Why is the transition from 3P1 to 3S1 in Helium forbidden?

In summary, the transition from 3P1 (m=0) to 3S1 (m=0) in Helium's n=2 state is forbidden. However, this fact was used by a group in a lecture to measure the fine structure constant with high precision. While it is expected to be an acceptable E1 transition according to atomic selection rules, the Clebsch Gordan coefficient for 1+1=1 with all m=0 vanishes, making the transition forbidden. This is due to the antisymmetric nature of 1+1=1, which is not accounted for in the selection rules table.
  • #1
Chaosmarch
7
0
The transition in the n=2 state of Helium from 3P1 (m=0) to 3S1 (m=0) is forbidden. I know this, because I went to a lecture where a group measured the fine structure constant very precisely using this fact. However, as far as I can see, according to atomic selection rules (available here: http://en.wikipedia.org/wiki/Selection_rule#Summary_table) this should be a perfectly acceptable E1 transition. Can anyone explain why this transition is forbidden?

For reference, the 3P1 (m=0) to 3S1 (m=+/-1) transition is allowed.
 
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  • #2
The Clebsch Gordan coefficient for 1+1=1 with all m=0 vanishes.
This is because 1+1=1 is antisymmetric.
 
  • #3
So the selection rules in that table do not account for matrix elements vanishing? I was under the impression the selection rules were derived from the CG coefficients.
 
  • #4
Wikipedia just left some out. Not unusual for W.
 

1. What are selection rules in chemistry?

Selection rules in chemistry refer to the set of rules that govern the transitions between different energy levels of atoms or molecules. These rules determine which transitions are allowed or forbidden, and they are based on principles of quantum mechanics.

2. Why are selection rules important?

Selection rules are important because they help us understand and predict the behavior of atoms and molecules. By following these rules, we can determine which transitions are likely to occur and which are not, which is crucial for understanding the properties and reactions of chemical systems.

3. What are the different types of selection rules?

There are two main types of selection rules: electric dipole selection rules and magnetic dipole selection rules. Electric dipole selection rules govern transitions that involve a change in the electric dipole moment of the system, while magnetic dipole selection rules govern transitions that involve a change in the magnetic dipole moment.

4. How do selection rules affect spectroscopy?

Selection rules play a crucial role in spectroscopy, as they determine which transitions can be observed and measured. For example, in infrared spectroscopy, only transitions that result in a change in the electric dipole moment are allowed, while in nuclear magnetic resonance spectroscopy, only transitions that result in a change in the magnetic dipole moment are allowed.

5. Can selection rules be violated?

Yes, selection rules can be violated under certain conditions. This usually occurs in highly symmetrical molecules, where the electric or magnetic dipole moment does not change due to the symmetry of the molecule. In these cases, forbidden transitions can still occur, but they are less likely and have lower intensities compared to allowed transitions.

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