Particle in a One Dimensional Box
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Particle in a One Dimensional Box
V= V=V=0
b-b 0x
Infinitely high, thick,impenetrable walls
Particle inside box. Can’t get out because of impenetrable walls.
Classically E is continuous. E can be zero. One D racquet ball court.
Q.M. /2x p E can’t be zero.
Schrödinger Equation
H E Energy eigenvalue problem
+2 2
2
( )( ) ( ) ( )
2
d xV x x E x
m d x
( ) 0V x x b
( )V x x b Copyright – Michael D. Fayer, 2007
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x bFor
2 2
2
( )( )
2
d xE x
m d x
Want to solve differential Equation, butSolution must by physically acceptable.
Born Condition on Wavefunction to make physically meaningful.
1. The wave function must be finite everywhere.
2. The wave function must be single valued.
3. The wave function must be continuous.
4. First derivative of wave function must be
continuous.
Copyright – Michael D. Fayer, 2007
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2
2 2
( ) 2( )
d x mEx
dx
Second derivative of a function equals anegative constant times the same function.
22
2
sin( )sin( )
d axa ax
dx
22cos( )cos( )
d axa ax
dx
Functions with this property sin and cos.
22
2mEa
These are solutions provided
Copyright – Michael D. Fayer, 2007
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Solutions with any value of a don’t obey Born conditions.
b-b
0
x0
Well is infinitely deep.Particle has zero probability of being
found outside the box.
x b = 0 for
Function as drawn discontinuous at x b
To be an acceptable wavefunction
sin and cos 0 at x b
Copyright – Michael D. Fayer, 2007
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will vanish at ifx b
2 n
na a
b
n is an integer
cos 1,3,5...na x n
sin 2,4,6...na x n
2 22
2 2
2
4n
n mEa
b
Have two conditions for a2.
Solve for E.
2 2 2 2 2
2 28 8n
n n hE
mb mL
Energy eigenvalues – energy levels, not continuous.
L = 2b – length of box.
Integral number of half wavelengthsin box. Zero at walls.
b–b
0
Copyright – Michael D. Fayer, 2007
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Energy levels are quantized.Lowest energy not zero.
2 2 2 2 2
2 28 8n
n n hE
mb mL
L = 2b – length of box.
b-b
0
0
0
n =1
n =2
n =3
0x
First few wavefunctions.
Quantization forced by Born conditions(boundary conditions)
Forth Born condition not met – first derivative not continuous.Physically unrealistic problem because
the potential is discontinuous.
Like classical string – has “fundamental”and harmonics.
1
21( ) cos 1,3,5
2n
n xx x b n
b b
1
21( ) sin 2,4,6
2n
n xx x b n
b b
wavefunctions includingnormalization constants
Copyright – Michael D. Fayer, 2007
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Particle in a Box Simple model of molecular energy levels.
Anthracene
L 6 AL
electrons – consider “free”in box of length L.
Ignore all coulomb interactions.
2
2 1 2
3
8
hE E E
mL
E2
E1 S0
S1
E
Calculate wavelength of absorption of light.Form particle in box energy level formula
31
1
19
0
34
9 10 kg
6 A 6 10 m
h 6.6 1
E 5.04 10 J
0 Js
em m
L
14
393 nm
E h
E / h 7.64 10 Hz
c / blue-violet
Experiment 400 nm
Copyright – Michael D. Fayer, 2007
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Anthracene particularly good agreement.Other molecules, naphthalene, benzene, agreement much worse.
Important pointConfine a particle with “size” of electron to boxsize of a molecule
Get energy level separation, light absorption, in visible and UV.
Molecular structure, realistic potentialgive accurate calculation, but
It is the mass and size alone that set scale.
Big molecules absorb in red.Small molecules absorb in UV.
Copyright – Michael D. Fayer, 2007
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Particle in a Finite Box – Tunneling and Ionization
-b b0
V(x)=0
V(x)=V V(x)=V
x
Box with finite walls.
+2 2
2
( )( ) ( ) ( )
2
d xV x x E x
m d x
Time independent Schrödinger Eq.
