## Clarification of notation

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?

 PhysOrg.com chemistry news on PhysOrg.com >> Non-wetting fabric drains sweat>> Protein study suggests drug side effects are inevitable>> RNA capable of catalyzing electron transfer on early earth with iron's help, study says