Solving Ladderoperator Problem for c_+ Expressed in j and m

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Discussion Overview

The discussion revolves around deriving the expression for the constant \( c_+ \) in the context of quantum mechanics, specifically related to the ladder operator \( \hat{J}_+ \) acting on spherical harmonics \( |Y_{jm}\rangle \). The scope includes mathematical reasoning and technical explanations related to quantum mechanics.

Discussion Character

  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • Daniel seeks assistance in finding the expression for the constant \( c_+ \) in the equation \( \hat{J}_+|Y_{jm}\rangle = c_+|Y_{jm+1}\rangle \).
  • One participant suggests consulting a quantum mechanics textbook, specifically mentioning Cohen-Tanoudji, implying that the problem is a classical one within the field.
  • Another participant provides a derivation involving the adjoint of the ladder operator and inner products, leading to the expression \( C = h \sqrt{j(j+1) - m^2 - m} \) for \( c_+ \).

Areas of Agreement / Disagreement

The discussion does not present a consensus, as it includes a request for help, a suggestion to consult literature, and a mathematical derivation that may not be universally accepted or verified by all participants.

Contextual Notes

The derivation involves several assumptions about the properties of the operators and the states, including the use of adjoint operators and inner products, which may depend on specific definitions and conventions in quantum mechanics.

danja347
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I need help figuring out the expression for the constant [tex]c_+[/tex]
expressed in j and m in the following equation:

[tex]\hat J_+|Y_{jm}>=c_+|Y_{jm+1}>[/tex]

Y is just spherical harmonics and [tex]\hat J_+=\hat J_x + i\hat J_y[/tex] is a ladderoperator.

/Daniel
 
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danja347 said:
I need help figuring out the expression for the constant [tex]c_+[/tex]
expressed in j and m in the following equation:

[tex]\hat J_+|Y_{jm}>=c_+|Y_{jm+1}>[/tex]

Y is just spherical harmonics and [tex]\hat J_+=\hat J_x + i\hat J_y[/tex] is a ladderoperator.

/Daniel

1.Have u tried to look it into your QM book?It's something pretty "classical".Try Cohen-Tanoudji.
2.I would have given u alink,but the server at univ texas at austin is dead.Anyway...I would have actually wanted to upload that chapter from the course,but the server wouldn't accept anything more than 50KB.

Good Luck!
 
Thanks... its all clear now! :-/
 
trying to recall...oh yeah:

J+|jm>=C|j(m+1)>

<jm|adjoint(J+)=<j(m+1)|C*

J+=Jx+iJy
adjoint(J+)=Jx-iJy=J- (since J is Hermitian)

<jm|adjoint(J+)=<jm|J-

So taking the inner product:

<jm|J-J+|jm> = CC*<j(m+1)|j(m+1)> = CC*

J-J+=(Jx-iJy)(Jx+iJy)=JxJx+JyJy+i[Jx,Jy]=J^2 - (Jz)^2 -hJz

<jm|J-J+|jm>=<jm|J^2 - (Jz)^2 -hJz}jm>=j(j+1)h^2 -m^2 h^2 - h^2 m = CC*

So taking the square root:

C=h sqrt(j(j+1) -m^2 -m)
 
Last edited:

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