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Question: Derive the fine structure formula (Equation 6.66) from the relativistic correction (Equation 6.57) and the spin-orbit coupling (Equation 6.65). Hint: Note tha j=l12t; treat the plus sign and the minus sign separately, and you'll find that you get the same final answer either way.

Short Answer

Expert verified

The fine structure formula is Er'=(En)22mc2[3-4n(j+12)].

Step by step solution

01

Formula Used

The relativistic correction in the energy levels is:

Er'=-(En)2(2mc2)4nl+12-3

And, the spin-orbit coupling: E50'=(En)22mc2[n[j(j+1)-l(l+1)-34]l(l+12)(l+1)]

Where,

j=l12l=j12

The equations 6.65, 6.66, 6.67 are

role="math" localid="1658296542883" Es01=(En)2mc2[n[j(j+1)-l(l+1)-34]l(l+12)(l+1)]......(6.65)Efs1=(En)22mc2[3-4nj+12]....(6.66)E=13.6eVn2[a+2n2(4nj+12-34)]....(6.67)

02

The Spin-orbit coupling form

Takel=j-12 and substitute into the relativistic correction equation:

Er'=-(En)22mc2[4nl-12+12-3]

And the spin-orbit coupling has the form:

Eso'=(En)2mc2[n[j(j+1)-j-12(j-12+1)]-34(j-12)(j-12+12)(j-12+1)]=(En)2mc2[n[j(j+1)-(j-12)(j-12)]-34j(j-12)(j+12)]=(En)2mc2[n[j2+j-(j2-14)]-34j(j2+14)]=(En)2mc2[n[j-12]j(j-12)(j+12)]=(En)2mc2[nj(j-12)]

03

The fine structure formula

Now, calculate the fine structure formula

Efs'=Er'+Eso'=(En)2mc24nj-3+(En)2mc22njj+12=(En)2mc22njj+12-4nj+3=(En)2mc22nj-4nj+12j2j+12+3

Solve further the equation

Efs'=(En)2mc22nj-4n2-2njj2j+12+3=(En)2mc23-4nj+12

04

Again, calculate spin-orbit coupling

Now take l=j+12 and substitute into the relativistic correction equation:

Er'=-(En)22mc24nj+12+12-3=-(En)22mc24nj+1-3

And the spin-orbit coupling has the form,

Eso'=(En)2mc2njj+1-j+12j+12+1-34j+12j+12+12j+12+1=(En)2mc2nj2+j-j2-32j-12j-34-34j+12j+1j+32=(En)2mc2n-j-32j+12j+1j+32=(En)2mc2-nj+32j+12j+1j+32=(En)2mc2-nj+12j+1

05

The fine structure formula

Now, calculate the fine structure formula:

Efs'=Er'+Eso'=-(En)22mc24nj+1-3-(En)22mc2-2nj+1j+12=(En)22mc23-4nj+1+2nj+1j+12=(En)22mc23-4nj+1j+12+2nj+1j+1j+1j+12

Solve the equation further

=(En)22mc23-4n+2n+2nj+1j+12=(En)22mc23-4nj+1j+1j+12=(En)22mc23-4nj+12

This is the same answer for l=j-12.

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Most popular questions from this chapter

Suppose we put a delta-function bump in the center of the infinite square well:

H'=伪未(x-a/2)

whereais a constant.

(a) Find the first-order correction to the allowed energies. Explain why the energies are not perturbed for evenn.

(b) Find the first three nonzero terms in the expansion (Equation 6.13) of the correction to the ground state,11.

Problem 6.6 Let the two "good" unperturbed states be

0=a0+b0

whereandare determined (up to normalization) by Equation 6.22(orEquation6.24). Show explicitly that

(a)are orthogonal;role="math" localid="1655966589608" (+0-0=0);

(b) +0|H'|-0=0;

(c)0|H'|0=E1,withE1given by Equation 6.27.

Consider a particle of mass m that is free to move in a one-dimensional region of length L that closes on itself (for instance, a bead that slides frictionlessly on a circular wire of circumference L, as inProblem 2.46).

(a) Show that the stationary states can be written in the formn(x)=1Le2inx/L,(-L/2<x<L/2),

wheren=0,1,2,....and the allowed energies areEn=2mnL2.Notice that with the exception of the ground state (n = 0 ) 鈥 are all doubly degenerate.

(b) Now suppose we introduce the perturbation,H'=-V0e-x2/a2where aLa. (This puts a little 鈥渄imple鈥 in the potential at x = 0, as though we bent the wire slightly to make a 鈥渢rap鈥.) Find the first-order correction to En, using Equation 6.27. Hint: To evaluate the integrals, exploit the fact that aLato extend the limits from L/2toafter all, H鈥 is essentially zero outside -a<x<a.

E1=12Waa+WbbWaa-Wbb2+4Wab2(6.27).

(c) What are the 鈥済ood鈥 linear combinations ofnand-n, for this problem? Show that with these states you get the first-order correction using Equation 6.9.

En'=n0H'n0(6.9).

(d) Find a hermitian operator A that fits the requirements of the theorem, and show that the simultaneous Eigenstates ofH0and A are precisely the ones you used in (c).

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(the integration is over the usual range:0<<,0<<2). Use this result to demonstrate that

(3Sp.rSe.r-Sp.Ser3)=0

For states with I=0. Hint:r=蝉颈苍胃肠辞蝉蠒i+蝉颈苍胃蝉颈苍蠒+肠辞蝉胃k.

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