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Lecture VI
Many - electron atoms
dr hab. Ewa Popko
S-states probability
P-states probability
The Zeeman effect
The Zeeman effect is the splitting of atomic energy and the associated spectrum lines when the atoms are placed in a magnetic field. This effect confirms experimentally the quantization of angular momentum.
N
S
NS
potential energy
The potential energy of an object in a magnetic field depends on the magnetic moment of the object and the magnetic field at its location
U B
magnetic moment of a current loop
4321 00BnBn
ˆI2
ˆI2
ba
ba
BA
I
The magnetic moment of a wire loop carrying current depends on the current I in the loop and the area A of the loop.
IA
The Zeeman effect
Lme
rmme
rr
eA
Te
iAe
ee 2
v22
v 2
e
The orbiting electron is equivalent to a current loop with radius r and area . 2r
The average current I is the average charge per unit time T for one revolution, given by T=2r/v.
Suppose B is directed towards z-axis. The interaction energy of the atom magnetic moment with the field is:
BU z
where z is the z-component of the vector . On the other hand:
The Zeeman effect
zz Lm
e
2
and Lz=ml with . Thus .....3,2,1,0 lm
BmBm
emBU Bllz
2
ohr magneton
Zeeman effect
The values of ml range from –l to +l in steps of one, an energy level with a particlular value of the orbital quantum number l contains (2l+1) diffrent orbital states. Without a magnetic field these states all have the same energy; that is they are degenerate. The magnetic field removes this degeneracy. In the presence of a magnetic field thy are split into (2l+1) distinct energy levels:
BBmeU B )2/(
,...2,1,0 lBl mwithBmU
Adjacent levels differ in energy by
TeV5
eB 1079.5
2me
μ
Energy diagram forhydrogen, showing the splitting of energy levels resulting from the interaction of the magnetic moment of the electron’s orbital motion with an external magnetic field.
The Zeeman effect
The Zeeman effect
Splitting of the energy levels of a d state caused by an applied magnetic field, assuming only an orbital magnetic moment.
Selection rulesThe photon carries one unit ( ) of angular momentum. Therefore theallowed transitions: l must change by 1 and ml must change by 0 or 1
Solid lines – allowed transitions;dashed - forbidden Nine solid lines give only three energies:Ei-Ef ;Ei-Ef +B;Ei-Ef -B
The Zeeman effect
Conclusions: spectrum lines corresponding to transitions from one set of levels to another set are correspondingly split and appear as a series of three closely spaced spectrum lines replacing a single line.
Anomalous Zeeman effect
Spin angular momentum and magnetic moment
Electron posseses spin angular momentum Ls. With this momentum magnetic momentum is connected:
se
es Lme
g
2
where ge is the gyromagnetic ratio
For free electron ge=2
se
s Lme
Allowed values of the spin angular momentum are quantized :
)1( ssLs
spin quantum number s = ½ 2
3sL
Własny moment pędu - spin
The z – component of the spin angular momentum:
ssz mL
2
12
1
sm
Spin angular momentum and magnetic moment
Be
sz
esz
esz
me
me
Lme
2
21
21sm
21sm
Ls sz
The z- component of the spin magnetic moment
Electron in a magnetic field
BEE sz 0
21sm
21sm
To label completely the state of the electron in a hydrogen atom, 4 quantum numbers are need:
name label magnitude
Principal quantum number
n 1, 2, 3, ...
Orbital quantum number
l 0, 1, 2, ... n-1
magnetic quantum number
ml od –l do +l
Spin quantum number
ms ± 1/2
Many – electron atoms and the exclusion principle
Central field approximation:
- Electron is moving in the total electric field due to the nucleus and averaged – out cloud of all the other electrons.
- There is a corresponding spherically symmetric potential – energy function U( r).
Solving the Schrodinger equation the same 4 quantum numbers are obtained. However wave functions are different. Energy levels depend on both n and l.
• In the ground state of a complex atom the electrons cannot all be in the lowest energy state.
