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An electron in the n=3,l=0,m=0state of hydrogen decays by a sequence of (electric dipole) transitions to the ground state.

(a) What decay routes are open to it? Specify them in the following way:

|300|nlm|n'l'm'|100.

(b) If you had a bottle full of atoms in this state, what fraction of them would decay via each route?

(c) What is the lifetime of this state? Hint: Once it鈥檚 made the first transition, it鈥檚 no longer in the state |300\rangle鈭300鉄, so only the first step in each sequence is relevant in computing the lifetime.

Short Answer

Expert verified

(a)(|300|200and|300|1100)

(b)|211|r|300|2=2|211|x|300|2=K2/3

(c)鈥夆赌=1R=1.58107s

Step by step solution

01

(a) Specifying the decay routes.

(|300|200and|300|1100violatel=1rule.)

02

(b) Fraction of decay via each route. 

From Eq. 11.76:

{ifm'=m,鈥夆赌夆塼henn'l'm'|x|nlm=n'l'm'|y|nlm=0ifm'=m1,thenn'l'm'|x|nlm=i(n'l'm'|y|nlmandn'l'm'|z|nlm=0(11.76).

210|r|300=210|z|300k^211|r|300=211|x|300i^+211|y|300j^

211|x|300=i211|y|.Thus|210|r|300|2=|210|z|300|2鈥夆赌夆塧nd鈥夆赌夆|211|r|300|2=2|211|x|300|2

So there are really just two matrix elements to calculate.

21m=R21Y1m,鈥夆赌夆300=R30Y00.From Table 4.3:

Y10Y00cossindd=34140cos2sind02d=34(cos33)|0(2)=32(23)=13

(Y11)*Y00sin2cosdd=38140sin3d02coseid=1432(43)[02cos2di02cossind]=16(0)=16

From Table 4.7:

K0R21R30r3dr=124a3/2227a3/20raer/2a[123ra+227(ra)2]er/3ar3dr=192a3a40(123u+227u2)u4e5u/6du=a92[4!(65)5235!(65)6+2276!(65)7]

=a924!6556(52365+22763)=a924!6556=2734562a

So,

211|x|300=R21(Y11)*(rsincos)R30Y00r2sindrdd=K(16)

210|z|300=R21Y10(rcos)R30Y00r2sindrdd=K(13)

|210|r|300|2=|210|z|300|2=K2/3;|211|r|300|2=2|211|x|300|2=K2/3

Evidently the three transition rates are equal, and hence 1/3 go by each route.

03

(c) Lifetime of the state

For each mode,

A=3e2|r|230c3here,=E3E2=1(E19E14)=536E1,

so the total decay rate is

R=3(536E1)3e230c313(2734562a)2=6(25)9(E1mc2)2(ca)

=6(25)9(13.60.511106)2(31080.5291010)/s=6.32106/s=1R=1.58107s

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Most popular questions from this chapter

Suppose the perturbation takes the form of a delta function (in time):

H^'=U^(t);

Assume thatUaa=Ubb=0,andletUab=Uba+=if ca(-)=1and cb(-)=0,

find ca(t)andcb(t),and check that lc(t)l2+lcb(t)l2=1. What is the net probability(Pabfort) that a transition occurs? Hint: You might want to treat the delta function as the limit of a sequence of rectangles.

Answer:Pab=sin2(||lh)

As a mechanism for downward transitions, spontaneous emission competes with thermally stimulated emission (stimulated emission for which blackbody radiation is the source). Show that at room temperature (T = 300 K) thermal stimulation dominates for frequencies well below 51012Hz , whereas spontaneous emission dominates for frequencies well above . Which mechanism dominates for visible light?

Show that the spontaneous emission rate (Equation 9.56) for a transition from n,lton',l' in hydrogen is

e22l23o0hc3{l+12l+1ifl'=l+1l2l-1ifl'=l-1

where

l=0r3Rnl(r)Rn'J'(r)dr

(The atom starts out with a specific value of m, and it goes toamyof the state鈥檚 mconsistent with the selection rules:m'=m+1,m or m -1 . Notice that the answer is independent of m .) Hint: First calculate all the nonzero matrix elements of x,y,and z between role="math" localid="1658313179553" |n|m>andn'l'm'>for the case . From these, determine the quantity

|n'.1+1.m+1rn|m|2+|n'.1+1,mr|nm|2+|n'.1+1,m-1r|nm|2

Then do the same forl'=l-1.

Calculate ca(t)andcb(t), to second order, for a time-independent perturbation in Problem 9.2. Compare your answer with the exact result.

Solve Equation 9.13 for the case of a time-independent perturbation, assumingthatandcheck that

. Comment: Ostensibly, this system oscillates between 鈥溾 Doesn鈥檛 this contradict my general assertion that no transitions occur for time-independent perturbations? No, but the reason is rather subtle: In this are not, and never were, Eigen states of the Hamiltonian鈥攁 measurement of the energy never yields. In time-dependent perturbation theory we typically contemplate turning on the perturbation for a while, and then turning it off again, in order to examine the system. At the beginning, and at the end,are Eigen states of the exact Hamiltonian, and only in this context does it make sense to say that the system underwent a transition from one to the other. For the present problem, then, assume that the perturbation was turned on at time t = 0, and off again at time T 鈥攖his doesn鈥檛 affect the calculations, but it allows for a more sensible interpretation of the result.

ca=-ihHabeigtcb,cb=-ihHbaeigtca 鈥(9.13).

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