# Thread: Regarding Hg emission spectra

1. My question is divided into two:
a. One of Hg spectral lines is a strong one, 253.7 nm, emitted by transition from 3P1 (triplet) to 1S0 (singlet). I'm afraid I don't read the 1st state right, since it says S=1 (due to 2S+1=3) & J=1, hence (?) L=0. However, P means L=1, isn't it ? What am I missing here ?
b. In LS coupling (mainly light atoms) we add all Ls & all Ss separately 1st, and only then add the sums to get J , while for very heavy atoms (e.g., Hg), the right model is jj coupling, hence the order of addition differs: we combine 1st pairs of L & S, then add the sums together to get J. I know that the reason behind the different procedures is the change in the relative strength between LL/SS interactions & LS interaction (the latter being stronger in heavy atoms). What I don't understand is why it is so. Namely, why one type of interaction necessitates the specific procedure used, but the other is wrong ? What's the relation between a dominant interaction & its appropriate way of vector addition ?
Thanks, BC

2. Originally Posted by BentzyCo
My question is divided into two:
a. One of Hg spectral lines is a strong one, 253.7 nm, emitted by transition from 3P1 (triplet) to 1S0 (singlet). I'm afraid I don't read the 1st state right, since it says S=1 (due to 2S+1=3) & J=1, hence (?) L=0. However, P means L=1, isn't it ? What am I missing here ?
b. In LS coupling (mainly light atoms) we add all Ls & all Ss separately 1st, and only then add the sums to get J , while for very heavy atoms (e.g., Hg), the right model is jj coupling, hence the order of addition differs: we combine 1st pairs of L & S, then add the sums together to get J. I know that the reason behind the different procedures is the change in the relative strength between LL/SS interactions & LS interaction (the latter being stronger in heavy atoms). What I don't understand is why it is so. Namely, why one type of interaction necessitates the specific procedure used, but the other is wrong ? What's the relation between a dominant interaction & its appropriate way of vector addition ?
Thanks, BC
The LS coupling approximation is just that -- an approximation. By its selection rules, the 253.65nm line would be forbidden (only triplet-triplet and singlet-singlet transitions would be allowed). The strength of the "forbidden" triplet-singlet transition line reveals the extent to which the approximation holds.

3. Originally Posted by tk421
The LS coupling approximation is just that -- an approximation. By its selection rules, the 253.65nm line would be forbidden (only triplet-triplet and singlet-singlet transitions would be allowed). The strength of the "forbidden" triplet-singlet transition line reveals the extent to which the approximation holds.
I know all that, and also didn't ask the nature of this approximation, though the type of the approximation is relevant to the atomic change itself. My main concerns are, as I said: (1) figuring about the term symbol 3P1, as I described in my original message; and (2) the relation between LS / jj couplings and the corresponding procedure of adding the angular momenta of the atom system. My questions are well defined, and I hope they'll be addressed accordingly. Best wishes to all of us.

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