Chapter 9: Q13P (page 363)
Close the 鈥渓oophole鈥 in Equation 9.78 by showing that ifthen
Short Answer
Showedthat If then
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Chapter 9: Q13P (page 363)
Close the 鈥渓oophole鈥 in Equation 9.78 by showing that ifthen
Showedthat If then
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We have encountered stimulated emission, (stimulated) absorption, and spontaneous emission. How come there is no such thing as spontaneous absorption?
Suppose the perturbation takes the form of a delta function (in time):
;
Assume thatif and
find and check that . What is the net probability that a transition occurs? Hint: You might want to treat the delta function as the limit of a sequence of rectangles.
Answer:
Magnetic resonance. A spin-1/2 particle with gyromagnetic ratio at rest in a static magnetic field precesses at the Larmor frequency (Example 4.3). Now we turn on a small transverse radiofrequency (rf) field, so that the total field is
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(a) Construct the Hamiltonian matrix (Equation 4.158) for this system.
(b) If is the spin state at time , show that
where is related to the strength of the rf field.
(c) Check that the general solution for and in terms of their initial values and is
role="math" localid="1659004637631"
Where
(d) If the particle starts out with spin up (i.e. ,), find the probability of a transition to spin down, as a function of time. Answer:
(e) Sketch the resonance curve,
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as a function of the driving frequency (for fixed and ). Note that the maximum occurs at Find the "full width at half maximum,"
(f) Since we can use the experimentally observed resonance to determine the magnetic dipole moment of the particle. In a nuclear magnetic resonance (nmr) experiment the factor of the proton is to be measured, using a static field of 10,000 gauss and an rf field of amplitude gauss. What will the resonant frequency be? (See Section for the magnetic moment of the proton.) Find the width of the resonance curve. (Give your answers in Hz.)
The half-life of an excited state is the time it would take for half the atoms in a large sample to make a transition. Find the relation betweenrole="math" localid="1658300900358" (the 鈥渓ife time鈥 of the state).
In Equation 9.31 assumed that the atom is so small (in comparison to the wavelength of light) that spatial variations in the field can be ignored. The true electric field would be
If the atom is centered at the origin, then over the relevant volume, so and that's why we could afford to drop this term. Suppose we keep the first-order correction:
The first term gives rise to the allowed (electric dipole) transitions we considered in the text; the second leads to so-called forbidden (magnetic dipole and electric quadrupole) transitions (higher powers of lead to even more "forbidden" transitions, associated with higher multipole moments).
(a) Obtain the spontaneous emission rate for forbidden transitions (don't bother to average over polarization and propagation directions, though this should really be done to complete the calculation). Answer:role="math" localid="1659008133999"
(b) Show that for a one-dimensional oscillator the forbidden transitions go from level to levelrole="math" localid="1659008239387" and the transition rate (suitably averaged over ) is
(Note: Here is the frequency of the photon, not the oscillator.) Find the ratio of the "forbidden" rate to the "allowed" rate, and comment on the terminology.
(c) Show that the transition in hydrogen is not possible even by a "forbidden" transition. (As it turns out, this is true for all the higher multipoles as well; the dominant decay is in fact by two-photon emission, and the lifetime it is about a tenth of a second
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