( ) 0V x x b
( )V x V x b
Inside Box V = 0
2
2 2
( ) 2( )
d x mEx
dx
2 2
2
( )( )
2
d xE x
m d x
Second derivative of function equalsnegative constant times same function.
Solutions – sin and cos.Copyright – Michael D. Fayer, 2007
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Solutions inside box
1 2
2( ) sin
mEx q x
2 2
2( ) cos
mEx q x
or
Outside Box
2
2 2
( ) 2 ( )( )
d x m E Vx
dx
Two cases: Bound states, E < V Unbound states, E > V
Bound States
2
2 2
( ) 2 ( )( )
d x m V Ex
dx
Second derivative of function equalspositive constant times same function.
Not oscillatory.
Copyright – Michael D. Fayer, 2007
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Try solutions
exp ax2
22
axa xd e
a ed x
Second derivative of function equals
positive constant times same function.
Then, solutions outside the box
x b1 / 2
2
2 ( )
( )m V E
x
x e
Solutions must obey Born Conditions
x ( )x can’t blow up as Therefore,
1 / 2
2
2 ( )
1( )m V E
x
x r e x b
1 / 2
2
2 ( )
2( )m V E
x
x r e x b
Outside box exp. decaysInside box oscillatory
Copyright – Michael D. Fayer, 2007
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b-b
0
x0
Outside box exp. decaysInside box oscillatory
The wavefunction and its first derivativecontinuous at walls – Born Conditions.
bx
0
probability 0
Expanded viewFor a finite distance into the material
finite probability of finding particle.
Classically forbidden regionV > E.
Copyright – Michael D. Fayer, 2007
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Tunneling - Qualitative Discussion
0
X
Classically forbidden region.
Wavefunction not zero at farside of wall.
Probability of finding particle smallbut finite outside box.
A particle placed inside of box with not enough energy to go over the wallcan Tunnel Through the Wall.
Formula derived in book
mass = me
E = 1000 cm-1
V = 2000 cm-1
Wall thickness (d) 1 Å 10 Å 100 Åprobability 0.68 0.02 3 10-17
2 1/22 [2 ( ) / ]d m V Ee Ratio probs - outsidevs. inside edges of wall.
Copyright – Michael D. Fayer, 2007
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q
reactants
products
Chemical Reaction
not enough energy to go over barrier
ETemperature dependence of somechemical reactions shows to muchproduct at low T. E > kT .
2 1/ 2(2 / )d mV
Decay of wavefunction in classically forbidden region for parabolic potential.
e-
tunneling distance mass barrier heightparameter
light particles tunnel
Copyright – Michael D. Fayer, 2007
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Methyl Rotation
C HH
H
Methyl groups rotate evenat very low T.
Radioactive Decay
repulsive Coulomb interaction
nuclear attractionstrong interaction
some probability outside nucleus
Copyright – Michael D. Fayer, 2007
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Unbound States and Ionization
-b b
bound states
V(x) = VIf E > V - unbound states
E large enough - ionization
E < V
1 2
2( ) sin
mEx q x
Inside the box (between -b and b)V = 0
2 2
2( ) cos
mEx q x
Solutions oscillatory
2
2 2
( ) 2 ( )( )
d x m E Vx
dx
1 2
2 ( )( ) sin
m E Vx s x
Outside the box (x > |b|)E > V
2 2
2 ( )( ) cos
m E Vx s x
Solutions oscillatory
Copyright – Michael D. Fayer, 2007
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-b b
-b b
-b b
unbound state
1 2
2sin
mEq b
1 2
2 ( )sin
m E Vs b
To solve (numerically)Wavefunction and first derivativeequal at walls, for example at x = b
In limit
1 1
( )
E V
E V E
q s
Wavefunction has equalamplitude everywhere.
Copyright – Michael D. Fayer, 2007
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For E >> V
2
2( ) sin
mEx x
for all x
As if there is no wall.
Continuous range of energies - free particle
Particle has been ionized.
2 2 2
2 2 2
2 2
2
mE mp kk
m
( ) sinx k x Free particle wavefunction
Copyright – Michael D. Fayer, 2007
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In real world potential barriers are finite.
Tunneling
Ionization
Copyright – Michael D. Fayer, 2007