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Lecture 13
Space quantization and spin
(c) So Hirata, Department of Chemistry, University of Illinois at Urbana-Champaign. This material has
been developed and made available online by work supported jointly by University of Illinois, the
National Science Foundation under Grant CHE-1118616 (CAREER), and the Camille & Henry Dreyfus
Foundation, Inc. through the Camille Dreyfus Teacher-Scholar program. Any opinions, findings, and
conclusions or recommendations expressed in this material are those of the author(s) and do not
necessarily reflect the views of the sponsoring agencies.
z-component angular momentum

The z-component of the angular momentum
operator depends only on φ
ˆl = -i ¶
z
¶j

The “particle on a sphere” wave function is
the eigenfunction of the lz operator.
lˆz ( ,  )  Nlml lml ( )lˆz  ml ( )  ml ( ,  )
imlj
e
Angular
momentum
Total angular momentum

The total angular momentum operator is
2
æ
1
¶
1 ¶
¶ö
ˆl 2 = - 2
total
çè sin 2 q ¶j 2 + sinq ¶q sinq ¶q ÷ø
2
æ
1
¶
1 ¶
¶ö
ˆ
H =+
sin q ÷
2 ç
2
2
2mr è sin q ¶j
sinq ¶q
¶q ø
2
2
l(l
+
1)
Hˆ Y =
Y
2
2mr
Total angular momentum

The “particle on a sphere” wave function is
also the eigenfunction of the l2total operator.
ˆl 2 Y(q ,j ) = l(l +1) 2 Y(q ,j )
total
Angular momentum squared
2
æ
1
¶
1 ¶
¶ö
ˆ
H =+
sin q ÷
2 ç
2
2
2mr è sin q ¶j
sinq ¶q
¶q ø
2
2
l(l
+
1)
Hˆ Y =
Y
2
2mr
z-component and total angular
momenta


The “particle on a sphere” wave function,
therefore, has well-defined total energy and
total angular momentum:
2
l (l  1)
El 
 ltotal  l (l  1)
2
2m r
… and well-defined z-component energy and
z-component angular momentum:
2
l
2
m
Eml 
 l z  ml 
2
2m r
x- and y-component angular
momenta

This is because lz and ltotal commute.
2
élˆz , lˆtotal
ù=0
ë
û
and ly operators depend on both θ and φ.
They do not commute with lz or ltotal or H.
 lx
ˆl = i æ sin j ¶ + cosj ¶ ö
x
çè
¶q tanq ¶j ÷ø
ˆl = i æ - cosj ¶ + sin j ¶ ö
y
çè
¶q tanq ¶j ÷ø
élˆz , lˆx ù = i
ë
û
élˆy , lˆz ù = i
ë
û
lˆy ¹ 0
lˆx ¹ 0
Uncertainty principle
Observable
Determined
Simultaneously?
Energy
Yes
x angular momentum
y angular momentum
Yes only for z
No for x and y
z angular momentum
total angular momentum
Yes
Space quantization


If all of x, y, and z
components were known, we
knew the angular momentum
vector (length and direction)
exactly and hence the circular
trajectory perpendicular to it.
The uncertainty in x and y
components indicates the
precise trajectory cannot be
known.
Space quantization


We can only know the
total ( l (l 1) )
and z component
( ml  ).
x and y components
remain undetermined,
so we do not know the
precise trajectory.
Space quantization


Even when ml = l, zcomponent does not exhaust
the total angular momentum
because l (l  1)  l .
If it were not for “+1”, ml = l
would leave nothing for x and
y components and precisely
determine all x, y, and z
components simultaneously,
violating uncertainty principle!
Angular momentum as a magnet



Rotational motion of a charged
particle (such as an electron) gives
rise to a magnetic field.
Angular momentum is proportional
to the magnetic moment.
Applying an external magnetic field
(along z axis) and measuring the
interaction, one can determine the
(z-component) angular momentum
of an electron.
Stern-Gerlach experiment


The trajectories of electrons in an
inhomogeneous magnetic field are bent.
The trajectories are “quantized” – the proof of
the quantization of angular momentum
orientation, namely, space quantization.
Summary


The total angular momentum and only one of the
three Cartesian components (z-component) can
be determined exactly simultaneously. One
component cannot exhaust the total momentum
(because of “+1” in l(l +1) ).
The angular momentum is quantized in both its
length and orientation – it cannot point at any
arbitrary direction (space quantization).
Stern-Gerlach experiment



Just two trajectories observed for electrons.
This suggests l = ½ and m = ½ and –½.
This cannot exactly be a particle on a sphere
(where l and m must be full integers)?
Spin



It is the “spin” angular momentum of an
electron.
It has been discovered that the particle has
intrinsic magnetic momentum. Its precise
derivation is beyond quantum chemistrty.
We can imagine the particle spinning and its
associated angular momentum acts like a
magnetic moment.
Spin




An electron has the spin quantum number
s = ½ (corresponding to l of particle on a
sphere). The total spin angular momentum
is s(s 1)  3  .
4
The spin magnetic quantum number ms
(corresponding to ml) can take s,…,–s (unit
interval). For an electron ms = ½ (spin up or α
spin) and –½ (spin down or β spin).
A proton or neutron also has s = ½.
A photon has s = 1.
Indistinguishable particles

Because of the uncertainty in the position and
momentum of a particle, in a microscopic
scale, two particles of the same kind (such as
two electrons) nearby are indistinguishable.
Indistinguishable particles

The probability of finding particle 1 at position
r1 and particle 2 at r2 is the same as that of
finding particle 2 at r1 and particle 1 at r2
(otherwise we can distinguish the two).
 (r1 , r2 )   (r2 , r1 )
2
2
There are at least
two immediate
possibilities
(r1 , r2 )  (r2 , r1 )
(r1 , r2 )  (r2 , r1 )
Fermions


Possibility 1: (r1 , r2 )  (r2 , r1 )
This is the case with particles having half
integer spin quantum numbers (such as
electrons). They are called fermions.
Two fermions cannot occupy the same
position in space (Pauli exclusion principle).
They form matter.
Y(r,r) = -Y(r,r)
Y(r,r) = 0
Bosons


Possibility 2: (r1 , r2 )  (r2 , r1 )
This is the case with particles having full
integer spin quantum numbers (such as
photons). They are called bosons.
Two bosons can occupy the same position in
space (e.g., photons can be superimposed to
become more intense). They tend to mediate
fundamental interactions.
Y(r,r) = Y(r,r)
Caveat



The derivation of the concepts of spin,
fermions, and bosons is beyond quantum
chemistry.
However, spin and the Pauli exclusion
principle for electrons (as fermions) are a
critical element of chemistry (whenever we
have more than one electrons).
We treat these as external postulates to
quantum chemistry.
Homework Challenge #3


Find the origin of spins. Explain why the spin
quantum number of an electron, proton, and
neutron is ½, whereas that of a photon is 1.
Explain why particles with half integer spin
quantum numbers are fermions and have
wave functions that are anti-symmetric with
respect to particle interchange. Explain why
particles with full integer spin quantum
numbers are bosons and their wave functions
are symmetric with respect to interchange.

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