课程名称︰量子力学（二）
课程性质︰【必修】物理学研究所；【选修】天文物理所、应用物理所、物理学系
课程教师︰管希圣
开课学院：理学院
开课系所︰物理学研究所
考试日期（年月日）︰2017/05/02
考试时限（分钟）：210 min
试题 :
Midterm Examination
PHYS M1420: Quantum Mechanics(II)
Tuesday; May 2, 2017
Problem 1.(22 points)
Consider the problem of a spinless particle of mass m in an attractive
spherical square well potential,
┌ -V_0 ,r≦a
V(r) = │ ,
└ 0 ,r＞a
(a) write down the radial wave functions in the two regins of 0≦r≦a and r＞a,
respectively, in terms of the spherical Bessel functions j_l(ρ), spherical
Neumann functions n_l(ρ), and/or spherical Hankel functions h_l^(1) (ρ),
h_l^(2) (ρ) for
(i) the positive energy case (E＞0), (4 points)
(ii) the negative energy case (E＜0). (4 points)
(b) Show that the quantization condition for zero angular momentum (l=0) bound
state (E＜0) is κ＝－k' cot(k'a), where k' is the wave number inside the
well and iκ is the complex wave number for the exponential tail outside.
(8 points)
(c) From part (b), show that a bound state exists only if the magnitude of the
depth of the well has at least a certain minimum value of
π^2 (hbar)^2
V_0≧────────
8ma^2
(6 points)
Problem 2.(25 points)
(a) For a spin 1/2 electron, find the state ∣\hat{n}, ±〉 with spin up/down
the direction of the nuit vector \hat{n}=(n_x, n_y, n_z)=(sinθ cosφ,
sinθ sinφ , cosθ), i.e., find the eigenvecotrs of the spin operator
\hat{n}‧\vector{S} along the direction of \hat{n}. (7 points)
(b) Suppose that an spin 1/2 electron is in a uniform magnetic field
\vector{B_0} = B_0 \hat{z}. At time t=0, the electron spin is in the state
of ∣\hat{n}, -〉, claculate the expectation value of 〈\vector{S}(t)〉 at
time t. (8 points)
(c) An additional magnetic field
\vector{B_1} = B_1 cos(ωt) \hat{x} + B_1 sin(ωt) \hat{y}
is now applied. If an electron in the combined field \vector{B_0} +
\vector{B_1} has spin pointing along the +\hat{z} axis at time t=0, what is
the probability that will have flipped to -\hat{z} axis at time t.
(10 points)
Problem 3.(22 points)
If an electron bound to a proton through Coulomb potential (hydrogen atom) is
in a state of orbital angular momentum l and is subjected to an additional
spin-orbital Hamiltonian of η\vector{L}‧\vector{S}, where \vector{L} and
\vector{S} are the orbital angular momentum and spin operators, respectively,
and η is a constant.
(a) Find the total angular momentum j-states(eigenstate) in terms of the
product state of ∣l m_l〉 and ∣s m_s〉 [i.e., find the Clebsch-Gordon
(C-G) coefficients for the states of thr total angular momentum
\vector{J} = \vector{L} + \vector{S} which can have values of j = l±l/2],
where l and s are the values of the orbital and spin angular momenta of
the electron, respectively, and m_l hbar and m_s hbar are their
corresponding z-component values (If you don't know how to find the answer
for the general case of any value l, you may try to solve the problem for a
special case of l=1 to get partial credits). (12 points)
(b) Find the eigenenergies of the electron and the corresponding eigenfunctions
in the spherical corrdinate basis and spinor (representation). (10 points)
Problem 4.(16 points)
(a) Using Wigner-Eckert theorem to derive angular momentum selection rules for
an irreducible
(i) first-rank tensor operator (vector operator) and (4 points)
(ii) second-rank (e.g. quadrupole) tensor operator. (4 points)
(b) Explain
(i) what the accidental degeneracy is, (4 points)
(ii) why the accidental degeneracy observed in hydrogen atom is lost in
multielectron atoms. (4 points)
Problem 5.(20 points)
Estimate the approximate values of the two lowest energy levels of a partical
of mass m in a one-dimensional potential
┌ ＋∞, x≦0
V(x)＝│ 1
└ ─ mω^2 x^2, x＞0
2
by means of
(a) the variational method (10 points)
(b) the WKB method (10 points).