I Selection Rules for a Diatomic Molecule?

[Sebacide]

Hello!
I have a question about selection rules and electronic transitions of a diatomic molecule: I can't find a good explanation about that, and there's so much confusion about the right rules to use.
I studied that on Bransden-Joachain's "Physics of atoms and molecules", but there (to whom have the book, paragraphs 11.1 and 11.3 of 2nd edition) is said that diatomic molecules are forced to have a transition with ΔΛ=0, and, some pages on, that there is a rule ΔΛ=0,+-1; so, which is the right one?
Surely they are referred to different situations, but it is not explained very clearly.
Thanks for the answers!

 

[DrClaude]

I don't really understand what Bransden and Joachain are saying in 11.1. They are discussing pure rotational spectra, so I don't see why they invoke $\Delta \Lambda$.

In any case, 11.3 is correct and $\Delta \Lambda = 0, \pm 1$.

 

[Sebacide]

Hi, thank you for your help: I thought the same, but during class it was mentioned the $\Delta \Lambda=0$ only.
So, if I have to describe the possible transition between two fixed electonic states, I have to separate the case in which one of the two lamdas is not zero and the one they are zero?

 

[DrClaude]

Hi, thank you for your help: I thought the same, but during class it was mentioned the $\Delta \Lambda=0$ only.
So, if I have to describe the possible transition between two fixed electonic states, I have to separate the case in which one of the two lamdas is not zero and the one they are zero?

The selection rules on $\Delta K$ is not the same for $\Lambda = 0 \leftrightarrow \Lambda = 0$ than for $\Lambda \neq 0 \leftrightarrow \Lambda \neq 0$

 

[Hyperfine]

I have a question about selection rules and electronic transitions of a diatomic molecule: I can't find a good explanation about that, and there's so much confusion about the right rules to use.

The following books provide definitive discussions of the selection rules:

1. G. Herzberg, "Molecular Spectra and Molecular Structure. Volume I: Spectra of Diatomic Molecules. Second Edition" D. Van Nostrand, 1950.

2. C. H. Townes and A. L. Scawlow, "Microwave Spectroscopy", McGraw Hill, 1955.

While not new books, they are classic discussions of the fundamental concepts.