Are certain combinations of quantum numbers (basis vectors) forbidden?

Click For Summary
SUMMARY

The discussion centers on the validity of the quantum state representation ##\ket{n l s m_l m_j}## in comparison to the more commonly used ##\ket{n l s m_l m_s}## and ##\ket{n l s j m_j}##. It is established that while ##\ket{n l s m_l m_j}## is not strictly forbidden, it is less desirable due to the constraints imposed by the relationship ##m_j = m_l + m_s##, particularly for spin-1/2 particles. The conversation highlights the limitations in determining quantum numbers based on known values, emphasizing that knowing all three angular momentum quantum numbers allows for the determination of at most one magnetic quantum number.

PREREQUISITES
  • Understanding of quantum mechanics and wavefunctions
  • Familiarity with angular momentum quantum numbers
  • Knowledge of Clebsch-Gordan coefficients
  • Basic concepts of spin and magnetic quantum numbers
NEXT STEPS
  • Study the application of Clebsch-Gordan coefficients in quantum mechanics
  • Research the implications of the total angular momentum quantum number ##J = L + S##
  • Explore the relationships between magnetic quantum numbers and angular momentum quantum numbers
  • Investigate the significance of intrinsic properties in quantum state representations
USEFUL FOR

Students and professionals in quantum mechanics, physicists specializing in atomic and subatomic systems, and anyone interested in the mathematical representation of quantum states.

Happiness
Messages
686
Reaction score
30
TL;DR
The electron's wavefunction is usually expressed in the standard basis {n, l, m_l, s, m_s}, but how to express it in the basis {n, l, m_l, s, m_j} ? (Note that m_s is replaced with m_j.) Or is it that certain combinations of quantum numbers are forbidden?
I've seen the hydrogen electron's wavefunction expressed in the basis ##\ket{n l s m_l m_s}## or ##\ket{n l s j m_j}##, but so far, never in ##\ket{n l s m_l m_j}##. My question is, are certain combinations of quantum numbers, eg, ##\ket{n l s m_l m_j}##, forbidden?

If ##\ket{n l s m_l m_j}## is not forbidden, how do we get it from the standard basis ##\ket{n l s m_l m_s}##?

I know how to get ##\ket{n l s j m_j}## from ##\ket{n l s m_l m_s}## using Clebsch-Gordan coefficients:
Screenshot 2024-07-12 at 5.29.38 AM.png


where ##J=L+S##.
##J## is the total angular momentum.

But other than that, I do not know how to express the wavefunction in other bases.
 
Last edited:
Physics news on Phys.org
##|nlsm_lm_s\rangle## is equivalent to ##|nlsm_lm_j\rangle## due to the relation ##m_j=m_l+m_s##. This relation does render some ##|nlsm_lm_j\rangle## states invalid, though, eg., for a spin-1/2 particle we have the requirement ##m_j = m_l \pm 1/2##. I think this just makes ##|nlsm_lm_j\rangle## a less-desirable way to write out the state, though it is equivalent.

In general, with the addition of angular momentum, if you know all three angular momentum quantum numbers (i.e. l, s, and j), you may know at most one magnetic quantum number. If you know all three magnetic quantum numbers, you may know at most two angular momentum quantum numbers. You can know less information, but not more (with the exception of the "top" or "bottom" states, where ##j=l+s## and ##m_j = \pm j##).

Also, in the case of adding orbital angular momentum and spin, we always know the s quantum number, because it is an intrinsic property of the particle, so that also effectively limits what combination of quantum numbers you can have.
 
  • Like
Likes   Reactions: Nugatory

Similar threads

  • · Replies 1 ·
Replies
1
Views
1K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 18 ·
Replies
18
Views
3K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 6 ·
Replies
6
Views
4K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K