are 1A_g and 1B_u excitons?

I was going through a paper on organic LED chemistry [1]. I saw that the authors referred to "$1A_g$" and "$1B_u$" excitons (and also "$2A_g$" and "$2B_u$" excitons), terms which I don't really understand because I am from an engineering background. The terms are used on the 2nd-to-last paragraph of page 4 of the paper.

Can anyone help me understand this terminology?

• What is the paper? Please consider editing your question to include at least a link to it. Even better would be to both link and then excerpt a paragraph that uses the terms you are confused by. – Curt F. Mar 6 '16 at 17:43
• Also, it appears you cross-posted your question to physics.se: physics.stackexchange.com/questions/241852/… Please DO NOT do that. Ask only in one place. – Curt F. Mar 6 '16 at 18:09
• You're looking at symbols that describe the symmetry of the excitons. Have a look here for so-called character tables that decipher the symbols. All of this falls under the broader category of group theory. – Todd Minehardt Mar 6 '16 at 20:32