1. WKB harmonic oscillator
1. WKB harmonic oscillator
1. WKB harmonic oscillator
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
HW6.nb 1<br />
HW 6<br />
<strong>1.</strong> <strong>WKB</strong> <strong>harmonic</strong> <strong>oscillator</strong><br />
a) Energy levels<br />
(Hitoshi does this problem in his <strong>WKB</strong> notes, on page 8.)<br />
The classical turning points a, b are where KE=0, i.e.<br />
E Vx 1<br />
mΩ 2 2 x2 so<br />
a, b 2E<br />
m Ω2 .<br />
We apply or Vx and the turning points to take the energy condition<br />
b <br />
2mE Vx x n <br />
a<br />
1<br />
Π 2<br />
and integrate the l.h.s.:<br />
We find E Π<br />
a0 2E0<br />
; b0 a0;<br />
m Ω2 V0x_ m Ω2 x2 ;<br />
2<br />
IntegrateSqrt2mE0 V0x, x, a0, b0, Assumptions m 0, Ω 0, E0 0<br />
E0 Π<br />
<br />
Ω<br />
n Ω 1<br />
Π 2<br />
such that<br />
E n 1<br />
Ω 2<br />
as desired.<br />
b) SHO <strong>WKB</strong> wavefunctions<br />
(Let us proceed in the units m Ω <strong>1.</strong>)<br />
The trick here is that we want to use the classically−allowed <strong>WKB</strong> wavefunction for SHO in between the turning points,<br />
and the "tunneling" <strong>WKB</strong> wavefunctions in the forbidden regions, which extend to . This can be seen easily on a plot<br />
of the ground state, where the turning points are at 1:
HW6.nb 2<br />
PlotΠ14 Exp x2 x2<br />
, , x, 2, 2, PlotStyle Dashing, Dashing.05, .05;<br />
2 2<br />
2<br />
<strong>1.</strong>5<br />
1<br />
0.5<br />
-2 -1 1 2<br />
Our SHO energies (from part (a)), potential function, and classical turning points are:<br />
Enn_ n 1<br />
<br />
2 ;<br />
Vx_ x2<br />
;<br />
2<br />
an_ <br />
2 Enn ;<br />
bn_ an;<br />
We construct the wavefunction in allowed region using Eq. 30 in the notes. We split up the integral around x 0 and<br />
consider the two turning points a and b separately, so Mathematica doesn’t give a trivial result:<br />
Ψa1x_, n_ <br />
2AbsV ’an<br />
<br />
13<br />
<br />
Π <br />
<br />
CosIntegrate<br />
2Enn Vx <br />
<br />
2Enn Vt ,<br />
t, an, x, Assumptions n 0, an x, x 0 Π<br />
;<br />
4<br />
12<br />
12<br />
Ψa2x_, n_ <br />
2AbsV ’bn<br />
<br />
13<br />
<br />
Π <br />
<br />
CosIntegrate<br />
2Enn Vx <br />
<br />
2Enn Vt , t, x, bn, Assumptions n 0, x 0, x bn Π<br />
;<br />
4<br />
Similarly, for the forbidden regions we use Eq. 31:<br />
12<br />
Ψf1x_, n_ <br />
2AbsV ’an<br />
<br />
13<br />
<br />
Π <br />
<br />
<br />
2Vx Enn <br />
1<br />
<br />
2<br />
ExpIntegrate <br />
2Vt Enn , t, x, an, Assumptions n 0, x an;<br />
12<br />
Ψf2x_, n_ <br />
2AbsV ’bn<br />
<br />
13<br />
<br />
Π <br />
<br />
<br />
2Vx Enn <br />
1<br />
<br />
2<br />
ExpIntegrate <br />
2Vt Enn , t, bn, x, Assumptions n 0, x bn;
HW6.nb 3<br />
Ok, now what about the regions right around the classical turning points a and b? We know that the wavefunctions above<br />
blow up near these points. So, let’s define a radius about the turning points inside which we treat specially:<br />
Ε 0.5;<br />
Next, we construct a piecewise function from the asymptotic wavefunctions above outside these regions (using ’UnitStep’):<br />
Ψwkbasyx_, n_ 1 n Ψf1x, n1 UnitStepx an Ε <br />
1 n Ψa1x, nUnitStepx an Ε1 UnitStepx <br />
Ψa2x, nUnitStepx1 UnitStepx bn Ε Ψf2x, nUnitStepx bn Ε;<br />
Now, in the special regions, we use the Airy functions themselves, and again construct a piecewise function:<br />
Ψwkbtpax_, n_ 1 n AiryAi2AbsV ’an 13 x anUnitStepx an Ε<br />
1 UnitStepx an Ε AiryAi2AbsV ’bn 13 x bn<br />
UnitStepx bn Ε1 UnitStepx bn Ε;<br />
Finally, we add these two chunks to get the whole function:<br />
