p460 - real h atom1 the real hydrogen atom solve se and in first order get (independent of l): can...
Post on 20-Dec-2015
221 views
TRANSCRIPT
P460 - real H atom 1
The Real Hydrogen Atom• Solve SE and in first order get (independent of L):
• can use perturbation theory to determine:
magnetic effects (spin-orbit and hyperfine e-A)
relativistic corrections
• Also have Lamb shift due to electron “self-interaction”. Need QED (Dirac eq.) and depends on H wavefunction at r=0 (source of electric field). Very small and skip in this course (first calculated by Bethe using perturbation theory on train from Long Island to Ithaca. Bethe also in film Fat Man and Little Boy....)
2
6.13
n
eVEn
P460 - real H atom 2
Spin-Orbit Interactions • A non-zero orbital angular momentum L produces
a magnetic field
• electron sees it. Its magnetic moment interacts giving energy shift
• in rest frame of electron, B field is (see book/ER):
• convert back to lab frame (Thomas precession due to non-inertial frame gives a factor of 2 – Dirac eq gives directly). Energy depends on spin-orbit coupling
BSg
BF
Ldr
rdV
remcB
bs
)(112
EvB
rvAB
use
c
21
LSdr
dV
r
g
emcE bs
1
2
12
P460 - real H atom 3
Spin-Orbit: Quantum Numbers • The spin-orbit coupling (L*S) causes ml and ms to
no longer be “good” quantum numbers
• spin-orbit interactions changes energy.
• In atomic physics, small perturbation, and can still use H spatial and spin wave function as very good starting point. Large effects in nuclear physics (and will see energy ordering very different due to couplings). So in atomic just need expectation value of additional interaction
0,
,:
)(2
22
2
HLHLLH
LLionseigenfunctalso
EHandrVHif
zzz
z
m
commutedonotLLLas
LSLSLSLLSL
SLaHH
zyx
zzzyyxxz
,,0
,,
P460 - real H atom 4
SL Expectation value • Determine expectation value of the spin-orbit
interaction using perturbation theory. Assume J,L,S are all “good” quantum numbers (which isn’t true)
• assume H wave function is ~eigenfunction of perturbed potential
)(
)(
2
0,
22221
22221
222
SLJSL
SLJSL
orSSLLJ
SLifSLJ
43
21
21
2
)1(,
:20
)1()1()1(2
sswithllj
valueslfor
sslljjSL
P460 - real H atom 5
SL Expectation value • To determine the energy shift, also need the
expectation value of the radial terms using Laguerre polynomials
• put all the terms together to get spin-orbit energy shift. =0 if l=0
0)12)(1(
221*1
14
1
330
33
330
2
ldrr
needas
lllnarr
rre
drdV
r
)12)(1(
))1()1((3
432
0
llln
lljjEE
SL
n=2
L=0,1
J=3/2, L=1
J=1/2 L=0
J=1/2 L=1
with relativistic
j=3/2
j=1/2
P460 - real H atom 6
Numerology • have
• but
• and so
300
2
22
2
42
1
a
e
cme
22
00
2
2
20
0
2
1
4
14
cmE
c
e
cmcma
e
ee
2
2
02
222
3
333
0
2
22300
2
22
22
2
4
12
42
1
Emc
cme
cma
e
cm ee
P460 - real H atom 7
Spin Orbit energy shift • For 2P state. N=2, L=1, J= 3/2 or 1/2
• and so energy split between 2 levels is
L=1
J=3/2
j=1/2eVE
EEE
split
split
5105.4
1371
21
23
48
2
48
)2(
485218
)21(
)12)(1(
)1()1((
204
323
212
0
204
325
232
0
3432
0
21
23
EEE
EEE
llln
lljjEE
SL
P460 - real H atom 8
Relativistic Effects
• Solved using non-relativistic S.E. can treat relativistic term (Krel) as a perturbation
• <V> can use virial theorem
23
4
23
42
24222
882 cm
pK
cm
p
m
p
mccmpcmET
rel
VEVEEEVEbut
VVEEK
VEK
ESEV
KEvEE
nn
mcrel
mcmp
mcrel
mp
relnn
,0,
)2(
)()(
.).(
22
22
21
2
212
221
2
2
2
2
2
2
nnaZe
rZe EV
o22
0
2
0
2
41
4
P460 - real H atom 9
Relativistic+spin-orbit Effects• by integrating over the radial wave function
• combine spin-orbit and relativistic corrections
• energy levels depend on only n+j (!). Dirac equation gives directly (not as perturbation). For n=2 have:
)12(22
412
42
30
2
20
2
)()(
lna
Zer
Zeo
V
)( 83
1212
3
44
nlenZ
rel cmK
):()( 2
143
122
43
122
)12)(1()1()1(
3
20
43
3
20
ljuse
EE
njn
E
nlllllljj
n
E
relSL
8
5
8
3
12
2
8
1
8
3
12
2
21
23
P460 - real H atom 10
Energy Levels in Hydrogen• Degeneracy = 2j+1
• spectroscopic notation: nLj with L=0 S=state, L=1 P-state, L=2 D-state
• also can note spin “doublet” is single electron with s=1/2
E
N=1
N=3
N=2
223,31,0
443,32,1
632
21
21
23
23
25
21
23
25
PSjl
DPjl
Djl
222,21,0
421
21
21
23
21
23
PSjl
Pjl
210212
1 Sjl
# states
21
2 S
P460 - real H atom 11
Zeeman Effect:External B Field
• Energy shift depends on mj and removes any remaining degeneracy. Now two fields (internal and external) and details of splitting depends on relative strengths
• Unless S=0, the magnetic moment and the total angular momentum are not in the same direction (and aren’t in B direction). For weak external field, manipulating the dot products gives
......
)2(
2121 SSSandLLLwith
SLandSLJ b
)1(2
)1()1()1(1
jj
llssjjg
mBgBE jb
23
2P
23
21
21
23
j
j
j
j
m
m
m
mB=0 B>0
P460 - real H atom 13
Zeeman Effect:External B Field
• Assume that weak B field (if strong then L and S won’t couple)
• B field off 1 photon energy B field on 6 photon energies (with their energy depending on the g factor and on the B field
• One of the first indicators that the electron had intrinsic angular momentum s=1/2
212
21 sS
23
21
21
23
j
j
j
j
m
m
m
mB=0B>0
212 ,
23 sP
21
21
j
j
m
m
3
4
2
212
21
415
43
415
43
43
43
Ps gg
0n
P460 - real H atom 14
Hyperfine Splitting• Many nuclei also have spin
• p,n have S=1/2. Made from 3 S=1/2 quarks (plus additional quarks and antiquarks and gluons). G-factors are 5.58 and -3.8 from this (-2 for electron).
• Nuclear g-factors/magnetic moments complicated. Usually just use experimental number
• for Hydrogen. Let I be the nuclear spin (1/2)
• have added terms to energy. For S-states, L=0 and can ignore that term
Bp
epp
ppp m
mgI
g
58.5
eppnuc bLaE
P460 - real H atom 15
Hyperfine Splitting• Electron spin couples to nuclear spin
• so energy difference between spins opposite and aligned. Gives 21 cm line for hydrogen (and is basis of NMR/MRI)
)2
3
2
122(
2,1
2
3
2
12
2,0
))1()1()1((2
2
2
2
ISfalignedspins
ISfoppositespins
iissffIS
SIFletISep
eVffE 6106)]0()1[(