GR8677 #92



Comments 
casseverhart13 20190819 09:43:53  This problem helps me more. Thanks for your sharing! Tree Solutions Lake Mary   whereami 20180409 03:39:50  My memory goes that selection rule regards only delta l and delta ml. I didn\'t even believe selection rule works on those aspects. \r\nTurns out that delta s =0 is one of the selection rules. And there are none regarding n values. My thought against A is that n could be 0. But of course that is not forbidden by the selection rules.   kevintah 20150909 09:05:11  This site has more information including the full derivation and results. http://quantummechanics.ucsd.edu/ph130a/130_notes/node422.html\r\n There is no selection rule for spin as mentioned\r\n
sam16 20151020 14:12:33 
page not found!

NoPhysicist2 20170319 11:36:32 
http://quantummechanics.ucsd.edu/ph130a/130_notes/node422.html

  flyboy621 20101115 19:54:48  is always 0, and is always +/ 1. Therefore is also +/ 1. Thus (B) and (E) are compatible and (D) is not.   dvarjas 20101006 07:52:54  My oppinion after much thougth about why D is incorrect and E correct: If there are two electrons on the same orbit, their spatial wave functions are symmetric, therefore they form a spin singlet with s=0. It could happen, that an unpaired electron with s=1/2 jumps to a half filled orbit with another electron with s=1/2, making a transition from s=1/2+1/2=1 to s=0, or the reverse process could also happen. But the total angular momentum j is conserved for the whole system, photon takes away j=1, so as in (E). But and cannot be greater than 1 for a single electron process, is needed for conservation of total angular momentum. So the only process allowed is that an unpaired electron jumps to an empty orbit.
gman 20101111 23:11:59 
How about this...
When the electron transitions, . Since it's an electron, and it doesn't change spin, . So .
Now the photon has spin 1, and has no orbital angular momentum (right? what's it.. orbiting?) So , , and thus . All consistent with the above selection rules. The orbital angular momentum lost becomes spin angular momentum. Thus,

  hanmas 20080411 20:48:21  s always equals to 1/2 for electron, so there is no such thing as = 1.
lattes 20080805 22:40:54 
Good observation!

Poop Loops 20081105 21:43:06 
But can't you go +1/2 or 1/2? I thought that's what it meant. Either way, it still doesn't work so that's the answer.

wittensdog 20090728 08:17:19 
Indeed, s is the magnitude of spin, and m_s would be the value of it's projection along a given axis, often denoted z by convention. It's the same as how l is the magnitude of orbital angular momentum, and m_l is it's projection along an axis. Since in the answers they did make note of l vs. m_l (and not simply just m), I believe they meant to distinguish between s and m_s. While m_s can change for an electron, between 1/2 and 1/2, the MAGNITUDE of spin for any particle is fixed, so having any change in s is nonsensical. I actually didn't even notice this myself, and was originally thinking it meant m_s. Good observation!

  keflavich 20051110 11:04:25  Since , and is unrestrictied (i.e. it can equal zero), E is compatible with electric dipole selection rules. This whole question is about photon emission: photons are emitted when electrons change energy levels, and it\'s the change in energy level\'s that\'s being restricted.
yosun 20051110 11:33:18 
if and is unrestricted, then shouldn't also be unrestricted?

keflavich 20051110 12:05:46 
Maybe my logic was wrong. In retrospect, I think any restriction should be incompatible with electric dipole emission since changes in the spin are related to hyperfine splitting. If you were to consider only electric dipole emission by excited states of atoms, I think you must assume that spin doesn't change and therefore . Any would not involve or be involved in a transition between excitation states of an atom. I think it would require two emissions  one hyperfine, one electric dipole  to change both and .
Anyway, certainly wouldn't violate any selection rules, but would prevent any transition that did NOT change the spin of an electron, which is not a condition of electric dipole emission.

keflavich 20051110 17:48:31 
Right, valid point (though maybe you're refering to C). Instead of arguing more, I decided to look it up... http://hyperphysics.phyastr.gsu.edu/hbase/quantum/hydazi.html />
is a valid restriction for some case (though I don't really understand what a j=0 state would be since ), there is no spin restriction. As it turns out, hyperphysics claims that in photon emission the electron spin quantum number doesn't change, so perhaps the C transition (in 0177:82) is allowed because ?

keflavich 20051110 17:53:07 
Ignore my comments, just check out the website. I've contradicted myself enough on this problem that my answers are probably unreliable.

mhas035 20070412 23:10:09 
The electric dipole operator is not dependant on the spin of the electron so for electirc dipole transitions, transitions between different spin states will give a probability of zero

  Buli 20051108 21:14:27  If there is no selection rule for spin, then there would be no for j too. Recall that j=l+s. I was wondering this is because the photon emission obeys the conservation of spin momentum so that delta s is zero. Can anybody tell your opinion? Thanks.
mpdude8 20120419 16:56:25 
You said it yourself: j = l +s, there IS a selection rule for l (it must change by 1) but no selection rule for s, thus j must change with l. If there was a selection rule for s, you may be able to have the change in l and s cancel perfectly.

  snimi1 20051105 00:54:17  The official answer is D, which I tend to agree because there is no spin related selection rule.
yosun 20051105 23:09:46 
Thanks snimi1 for the correction!

athithi 20080410 22:50:20 
i am wondering about the option A ..how n can take negative values ?

thebigshow500 20081014 10:56:46 
Same question on choice (A)...the subject GRE test is coming soon, anyone care to explain?

kobayashi_maru 20081015 07:31:37 
The problem specifies emission of photons by excited states. For an electron to emit a photon, it has to drop from a higher excited state to a lower one (or ground state). Thus, is negative.

thebigshow500 20081015 18:45:29 
I am an idiot. I thought has to be a positive value. But it should be = negative value.
= lower state
= higher state
Thanks a lot!

 

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