Ψwkbx_, n_ Ψwkbasyx, n Ψwkbtpax, n;<br />
WAIT: If we’re using the asymptotic wavefunctions mated to the actual Airy functions in the transition regions, why not<br />
just use the Airy functions everywhere, expanding about the turning points? A moment’s reflection gives the answer: this<br />
would work great coming in from and passing through the turning points, but as you move into the classically−allowed<br />
region, the Airy function would not reach its cosine−shaped asymptote fast enough! The reason we do it piecewise is that<br />
we want accurate results where E V , and live with nastiness in the transition regions. (In the classically−forbidden<br />
regions, the Airy function should decay fast enough to match the asymptotic solution.)<br />
To see this, let’s construct this all−Airy function wavefunction by expanding around the two turning points:<br />
Ψairx_, n_ 1 n AiryAi2AbsV ’an 13 x an1 UnitStepx <br />
AiryAi2AbsV ’bn 13 x bnUnitStepx;<br />
Finally, we wish to compare to the exact results:<br />
1<br />
Ψexax_, n_ <br />
<br />
2n Π<br />
Factorialn<br />
14 Exp x2<br />
HermiteHn, x;<br />
2<br />
Let’s compare the exact and <strong>WKB</strong> solutions for n 1:
HW6.nb 4<br />
PlotΨexax, 1, Ψwkbx, 1, x, 4, 4, PlotStyle GrayLevel0, Hue0;<br />
0.6<br />
0.4<br />
0.2<br />
-4 -2 2 4<br />
-0.2<br />
-0.4<br />
-0.6<br />
Our <strong>WKB</strong> solutions looks great at x and x 0 where E V and E V respectively, as expected. The Airy<br />
function matches nicely with the decaying wavefunctions in the forbidden regions, but does indeed mismatch the wavefunction<br />
in the allowed region.<br />
Now, let’s compare the exact and all−Airy solutions:<br />
PlotΨexax, 1, Ψairx, 1, x, 4, 4, PlotStyle GrayLevel0, Hue0;<br />
0.6<br />
0.4<br />
0.2<br />
-4 -2 2 4<br />
-0.2<br />
-0.4<br />
-0.6<br />
As expected, we get the opposite behavior. Coming in from , the wavefunction matches the exact solution nicely.<br />
However, as we move toward x 0, the discrepancy builds up until you get a mismatch.<br />
Now let’s look at n 10:
HW6.nb 5<br />
PlotΨexax, 10, Ψwkbx, 10, x, 7, 7, PlotStyle GrayLevel0, Hue0;<br />
0.4<br />
0.2<br />
-6 -4 -2 2 4 6<br />
-0.2<br />
-0.4<br />
Fantastic! Because the energy is much higher, the approximation is far more accurate (i.e, the state is much more classical),<br />
and on this plot the mismatch between the Airy function in the transition region and the wavefunction in the allowed region<br />
is not even visible.<br />
Now let’s look at the all−Airy solution for n 10:<br />
PlotΨexax, 10, Ψairx, 10, x, 7, 7, PlotStyle GrayLevel0, Hue0;<br />
0.4<br />
0.2<br />
-6 -4 -2 2 4 6<br />
-0.2<br />
-0.4<br />
A bit better than n <strong>1.</strong> The discrepancy builds up again coming toward x 0 from the turning points; however, because n<br />
is even, there is no mismatch at x 0, just a sharp point.<br />
Finally, let us look at n 20:
HW6.nb 6<br />
PlotΨexax, 20, Ψwkbx, 20, x, 10, 10, PlotStyle GrayLevel0, Hue0;<br />
0.4<br />
0.2<br />
-10 -5 5 10<br />
-0.2<br />
-0.4<br />
We see that the <strong>WKB</strong> solution has a bit of overshoot due to the higher energy; otherwise the agreement is quite good.<br />
Finally, let us compare the exact and all−Airy solutions for n 20:<br />
PlotΨexax, 20, Ψairx, 20, x, 10, 10, PlotStyle GrayLevel0, Hue0;<br />
0.4<br />
0.2<br />
-10 -5 5 10<br />
-0.2<br />
-0.4<br />
The overshoot is still there, but the discrepancy is less approaching x 0 as one would expect with the higher energy.<br />
N.B.: Of course, one could just use polynomial to handle the transition regions. But, if we’re going to use numerical<br />
