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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 you don鈥檛 assume Haa=Hbb=0

(a) Find ca(t)and cb(t) in first-order perturbation theory, for the case

.show that , to first order in .

(b) There is a nicer way to handle this problem. Let

.

Show that

where

So the equations for are identical in structure to Equation 11.17 (with an extra

(c) Use the method in part (b) to obtain in first-order
perturbation theory, and compare your answer to (a). Comment on any discrepancies.

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)

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).

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

A particle starts out (at time t=0 ) in the Nth state of the infinite square well. Now the 鈥渇loor鈥 of the well rises temporarily (maybe water leaks in, and then drains out again), so that the potential inside is uniform but time dependent:V0(t),withV0(0)=V0(T)=0.

(a) Solve for the exact cm(t), using Equation 11.116, and show that the wave function changes phase, but no transitions occur. Find the phase change, role="math" localid="1658378247097" (T), in terms of the function V0(t)

(b) Analyze the same problem in first-order perturbation theory, and compare your answers. Compare your answers.
Comment: The same result holds whenever the perturbation simply adds a constant (constant in x, that is, not in to the potential; it has nothing to do with the infinite square well, as such. Compare Problem 1.8.

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