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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.

Short Answer

Expert verified

The first-order correction to the allowed energies=2asin2苍蟺2

The the first three nonzero terms in the expansion of the correction to the ground state,11is=a2尘伪22sin3蟺虫a-13sin5蟺虫a+16sin7蟺虫a

Step by step solution

01

Stationary state of a one-dimensional infinite square well.

The stationary state of a one-dimensional infinite square wellis:

n0=2asin(苍蟺ax)

02

Step 2: The first-order correction to the allowed energies.

a)

For the infinite square well:

H^'=x-a2,=const

Solve the problem by considering the stationary state of a one-dimensional infinite square well, that is:n0=2asin苍蟺axEn'=n0H^'n0=0aH^'^n0dX=2a0ax-a2sin2苍蟺虫adx=2伪补2sin苍蟺a.a2=2asin2苍蟺2-fornoddEn,=2asin2苍蟺2=2a-fornevenEn'=0

03

Step 3: The first three nonzero terms in the expansion.

b)

Use the formula and substitute each value.

n1=mnn0H^'n0En0-Em0m0

For n=1

n0H^'n0=2a0adxsin尘蟺虫ax-a2sin蟺虫a=2asin尘蟺a.a2sina.a2=2asin尘蟺2

Note that:m1,n=1,mn

form=0sin0=0

form=0sin尘蟺=0

The first three non- zero terms (odd)

m=3,5,7;n=1En0=n222ma2E10=222ma211=2aa2asin32222ma21-9sin3蟺虫a+sin52sin5蟺虫a222ma21-25+sinin52sin5蟺虫a222ma21-49=2aa2a2ma222ma22218sin3蟺虫a-124sin5蟺虫a+148sin7蟺虫a

Proceed further and obtain the result as,

=a2ma22sin3蟺虫a-13sin5蟺虫a+16sin7蟺虫a

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Most popular questions from this chapter

Van der Waals interaction. Consider two atoms a distanceapart. Because they are electrically neutral you might suppose there would be no force between them, but if they are polarizable there is in fact a weak attraction. To model this system, picture each atom as an electron (mass m , charge -e ) attached by a spring (spring constant k ) to the nucleus (charge +e ), as in Figure. We'll assume the nuclei are heavy, and essentially motionless. The Hamiltonian for the unperturbed system is

H0=12mp12+12kx12+12mp22+12kx22[6.96]

The Coulomb interaction between the atoms is

H'=14蟺系0(e2R-e2R-x1-e2R+x2+e2R-x1+x2 [6.97]

(a) Explain Equation6.97. Assuming that localid="1658203563220" |x1| and |x2|are both much less than, show that

localid="1658203513972" H'-e2x1x220R3 [6.98]

(b) Show that the total Hamiltonian (Equationplus Equation) separates into two harmonic oscillator Hamiltonians:

H=[12mp+2+12(k-e220R3x+2]+[+12mp-2+12(k+e220R3x-2] [6.99]

under the change of variables

x12(x1x2) Which entails p=12(p1p2) [6.100]

(c) The ground state energy for this Hamiltonian is evidently

E=12(++-) Where =k(e2/20R3)m [6.101]

Without the Coulomb interaction it would have been E0=0, where 0=k/m. Assuming that, show that

VE-E0-8m203(e220)21R6. [6.102]

Conclusion: There is an attractive potential between the atoms, proportional to the inverse sixth power of their separation. This is the van der Waals interaction between two neutral atoms.

(d) Now do the same calculation using second-order perturbation theory. Hint: The unperturbed states are of the form n1(x1)n2(x2), where n(x)is a one-particle oscillator wave function with mass mand spring constant k;Vis the second-order correction to the ground state energy, for the perturbation in Equation 6.98 (notice that the first-order correction is zero).

(a) Find the second-order correction to the energies(En2)for the potential in Problem 6.1. Comment: You can sum the series explicitly, obtaining -for odd n.

(b) Calculate the second-order correction to the ground state energy(E02)for the potential in Problem 6.2. Check that your result is consistent with the exact solution.

Prove Kramers' relation:

sn2rs-(2s+1)ars-1+s4[(2l+1)2-s2]a2rs-2=0

Which relates the expectation values of rto three different powers (s,s-1,ands-2),for an electron in the state n/mof hydrogen. Hint: Rewrite the radial equation (Equation) in the form

u''=[l(l+1)r2-2ar+1n2a2]u

And use it to expressrole="math" localid="1658192415441" (ursu'')drin terms of (rs),(rs-1)and(rs-2). Then use integration by parts to reduce the second derivative. Show that (ursu'')dr=-(s/2)(rs-1)and(u'rsu')dr=-[2/s+1](u''rs+1u')dr. Take it from there.

Let aand bbe two constant vectors. Show that

(a.r)(b.r)蝉颈苍胃诲胃诲蠒=43(a.b)

(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.

For the harmonic oscillator[Vx=1/2kx2], the allowed energies areEN=(n+1/2)魔蝇,(n=0.1.2,..),whererole="math" localid="1656044150836" =k/mis the classical frequency. Now suppose the spring constant increases slightly:k(1+')k(Perhaps we cool the spring, so it becomes less flexible.)

(a) Find the exact new energies (trivial, in this case). Expand your formula as a power series in,, up to second order.

(b) Now calculate the first-order perturbation in the energy, using Equation 6.9. What ishere? Compare your result with part (a).

Hint: It is not necessary - in fact, it is not permitted - to calculate a single integral in doing this problem.

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