methods, we might as well use the variational method instead of <strong>WKB</strong>. :P
HW6.nb 7<br />
2. Classical limit of hydrogen atom<br />
a) Correspondence between orbital motion and emitted photon<br />
One can derive the Bohr model by taking the energy of an electron circling a proton (CGS units)<br />
E 1<br />
m v 2 2 Z e2<br />
r<br />
and applying the quantization condition<br />
Ln m v r n .<br />
(This justification for this quantization is just units; we know that the ground state in fact has no angular momentum. One<br />
can also derive the Bohr model by requiring whole wavelengths to fit on a circle of radius r.)<br />
Substituting for v, we get<br />
E 1 n <br />
m 2 m r 2<br />
Z e2<br />
. r<br />
We require one more condition to eliminate r. Let us use the force balance<br />
m v2<br />
Z e2<br />
F <br />
r r2 which gives<br />
Z e2<br />
r m v2 Z e2<br />
m<br />
and finally<br />
rn n2 2<br />
m r<br />
n 2<br />
Z e2 m .<br />
Substituting this back in to the energy,<br />
En 1<br />
to obtain<br />
En Z2 e4 m<br />
<br />
Similarly,<br />
vn <br />
2 n2 2 m rn 2 Z e2<br />
rn n2 2 n <br />
<br />
m rn<br />
2n 2 2 .<br />
n <br />
<br />
m Z e2 m<br />
n2 2 <br />
<br />
2m Z e2 m<br />
<br />
Z e2<br />
<br />
n .<br />
n 2 2 2<br />
Z e2 Z e2 m<br />
n2 2 <br />
Having derived these basic quantities, we can express the frequency of orbital motion in state n as<br />
Νo vn<br />
Z e2<br />
<br />
2Π rn 2Π n <br />
n3 3 .<br />
On the other hand, we know that the frequency of a photon due to a change from state n to n k is<br />
ΝΓ En Enk 1<br />
<br />
2Π 2Π<br />
Z2 e4 m<br />
23 1<br />
n2 1<br />
<br />
nk 2 1<br />
2Π<br />
Z2 e4 m<br />
2n2 3 <br />
1<br />
<br />
1 k<br />
n 2 1 <br />
Approximating k n,<br />
.<br />
<br />
ΝΓ 1<br />
2Π<br />
Z2 e4 m<br />
2n2 3 1 2k<br />
1<br />
1 <br />
n 2Π Z2 e4 m<br />
n3 3 k.<br />
1<br />
Z e2 m<br />
n2 2 1<br />
2Π Z2 e4 m<br />
As indicated in the hint, we find that the frequency of the photon ΝΓ is an integer multiple of the frequency of orbital<br />
motion Νo .
HW6.nb 8<br />
b) Correspondence between classical radiation and mean lifetime<br />
Under classical dynamics the electron is being continually accelerated, so it should emit radiation. To know the mean<br />
lifetime Τ of a state n, we must know how fast it loses power. Since the model is semiclassical, we can use a classical<br />
radiation formula. We found above that the velocity goes like 1<br />
, so the motion at large n is non−relativistic and so we can<br />
n<br />
just use the Larmor power formula (again in CGS)<br />
.<br />
From above,<br />
P 2e2 a2 3c3 <br />
an Fn<br />
Substituting,<br />
m <br />
P 2Z6 e 14 m 2<br />
.<br />
3c 3 n 8 8<br />
Z e2<br />
<br />
m rn 2 <br />
The mean lifetime should be<br />
Rearranging,<br />
Z e2<br />
<br />
m Z e2 m<br />
n2 2 2<br />
Z3 e6 m<br />
n4 4 Τ En En1 P<br />
Z2 e4 m<br />
n3 2 1 3c3 n8 8 <br />
as expected.<br />
1<br />
Τ 2<br />
3 e2 Z e2<br />
c c 4 <br />
m c2<br />
<br />
1<br />
n5 2Z 6 e 14 m 2 3c3 n 5 6<br />
.<br />
<br />
Z 4 e 10 m .<br />
c) Comparison of classical and quantum lifetimes<br />
We implement the correct expression above (with in units of eV s):<br />
lifetimecn_ <br />
<br />
<br />
2<br />
ΑZ Α<br />
3 4 mec2 1<br />
<br />
hbar n5 <br />
<br />
Computing for n 2, 4, 6 we find respectively<br />
2, 4, 6 lifetimec<br />
1<br />
2.98388 10 9 , 9.54841 10 8 , 7.25082 10 7 <br />
. Α 137 1 , Z 1, mec2 511000, hbar 6.582 10 16 ;<br />
These are accurate to within an order of magnitude of the given quantum values, and seem to improve with increasing n as<br />
we would expect.
