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^{ 3}P_{1} (triplet) to ^{1}S_{0} (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** L**s & all** S**s 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