Pauli’s exclusion principle states that no two electrons can occupy the same quantum – mechanical state. That is, no two electrons in an atom can have the same values of all four quantum numbers (n, l, ml and ms )
Shells and orbitals
Nmax - maximum number of electrons occupying given orbital
n shell orbital
1 K 0 s
2 L 0 s
L 1 p
3 M 0 s
M 1 p
M 2 d
4 N NNN
01
23
sp
df
Nmax
2
2
2
6
6
6210
1014
Shells K, L, M
n 1 2 3
0 0 1 0 1 2
m 0 0 -1 0 1 0 -1 0 1 -2 -1 0 1 2
ms
N 2 8 18
N : number of allowed states state with ms = +1/2 state with ms = -1/2
1s22s22p2
1s22s22p4
carbon
oxygen
Hund’s rule - electrons occupying given shell initially set up their spins paralelly
The periodic table of elements
Atoms of helium, lithium and sodium
n =1, = 0 n =1, = 0 n =1, = 0
n =2, = 0 n =2, = 0n =2, = 0
n =2, = 1 n =2, = 1
n =3, = 0
Helium (Z = 2) Lithium(Z = 3) Sodium (Z= 11)
1s
2s
2p
3s
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10
6p6 7s2 6d10 5f14
110
25
15
23
22
21
26
162
43:
43:
43:
43:
43:
43:
43:
431:
sdCu
sdMn
sdCr
sdV
sdTi
sdSc
spCa
spsK
Electron configuration – the occupying of orbitals
Example: l = 1, s = ½
1 jjJ
21
21
23
21
21
23
21
21
23
21
, lub,,,
1lub1
jj mm
jj
j = 3/2 j = 1/2
SLLJ
Possible two magnitudes of j : l-sjslj or
jjjjmmJ jjz ,1,,1,,
Total angular momentum - J
TheStern-Gerlach experiment
Diamagnetics
.Diamagnetics Shells totally filled with electrons. Total magnetic moment equals zero. (In a filled orbital, the vectors for both the orbital angular momentum and the spin angular momentun point in all posible directions and thus cancel).
• Noble gas
- He, Ne, Ar…..• diatomic molecule gas
- H2, N2…..
• solid states of ionic bonds
- NaCl(Na+, Cl-)…• solid states of covalent bonds
- C(diamond), Si, Ge…..• most organic materials
. Paramagnetics Shells partially filled with electrons Total magnetic moment different from zero.
Paramagnetics
Bef JJg )1(
BJHef Mg,
The component of the magnetic moment directed towards external magnetic field
Fine and hyperfine structure
Line splittings resulting from magnetic inetractions are called fine structure.
The nucleus of the atom has also magnetic dipole moment that interacts with total magnetic moment of electrons. These effects are called hyperfine structure.
NMR ( nuclear magnetic resonance)
Like electrons, protons also posses magnetic moment due to orbital angular momentum and spin ( they are also spin-1/2 particles) angular momentum.
Spin flip experiment:
Protons, the nuclei of hydrogen atoms in the tissue under study, normally have random spin orientations. In the presence of a strong magnetic field, they become aligned with a component paralell to the field. A brief radio signal flips the spins; as their components reorient paralell to the field, they emit signals that are picked up by sensitive detectors. The differing magnetic environment in various regions permits reconstruction of an image showing the types of tissue present.
An electromagnet used for MRI imaging
Wilhelm Roentgen 1895
Roentgen lamp
2
maxmin
v
2e
AC
m hceV h
Roentgen 1895; X -ray: 10-12m – 10-9m
X-ray continuum spectra
2
maxmin
v
2e
AC
m hceV h
ACeV
hcmin
X-ray spectra and Moseley law
The continous –spectrum radiation is nearly independent of the target material.
Sharp peaks (characteristic spectra) depend on the accelerating voltage and the target element. Frequencies of the peaks as a function of the element’s atomic number Z:
215 )1)(1048.2( ZHzf
Moseley law
ACeV
hcmin
X-ray spectra and Moseley law - explanationCharacteristic x-ray radiation is emitted in transitions involving the inner shells of a complex atom.
Let us assume, that due to electric field one of the two K – electrons is knocked out of the K shell. The vacancy can be filled by another electron falling in from the outer shells. K is the transition from n=2 to n=1. As the electron drops down it is attracted by Z protons in the nucleus screened by the one remaining electron in the K shell. The energy before (Ei) an after (Ef) transition:
22 2/)6.13()1( eVZEi )6.13()1( 2 eVZE f
)2.10()1( 2 eVZEK
215 )1)(1047.2( ZHzh
Ef
X-ray diffraction pattern
X-ray diffraction pattern
Diffraction maxima: md sin2
X