12.E: Time-Dependent Perturbation Theory (Exercises)
( \newcommand{\kernel}{\mathrm{null}\,}\)
- 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⟩∗=12γℏexp(iωt), where e11, e22, γ, and ω are real. Show that idˆc1dt=γ2exp[+i(ω−ˆω21)t]ˆc2,idˆc2dt=γ2exp[−i(ω−ˆω21)t]ˆc1, where ˆc1=c1exp(ie11t/ℏ), ˆc2=c2exp(ie22t/ℏ), and ˆω21=E2+e22−E1−e11ℏ. 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+(ω−ˆω21)2sin2([γ2+(ω−ˆω21)2]1/2t2).
- Consider an atomic nucleus of spin-s and gyromagnetic ratio g placed in the magnetic field B=B0ez+B1[cos(ωt)ex−sin(ωt)ey], where B1≪B0. 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)ℏ2χs,m and Szχs,m=mℏχs,m, where −s≤m≤s. Furthermore, the instantaneous nuclear spin state is written χ=∑m=−s,scm(r)χs,m, where ∑m=−s,s|cm|2=1.
- Demonstrate that dcmdt=iγ2([s(s+1)−m(m−1)]1/2ei(ω−ω0)tcm−1+[s(s+1)−m(m+1)]1/2e−i(ω−ω0)tcm+1) for −s≤m≤s, where ω0=gμNB0/ℏ, γ=gμNB1/ℏ, and μN=eℏ/(2mp).
- Consider the case s=1/2. Demonstrate that if ω=ω0 and c1/2(0)=1 then c1/2(t)=cos(γt/2),c−1/2(t)=isin(γt/2).
- Consider the case s=1. Demonstrate that if ω=ω0 and c1(0)=1 then c1(t)=cos2(γt/2),c0(t)=i√2cos(γt/2)sin(γt/2),c−1(t)=−sin2(γt/2).
- 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)=i√3cos(γt/2)sin2(γt/2),c−1/2(t)=−√3cos2(γt/2)sin(γt/2),c−3/2(t)=−isin3(γt/2).
- 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)