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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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