Download Wednesday, March 3, 2010

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project

Document related concepts

Propagator wikipedia , lookup

EPR paradox wikipedia , lookup

Ensemble interpretation wikipedia , lookup

Tight binding wikipedia , lookup

Hidden variable theory wikipedia , lookup

Renormalization wikipedia , lookup

Canonical quantization wikipedia , lookup

Path integral formulation wikipedia , lookup

Symmetry in quantum mechanics wikipedia , lookup

Coherent states wikipedia , lookup

Quantum electrodynamics wikipedia , lookup

Wheeler's delayed choice experiment wikipedia , lookup

Schrödinger equation wikipedia , lookup

T-symmetry wikipedia , lookup

Identical particles wikipedia , lookup

Elementary particle wikipedia , lookup

Electron scattering wikipedia , lookup

Atomic theory wikipedia , lookup

Copenhagen interpretation wikipedia , lookup

Molecular Hamiltonian wikipedia , lookup

Relativistic quantum mechanics wikipedia , lookup

Aharonov–Bohm effect wikipedia , lookup

Bohr–Einstein debates wikipedia , lookup

Wave function wikipedia , lookup

Double-slit experiment wikipedia , lookup

Probability amplitude wikipedia , lookup

Particle in a box wikipedia , lookup

Wave–particle duality wikipedia , lookup

Matter wave wikipedia , lookup

Theoretical and experimental justification for the Schrödinger equation wikipedia , lookup

Transcript
Physics 3313 - Lecture 13
Wednesday March 3, 2010
Dr. Andrew Brandt
1.
2.
3.
4.
3/3/2010
Review Monday March 8 (HW 5 due)
Test Weds Mar 10
Wells+Barriers
Harmonic Oscillator
3313 Andrew Brandt
1
Finite Potential Well
• Classically if E < U than particle bounces off sides,
but quantum mechanically, particle can penetrate
into regions I and III
2
2
• For I  2  2m2  E  U   0 rewrite as  2  a 2  0
x
x
• With a  2m U  E 
2
• Solutions are  I  Ce ax  De ax and  III  Feax  Ge  ax
 2 2mE
 2  0
x 2
 II  eikx  Beikx
• What about in the box? Since U=0
with k  2mE (this is similar to infinite potential well, AKA particle in
2
box)
3/3/2010
3313 Andrew Brandt
2
Finite Well BC
D must be 0 ,so  I  Ce
•
At x=- =0 so for  I  Ce  De
•
ax
 ax


Fe

Ge
III
Similarly at x=+ =0 so for
F must be 0 and
 ax
 III  Ge
•
Finally what about boundary conditions for  II ? Is it 0 at x=0?
•
Nope at x=0
•
•
•
•
ax
 ax
ax
 I  C   II
 aL




