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?
[1] W. Barford, R. J. Bursill, and R. W. Smith, Phys. Rev. B 2002, 66, 115205