HW6.nb 9<br />
3. Spin precession in magnetic field<br />
a) Time evolution of eigenstates<br />
The Schrödinger equation is<br />
e<br />
e B<br />
i t Ψ H Ψ g S B Ψ g <br />
2m c 2m c Sz Ψ Ω Sz Ψ <br />
where we defined Ω g<br />
e B<br />
<br />
2m c .<br />
Then, for Ψ Sz ; we have<br />
i t Sz ; Ω<br />
<br />
2 Sz ; <br />
and for Ψ Sz ; we have<br />
i t Sz ; Ω<br />
<br />
2 Sz ; .<br />
So we have for the solutions<br />
Sz ; , t e<br />
Sz ; , t e<br />
Ω<br />
i <br />
i Ω<br />
2 t Sz ; <br />
2 t Sz ; .<br />
For a state aligned with the quantization axis, the "precession" is just a rotating phase.<br />
b) Dynamics of S x ; <br />
First, define the spin matrices:<br />
Sx <br />
<br />
2 0, 1, 1, 0;<br />
Sy <br />
<br />
2 0, I, I, 0;<br />
Sz <br />
<br />
2 1, 0, 0, 1;<br />
Find the eigenstates of Sx :<br />
eigenx EigensystemSx <br />
<br />
,<br />
2 <br />
, 1, 1, 1, 1<br />
2<br />
Our state Sx ; is then<br />
and so<br />
Sx ; 1<br />
Sz ; Sz ; <br />
2<br />
Sx ; , t 1<br />
Sz ; , t Sz ; , t .<br />
2<br />
In component form on the time−independent basis Sz ; , Sz ; ,
Our state Sx ; is then<br />
Sx ; 1<br />
Sz ; Sz ; <br />
HW6.nb 2 10<br />
and so<br />
Sx ; , t 1<br />
Sz ; , t Sz ; , t .<br />
2<br />
In component form on the time−independent basis Sz ; , Sz ; ,<br />
Sx ; , t 1<br />
<br />
2 <br />
e<br />
Ω<br />
ei<br />
2 t<br />
Ω<br />
i <br />
c) Showing precession<br />
2 t<br />
<br />
<br />
.<br />
Let us implement the above state in component form, as well as its conjugate−transpose:<br />
1<br />
Ψsxpt_ <br />
<br />
ExpI<br />
2<br />
Ω<br />
t, ExpI<br />
2 Ω<br />
t;<br />
2<br />
1<br />
Ψsxpctt_ <br />
<br />
ExpI<br />
2<br />
Ω<br />
t, ExpI<br />
2 Ω<br />
t;<br />
2<br />
We calculate Sx ; , t S <br />
Sx ; , t and put it into vector x, y, z form:<br />
ExpToTrigΨsxpctt.Sx .Ψsxpt1, 1,<br />
Ψsxpctt.Sy .Ψsxpt1, 1, Ψsxpctt.Sz .Ψsxpt1, 1<br />
1<br />
Cost Ω, <br />
2 1<br />
Sint Ω, 0<br />
2<br />
<br />
This is the clockwise precession about the z−axis with angular frequency Ω g of a vector of length . As<br />
2m c 2<br />
expected, there is no z−component. (The classical analogue is a gyroscope precessing clockwise in the plane; of course,<br />
here we can’t simultaneously have components of x/y and z.)<br />
4. Hamilton−Jacobi for light [optional]<br />
a) Apply <strong>WKB</strong> approximation to equation of motion<br />
Following the same steps we did for the Schrödinger equation, we first write the Maxwell’s equation with A 0 e i S :<br />
<br />
<br />
n2<br />
c2 2<br />
t2 2 <br />
<br />
ei S <br />
<br />
n2<br />
c2 <br />
i S<br />
<br />
<br />
<br />
i S <br />
<br />
<br />
<br />
2<br />
i 2 S<br />
<br />
<br />
<br />
i S <br />
<br />
<br />
2 <br />
<br />
ei S 0 .<br />
