physics 3313 - lecture 16
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Physics 3313 - Lecture 16. Wednesday April 1, 2009 Dr. Andrew Brandt. Hydrogen Atom Wave Function Angular Momentum Orbital and Magnetic Quantum Numbers Angular Momentum Operator TEST moved to 4/27. Hydrogen Atom Wave Function. - PowerPoint PPT PresentationTRANSCRIPT
3313 Andrew Brandt 1
Physics 3313 - Lecture 16
4/1/2009
Wednesday April 1, 2009Dr. Andrew Brandt
1. Hydrogen Atom Wave Function2. Angular Momentum3. Orbital and Magnetic Quantum Numbers4. Angular Momentum Operator5. TEST moved to 4/27
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Hydrogen Atom Wave Function
3/30/2009
• http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/hydwf.html#c1
• different orbital angular momentum states identified by a letter in orbital notation
, ,nlm nl lm mr R r
l value 0 1 2 3 4
Orbital s p d f g
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Angular Momentum
• Radial equation (above) should only be concerned with radial motion (towards and away from nucleus), but energy could have an orbital term
• If then this term would cancel out
• since L=r x p =mvr, so
4/1/2009
22
2 2 20
11 20
4
l ld dR m er E R
r dr dr r r
radial orbitalE KE KE U
2
2
1
2orbital
l lKE
mr
22
2
1
2 2orbital
LKE mv
mr
( 1)L l l
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Orbital (l) and Magnetic (ml) Quantum Numbers
• l is related to orbital angular momentum; angular momentum is quantized and conserved, but since h is so small, often don’t notice quantization
• Electron orbiting nucleus is a small current loop and has a magnetic field, so an electron with angular momentum interacts with an external magnetic field
• The magnetic quantum number ml specifies the direction of L (which is a vector—right hand rule) and gives the component of L in the direction of the magnetic field Lz
• Five ml values for l=2 correspond to five different orientations of angular momentum vector.
4/1/2009
( 1)L l l
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Angular Momentum• L cannot be aligned parallel with an external magnetic
field (B) because Lz is always smaller than L (except when l=0)
• In the absence of an external field the choice of the z axis is arbitrary (measure projection as in any direction)
• Why only Lz quantized? What about Lx and Ly?
• Suppose L were in z direction, then electron would be confined to x-y plane; this implies z position is known and pz is infinitely uncertain, which is not true if part of a hydrogen atom
• Therefore average values of Lx =Ly =0 and it is only necessary to specify L and Lz4/1/2009
( 1) ll l m
lm
( 1)l l l
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Precession of Angular Momentum
• The direction of L is thus continually changing as it precesses around the z axis
(note average values of Lx =Ly =0 )
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Angular Momentum Operator
• Consider angular momentum definition: so
• We can define the angular momentum operator in cartesian and spherical coordinates:
• with
gives similarly
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L r p ������������� �
x z yL yp zp Y x zL zp xp z y xL xp yp
xp ix
ˆz
d dL i x y i
dy dx
ˆ
z nlmL i
nl lm mi R r
ˆ ( )( )l lzL i im m
yp iy
zp i
z
limAe
2 2ˆ ( 1)L l l
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QM Modifications to Bohr Model
• In Bohr model electron has circular orbit around nucleus with
and =90o and changes with time
• QM mods:
1) No definite r, , and , but only probabilities due to wave nature of electron
2) |2| independent of time and varies from place to place, so can’t think of electron as orbiting
4/1/2009
20nr n a
2 2 2 2| | | | | | | |R 2 * 2| | ( )l lim imAe Ae A
probability constant independent of azimuthal angle (spherical symmetry)
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Radial Wave Function
• Radial part of wave function varies with r and differs for different n,l combinations
• R is maximum at r=0 (inside nucleus) for s states but approaches 0 at r=0 for l>0
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Probabilities• Probability of finding electron in
hydrogen atom in a spherical shell between r and r+dr is given by
• with
• since angular wave functions are normalized
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222 22
0 0sinP r dr r R dr d d
22r R dr
2
,| | dV
2 2 2 2| | | | | | | |R
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Probability Distributions
4/1/2009