## Spin Orbit Interaction

1. The problem statement, all variables and given/known data
(a) Consider a system composed of two electrons with orbital angular momentum
quantum numbers l_1 = 4 and l_2 = 2.
Give all the possible values of
(i) the total orbital angular momentum quantum number L,
(ii) the total angular momentum quantum number J. [8]
(b) Explain what is meant by the parity of an atomic or nuclear state. Show that
the state described by the wave-function $$\psi= r cos \theta exp(r/2a)$$ has parity
quantum number -1.

2. Relevant equations
$$J=L+S \\  j=l \pm s \\  L^2 = l(l+1) \\  P \psi = e^{i\theta}\psi$$

3. The attempt at a solution

I know this is probably extremely easy but I've been given no examples and I keep getting myself in a muddle. Are the answers for L and J suppose to come out as non integers?
$$L^2 = l(l+1) \\  L_1=\sqrt{20} = \pm 4.47  \\  L_2=\sqrt{6} = \pm 2.449 \\ L = -6.919, -2.021, 2.021, 6.919$$
Are the negative values valid?
 Mentor Blog Entries: 10 (i) Start by thinking about two cases: 1. the angular momentum vectors of the two electrons point in the same direction 2. they point in opposite directions What would be l (that's a lower-case "L") for the system in these two situations? Are any other values of l possible?
 l would be 6, 2, -2, -4 ?

Mentor
Blog Entries: 10

## Spin Orbit Interaction

Okay, except that l just takes on positive values or zero, so it would be 6 and 2.
Next, what other values could it have, given that the two vectors need not be aligned (i.e. not in the same direction or opposite direction)?

Note:
 $$L^2 = l(l+1)$$
which should probably be $$L^2 = l(l+1)\hbar^2$$

This isn't really needed here. L refers to the magnitude of the actual angular momentum. But it is much more common to refer to angular momentum simply by the quantum number, l. So for example, if l=2, we just say the orbital angular momentum is 2, rather than the actual value of
$$\sqrt{6} \ \hbar$$

When I read your questions, it seems they really want the quantum numbers. These will be integers (or, if spin is included, then possibly half-integers).

 Okay, except that l just takes on positive values or zero, so it would be 6 and 2. Next, what other values could it have, given that the two vectors need not be aligned (i.e. in the same direction or opposite direction)?
Sorry I don't understand, are you saying there are more values for l (lower L)?

Presumably when I have all the l's I just $$\pm 1/2$$ from each one for j?

For b. I understand I have to multiple it by $$e^{i\theta}$$ do I need to convert the cos to terms of $$e^{i\theta}$$

Mentor
Blog Entries: 10
You probably should review the rules for adding two angular momenta together. It should be explained in your textbook or class lectures.

 Quote by dq1 Sorry I don't understand, are you saying there are more values for l (lower L)?
Yes. So far, we have just found the maximum and minimum values, the ones we get if the two L's (vectors) point in the same direction (maximum, 4+2=6) or directly opposite (minimum, 4-2=2).

If the two L's are at some angle to each other, l will be somewhere in between 2 and 6.

 Presumably when I have all the l's I just $$\pm 1/2$$ from each one for j?
This one is more complicated. If there were just 1 electron, then yes you'd $$\pm 1/2$$ since one electron has a spin of 1/2. But in this case you need to $$\pm$$ the combined spin of the two electrons.

 For b. I understand I have to multiple it by $$e^{i\theta}$$ do I need to convert the cos to terms of $$e^{i\theta}$$
I'm not sure, it has been a while since I worked in this area.