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Spin•Electronic charge in motion - A current loop behaves as a magnetic dipole and has a magnetic moment. - Note the current direction is opposite to the electron velocity, and also the angular momentum direction is opposite to the magnetic moment•Two contributions to the electronic magnetic moment -An orbital magnetic moment due to orbital angular momentum -A spin magnetic moment due to electron spin
Electronic Magnetic Moments -Orbital Contribution
•The orbital motion of an electron around the nucleus may correspond to a current in a loop of wire having no resistance where m=(area of loop) (current)•Note that the angular momentum is continuous (not quantized), indicating a classical treatment of the problem
2)
/)2(()( 22
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evr
vr
er
erm
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00 )2
( pm
em Bm
ehm
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Electronic Magnetic Moments-Spin Contribution
Spin? - Was postulated in 1925 by Paul Dirac in order to explain certain features of optical spectra of hot gases subjected to a magnetic field(Zeeman effect) and later theoretical confirmation in wave mechanics - The root cause of magnetism and an intrinsic property, together with charge and mass, of subatomic particles of fermions (eg.electrons, protons and neutrons) and bosons (photons, pions)
It was found, theoretically and experimentally, that the magnetic moment due to electron spin is equal to,m0
BS m
ehm
4
Electronic Magnetic Moments -m vs. p
For a given angular momentum, the spin gives twice the magnetic moment of orbit
00
0 )2
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244
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ehp
m
eh
m
ehm
SS
S pm
eshp
m
eh
m
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Electronic Magnetic Moments-Total Moments
The total magnetic moment per electron is the vector sum of the orbital and spin magnetic
moments
SStot pm
ep
m
emmm 2)
2()
2( 00
tottot pm
egm )
2(
The term ‘g’ is called the Lande splitting factor — g=2 for spin only components — g=1 for orbital only components
Electronic Magnetic Moments-Lande’Equation
)1(2
)1()1()1(1
JJ
LLSSJJg
•Orbital is quenching : L=0, J=S g=2•Spin =0 : S=0, J=L g=1
Schrodinger Equation
Schrodinger Equation
m=hml ml= l(l+1)
Schrodinger Equation
Electronic Magnetic Moments-Quantum Mechanical
The orbital angular momentum quantum number (l)
)2
(0 h
lp
The spin angular momentum quantum number (l)
)2
(h
spS
The spin angular momentum quantum number (l)
)2
)(()2
(h
slh
JpJ
Hund’s Rule Empirical rules which determine the occupancy
of the Available electronics within an atom
Used to calculate L, S and J for an unfilled shell 1. Maximum total S=max Sz with Sz=imsi Obeying the Pauli exclusion principle 2. Maximum total L=maxLz with Lz= imli Minimizing the Coulomb interaction energy 3. Spin-orbit interaction: L -S if less than half-filled J= L+S if more than half-filled
Hund’s Rule - Examples
•Sm3+ ion having 5 electrons in its 4f shell (n=4, l=3) S=5/2, L=5, J=L-S=5/2
ml 3 2 1 0 –1 –2 –3 ms
occupancy s •Fe2+ ion having 6 electrons in its 3d shell (n=3, l=2) S=2, L=2, J=L+S=4. Actually, however, S=2, L=0 (quenched) J=S=2 ml 2 1 0 –1 –2 2 1 0 -1 -2 ms
occupancy s
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2
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1
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