In the limit S , we can drop terms of OS and keep those of OS 2 :<br />
n2 c2 2<br />
S <br />
S 2 0.<br />
This is the "Hamilton−Jacobi" equation.<br />
e B
HW6.nb 11<br />
(Of course, in the Maxwell equation, there is no notion of . What we are doing is a valid approximation when the variation<br />
of the phase is very fast compared to the variation of the index of refraction. It is called "eikonal approximation" in optics.)<br />
b) Equivalence to particle in given potential<br />
The Hamiltonian of free particle in potential Vx 1<br />
2m nx 2<br />
H p2<br />
2m<br />
n2<br />
2m .<br />
Applying this to the Hamilton−Jacobi equation, we obtain<br />
1<br />
2m <br />
2<br />
S<br />
n2<br />
2m<br />
S<br />
0. t<br />
Since the action has no explicit time dependence, we can make the Legendre transformation St, x S E, x E t to get<br />
and finally<br />
2<br />
1<br />
2m S <br />
n2<br />
E 0<br />
2m<br />
n2 <br />
S 2<br />
0.<br />
If n in part (a) has no time dependence, we can do the same Legendre transformation and obtain the equivalent "H−J"<br />
equation<br />
E<br />
n c 2<br />
<br />
S 2<br />
0.<br />
c) Separate variables, integrate "action" variable<br />
Assuming nx nx, the "Hamilton−Jacobi" equation does not have an explicit dependence on y or t. Writing<br />
Sx, y, t S <br />
x, py , E py y E t,<br />
the equation becomes<br />
n 2<br />
<br />
Therefore,<br />
c2 E2 d<br />
<br />
d S <br />
<br />
d x<br />
and hence<br />
d x S<br />
<br />
<br />
n2<br />
c2 E2 py 2<br />
S <br />
<br />
x<br />
2 py 2 0.<br />
<br />
E2 py 2 d x’.<br />
nx’ 2<br />
c2 Here, the possible x−dependence of the index of refraction is emphasized.<br />
is
HW6.nb 12<br />
S <br />
<br />
x<br />
<br />
E2 py 2 d x’.<br />
nx’ 2<br />
c2 Here, the possible x−dependence of the index of refraction is emphasized.<br />
d) Integral expressions for "angle" variables<br />
S<br />
S<br />
Using t , y , we find<br />
E<br />
t <br />
y <br />
x<br />
x<br />
py<br />
nx’ 2<br />
<br />
c 2 E<br />
d x’,<br />
<br />
nx’2 <br />
c2 E2 py 2<br />
py<br />
<br />
<br />
nx’2 <br />
c2 E2 py 2<br />
e) Work to Snell’s law<br />
d x’.<br />
Assuming that nx n1 for x 0 and nx n2 for x 0, and choosing the lower end of the integration at x 0,<br />
and<br />
y x <br />
y x <br />
py<br />
<br />
n 2<br />
<br />
1<br />
c2 E2 py 2<br />
py<br />
<br />
n 2<br />
<br />
2<br />
c2 E2 py 2<br />
x for x 0<br />
x for x 0.<br />
1<br />
<br />
cot2 Α1<br />
Using the trigonometric relation sin Α , we find<br />
and hence<br />
1<br />
c py<br />
sin Α n1 E ,<br />
n 2<br />
1 E2 py 2<br />
<br />
c2 1<br />
py 2<br />
1<br />
c py<br />
sin Α n2 E ,<br />
sin Α<br />
<br />
sin Α<br />
n 2<br />
2 E2 py 2<br />
<br />
c2 1<br />
,<br />
n2 n1<br />
py 2<br />
which is nothing but the Snell’s law of refraction.