# Unknown Symbol in Molecular Term Symbols

1. May 27, 2010

### luke

Hello,

I am interested in the electronic states of the N2 molecule. Now it is a homonuclear diatomic molecule and http://en.wikipedia.org/wiki/Molecular_term_symbol" [Broken]) but I do not understand what the Latin letters before the molecular term symbols mean. Here are some of the states

$$X^{1} \Sigma_{g}^{+}$$
$$A^{3} \Sigma_{u}^{+}$$
$$B^{3} \Pi_{g}}$$
$$W^3 \Delta_u$$
$$B'^3 \Sigma_u^{-}$$
$$C^3 \Pi_u$$

What do the X, A, B, W, B', and C stand for? I understand the rest of the symbols. Any pointers would be helpful.

Last edited by a moderator: May 4, 2017
2. May 27, 2010

### gabriels-horn

The letter X refers to the ground state, while A, B, C, etc. refer to higher electronic states of the same multiplicity. If lowercase, a, b, c, etc. then the electronic states have different multiplicity.

Last edited by a moderator: May 4, 2017
3. May 27, 2010

### luke

Thank you gabriels for the fast reply. I have a few follow up questions. If you could recommend a reference that uses this notation that might be easier then answering my questions. Either way is fine by me.

From what I have read it makes sense that the X would denote the ground state but it seem that you would be able to recognize the ground state without the X. Is the X just there to make it explicit which state is the ground state?

A, B, C denote electronic states with the same multiplicity as what? Do you mean that all states with an A in front have the same multiplicity? It seems like there should be some other hidden meaning to the letters since why would the paper I am reading choose to use X,A,B,W,B',C,E,D?

4. May 27, 2010

### gabriels-horn

a) My guess is that X is used by convention as you suggested, because the gorund state might not be apparent when filling N2 and O2, for example.

b) A,B,C,D etc. have the same multiplicity (2S+1) as the ground state, while a,b,c,d, etc. have a different multiplicity than the ground state. I have not seen the prime notation, however.

Last edited: May 27, 2010
5. May 27, 2010

### luke

Thank you for your help. I found a reference that give essentially the same information but might be helpful to someone else who stumbles upon this.

Molecular Spectra and Molecular Structure, Vol I by Herzberg. The appendix has a list of electronic structures and provides an explanation of notation. N2 seems to be a special case. The appendix says,

"The only exception to this rule is N2 where the custom of designating the excited singlet states a, b, ... and the excited triplet states A, B, ... appeared too well established to make a change. The order a, b, c, ... A, B, C, ... is usually the order of increasing energy. But in some cases, particularly H2 He2, exceptions were made in order to be in agreement with earlier designations."