Ge
II
III
And at x=L
ikx
 ikx
But  II  e  Be so too many unknowns! Should we quit?
Need other constraints. Derivatives must also be continuous (match
slopes)
After some math again get specific energy levels, but wavelengths a
little longer than infinite well, and, from De Broglie, this means
momentum and thus energy is smaller
3/3/2010
3313 Andrew Brandt
3
Penetration Depth
• The penetration depth is the distance outside the potential well where the
probability significantly decreases. It is given by
• It should not be surprising to find that the penetration distance that violates
classical physics is proportional to Planck’s constant.
3/3/2010
3313 Andrew Brandt
4
6.6: Simple Harmonic Oscillator
•
Simple harmonic oscillators describe many physical situations: springs, diatomic
molecules and atomic lattices.
Substituting
Let
U=
 x2
and
That’s a big improvement!
3/3/2010
into Schrodinger’s Equation yields:
2
which gives
.
3313 Andrew Brandt
5
Parabolic Potential Well
•
The wave function solutions are
polynomials of order n.
•
In contrast to the particle in a box, where the oscillatory wave function is a sinusoidal
curve, in this case the oscillatory behavior is due to the polynomial, which dominates at
small x. The exponential tail is provided by the Gaussian function, which dominates at
large x.
3/3/2010
where Hn(x) are Hermite
3313 Andrew Brandt
6
Analysis of the Parabolic Potential Well
• The energy levels are given by
• The zero point energy is called the Heisenberg limit:
• Classically, the probability of finding the mass is
greatest at the ends of motion and smallest at the
center (that is, proportional to the amount of time
the mass spends at each position).
• Contrary to the classical one, the largest probability
for this lowest energy state is for the particle to be
3/3/2010
3313 Andrew Brandt
7
at the center.
Simple Harmonic Oscillator
Classical
QM
3/3/2010
3313 Andrew Brandt
8
6.7: Barriers and Tunneling
•
•
•
Consider a particle of energy E approaching a potential barrier of height V0 and the
potential everywhere else is zero.
First consider the case when the energy is greater than the potential barrier.
In regions I and III the wave numbers are:
•
In the barrier region we have
3/3/2010
3313 Andrew Brandt
9
Reflection and Transmission
•
•
The wave function will consist of an incident wave, a reflected wave, and a
transmitted wave.
The potentials and the Schrödinger wave equation for the three regions are as
follows:
•
The corresponding solutions are:
•
As the wave moves from left to right, we can simplify the wave functions to:
3/3/2010
3313 Andrew Brandt
10
Probability of Reflection and Transmission
• The probability of the particles being reflected R or transmitted T is:
• The maximum kinetic energy of the photoelectrons depends on the value of the
light frequency f and not on the intensity.
• Because the particles must be either reflected or transmitted we have: R + T = 1.
• By applying the boundary conditions x → ±∞, x = 0, and x = L, we arrive at the
transmission probability:
• Notice that there is a situation in which the transmission probability is 1.
3/3/2010
3313 Andrew Brandt
11
Tunneling
•
Now we consider the situation where classically the particle does not have enough
energy to surmount the potential barrier, E < V0.
•
The quantum mechanical result, however, is one of the most remarkable features of
modern physics, and there is ample experimental proof of its existence. There is a small,
but finite, probability that the particle can penetrate the barrier and even emerge on the
other side.
The wave function in region II becomes
•
•
The transmission probability that
describes the phenomenon of tunneling is
3/3/2010
3313 Andrew Brandt
12
Uncertainty Explanation
•
Consider when κL >> 1 then the transmission probability becomes:
•
This violation allowed by the uncertainty principle is equal to the negative
kinetic energy required! The particle is allowed by quantum mechanics and
the uncertainty principle to penetrate into a classically forbidden region. The
minimum such kinetic energy is:
3/3/2010
3313 Andrew Brandt
13
Potential Well
•
•
Consider a particle passing through a potential well region rather than through a potential
barrier.
Classically, the particle would speed up passing the well region, because K = mv2 / 2 = E + V0.
According to quantum mechanics, reflection and transmission may occur, but the wavelength
inside the potential well is smaller than outside. When the width of the potential well is
precisely equal to half-integral or integral units of the wavelength, the reflected waves may be
out of phase or in phase with the original wave, and cancellations or resonances may occur.
The reflection/cancellation effects can lead to almost pure transmission or pure reflection for
certain wavelengths. For example, at the second boundary (x = L) for a wave passing to the
right, the wave may reflect and be out of phase with the incident wave. The effect would be a
cancellation inside the well.
3/3/2010
3313 Andrew Brandt
14
Alpha-Particle Decay
•
•
•
•
•
•
The phenomenon of tunneling explains the alpha-particle decay of heavy,
radioactive nuclei.
Inside the nucleus, an alpha particle feels the strong, short-range attractive nuclear
force as well as the repulsive Coulomb force.
The nuclear force dominates inside the nuclear radius where the potential is
approximately a square well.
The Coulomb force dominates
outside the nuclear radius.
The potential barrier at the nuclear
radius is several times greater than
the energy of an alpha particle.
According to quantum mechanics,
however, the alpha particle can
“tunnel” through the barrier. Hence
this is observed as radioactive decay.
2m
T  e2 L with  
U  E
2 
Ex. Alpha particles with a few MeV can escape a
potential
them
1038Brandt
tries!)
3/3/2010 well of 25 MeV! (might take 3313
Andrew
15
Laser
• Light Amplification by Stimulated Emission of Radiation
• laser light is monochromatic (one color), coherent (all in phase), can be
very intense, small divergence (shine laser on mirror left on moon)
[I knew I forgot something]
3/3/2010
3313 Andrew Brandt
16
Three Level Laser
3/3/2010
3313 Andrew Brandt
17