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Physics LibreTexts

12.E: Time-Dependent Perturbation Theory (Exercises)

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  1. Consider the two-state system examined in Section 1.3.[ex8.1] Suppose that 1|H1|1=e11,2|H1|2=e22,1|H1|2=2|H1|1=12γexp(iωt), where e11, e22, γ, and ω are real. Show that idˆc1dt=γ2exp[+i(ωˆω21)t]ˆc2,idˆc2dt=γ2exp[i(ωˆω21)t]ˆc1, where ˆc1=c1exp(ie11t/), ˆc2=c2exp(ie22t/), and ˆω21=E2+e22E1e11. Hence, deduce that if the system is definitely in state 1 at time t=0 then the probability of finding it in state 2 at some subsequent time, t, is P2(t)=γ2γ2+(ωˆω21)2sin2([γ2+(ωˆω21)2]1/2t2).
  2. Consider an atomic nucleus of spin-s and gyromagnetic ratio g placed in the magnetic field B=B0ez+B1[cos(ωt)exsin(ωt)ey], where B1B0. Let χs,m be a properly normalized simultaneous eigenstate of S2 and Sz, where S is the nuclear spin. Thus, S2χs,m=s(s+1)2χs,m and Szχs,m=mχs,m, where sms. Furthermore, the instantaneous nuclear spin state is written χ=m=s,scm(r)χs,m, where m=s,s|cm|2=1.
    1. Demonstrate that dcmdt=iγ2([s(s+1)m(m1)]1/2ei(ωω0)tcm1+[s(s+1)m(m+1)]1/2ei(ωω0)tcm+1) for sms, where ω0=gμNB0/, γ=gμNB1/, and μN=e/(2mp).
    2. Consider the case s=1/2. Demonstrate that if ω=ω0 and c1/2(0)=1 then c1/2(t)=cos(γt/2),c1/2(t)=isin(γt/2).
    3. Consider the case s=1. Demonstrate that if ω=ω0 and c1(0)=1 then c1(t)=cos2(γt/2),c0(t)=i2cos(γt/2)sin(γt/2),c1(t)=sin2(γt/2).
    4. Consider the case s=3/2. Demonstrate that if ω=ω0 and c3/2(0)=1 then c3/2(t)=cos3(γt/2),c1/2(t)=i3cos(γt/2)sin2(γt/2),c1/2(t)=3cos2(γt/2)sin(γt/2),c3/2(t)=isin3(γt/2).
  3. Demonstrate that a spontaneous transition between two atomic states of zero orbital angular momentum is absolutely forbidden. (Actually, a spontaneous transition between two zero orbital angular momentum states is possible via the simultaneous emission of two photons, but takes place at a very slow rate .)

Contributors and Attributions

  • Richard Fitzpatrick (Professor of Physics, The University of Texas at Austin)


This page titled 12.E: Time-Dependent Perturbation Theory (Exercises) is shared under a not declared license and was authored, remixed, and/or curated by Richard Fitzpatrick.

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