Clarification of notation

1. Oct 22, 2011

AxiomOfChoice

I'm trying to decipher exactly what it means to say the following things:

(1) The ground electronic state of NO$_2$ is $^{2}A_1$;

(2) The ground electronic state of ClO$_2$ is $^{2}B_1$;

(3) The ground electronic state of O$_2$ is $^{3}\Sigma^{-}_g$.

I really don't have a clue what these symbols are supposed to mean. Can someone please help?

2. Oct 23, 2011