Electron in a Box. A wave packet in a square well (an electron in a box) changing with time.

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Penelope Newman
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1 Electron in a Box A wave packet in a square well (an electron in a box) changing with time.

2 Last Time: Light Wave model: Interference pattern is in terms of wave intensity Photon model: Interference in terms of probability The probability of detecting a photon within a narrow region of width δx at position x is directly proportional to the square of the light wave amplitude function at that point. Prob(in δx at x) A(x) δx 2 Probability Density Function: Px ( ) A(x) 2 The probability density function is independent of the width, δx, and depends only on x. SI units are m -1.

3 Double Slit: Electrons A light analogy.. There is no electron wave so we assume an analogy to the electric wave and call it the wave function, psi, : Ψ( x) The intensity at a point on the screen is proportional to the square of the wave function at that point. Px ( ) = Ψ( x) 2 The Probability Density Function is the Reality!

4 Probability: Electrons The probability of detecting an electron within a narrow region of width δx at position x is directly proportional to the square of the wave function at that point: Prob(in δx at x) = Ψ(x) δx 2 Probability Density Function: Px ( ) = Ψ(x) 2 The probability density function is independent of the width, δx, and depends only on x. SI units are m -1. Note: The above is an equality, not a proportionality as with photons. This is because we are defining psi this way. Also note, P(x) is unique but psi in not since psi is also a solution. DEGENERACY.

5 If the strip isn t narrow, then we integrate the probability density function so that the probability that an electron lands somewhere between x L and x R is: x R Prob( x x x ) = P( x) dx = ψ ( x) dx L R x L x x R L 2 Most Probable: Normalization dp( x) dx = 0 P( x) dx = ψ ( x) dx = 1 2

6 Electron Waves leads to Quantum Theory Waves: De Broglie: 2 L λ =, n = 1,2,3... n n h λ = p 1 p E = mv 2 = 2 2m 2 Combine: E n 2 2 hn = Energy is Quantized! 8mL 2

Wave Packet: Making Particles out of Waves

7 Wave Packet: Making Particles out of Waves h p = c= λ f λ p = hf / c Superposition of waves to make a defined wave packet. The more waves used of different frequencies, the more localized. However, the more frequencies used, the less the momentum is known.

Heisenberg Uncertainty Principle You make a wave packet

The more waves you use, the more defined your packet and

However, the more waves you use of different frequencies

8 Heisenberg Uncertainty Principle You make a wave packet by wave superposition and interference. The more waves you use, the more defined your packet and the more defined the position of the particle. However, the more waves you use of different frequencies (energy or momentum) to specify the position, the less you specify the momentum!

10 E t > h/4π x p> h/4π

11 Little h bar!

12 Which of these particles, A or B, can you locate more precisely? A. A B. B C. Both can be located with same precision.

13 Which of these particles, A or B, can you locate more precisely? A. A B. B C. Both can be located with same precision.

That energy is transferred to the electron in an unpredictable way and the motion (momentum) becomes uncertain.

14 Heisenberg Microscope Small wavelength (gamma) of light must be used to find the electron because it is too small. But small wavelength means high energy. That energy is transferred to the electron in an unpredictable way and the motion (momentum) becomes uncertain. If you use long wavelength light (infrared), the motion is not as disturbed but the position is uncertain because the wavelength is too long to see the electron. This results in the Uncertainty Principle. x ~ λ p = h/ λ x p> h

15 The possible wavelengths for an electron in a box of length L. Electron in a Box x~ L p~ p~ h/ λ = h/ L If you squeeze the walls to decrease x, you increase p! x p ~ L h/ L> h

16 Improved technology will not save us from Quantum Uncertainty! Quantum Uncertainty comes from the particle-wave nature of matter and the mathematics (wave functions) used to describe them!

17 Heisenberg Uncertainty Trying to see what slit an electron goes through destroys the interference pattern. Electrons act like waves going through the slits but arrive at the detector like a particle.

18 Which Hole Did the Electron Go Through? If you make a very dim beam of electrons you can essentially send one electron at a time. If you try to set up a way to detect which hole it goes through you destroy the wave interference pattern. Conclusions: Trying to detect the electron, destroys the interference pattern. The electron and apparatus are in a quantum superposition of states. There is no objective reality.

19 Feynman s version of the Uncertainty Principle It is impossible to design an apparatus to determine which hole the electron passes through, that will not at the same time disturb the electrons enough to destroy the interference pattern.

20 General Principles

21 Where do the Wave Functions come from??? Solutions to the time-independent Schrödinger equation: 2 2 d ψ + 2 Uψ = Eψ 2m dx 2 d ψ dx = OR 2m U ( E) 2 2 ψ Where does that come from???

22 The Schrödinger Equation Consider an atomic particle with mass m and mechanical energy E in an environment characterized by a potential energy function U(x). The Schrödinger equation for the particle s wave function is Conditions the wave function must obey are 1. ψ(x) is a continuous function. 2. ψ(x) = 0 if x is in a region where it is physically impossible for the particle to be. 3. ψ(x) 0 as x + and x. 4. ψ(x) is a normalized function.

23 Wavefunction Fun Wave Function: Probability Amplitude: ψ ( x) Probability Density: Px ( ) = ψ ( x) 2 Probability: x R Prob( x x x ) = P( x) dx = ψ ( x) dx L R x L x x R L 2 Normalization P( x) dx = ψ ( x) dx = 1 2 Expectation average value: x = xp( x) dx = x ψ ( x) dx 2 Most Probable: dp( x) dx = 0

24 Quantum Cases Free Particle Particle in a rigid box Particle in a finite box quantum well Quantum Tunneling Harmonic Oscillator Hydrogen Atom (Chapter 42)

25 Wave Function of a Free Particle The wave function of a free particle moving along the x-axis can be written as ψ(x) = Ae i(kx-ωt) A is the constant amplitude k = 2π/λ is the angular wave number of the wave representing the particle A free particle must have a sinusoidal wavefunction because it is not confined. Although the wave function is often associated with the particle, it is more properly determined by the particle and its interaction with its environment Think of the system wave function instead of the particle wave function

26 Free Particle Problem A free electron has a wave function at t=0 ψ where x is in meters. ( ) ( 10 i x) x = Ae (a) Show that it satisfies the SE. (b) Find its de Broglie wavelength (c) Find its momentum

27 A Particle in a Rigid Box Consider a particle of mass m confined in a rigid, onedimensional box. The boundaries of the box are at x = 0 and x = L. 1. The particle can move freely between 0 and L at constant speed and thus with constant kinetic energy. 2. No matter how much kinetic energy the particle has, its turning points are at x = 0 and x = L. 3. The regions x < 0 and x > L are forbidden. The particle cannot leave the box. A potential-energy function that describes the particle in this situation is

29 A Particle in a Rigid Box The solutions to the Schrödinger equation for a particle in a rigid box are For a particle in a box, these energies are the only values of E for which there are physically meaningful solutions to the Schrödinger equation. The particle s energy is quantized.

30 A Particle in a Rigid Box The normalization condition, which we found in Chapter 40, is This condition determines the constants A: The normalized wave function for the particle in quantum state n is

31 A Particle in a Rigid Box

32 Schrödinger Equation Applied to a Particle in a Box Solving for the allowed energies gives E n h 8mL 2 = 2 n The allowed wave functions are given by 2 nπx 2 nπx ψn ( x) = A sin = sin L L L The second expression is the normalized wave function These match the original results for the particle in a box HW P.17

33 Graphical Representations for a Particle in a Box

34 Energy of a Particle in a Box We chose the potential energy of the particle to be zero inside the box Therefore, the energy of the particle is just its kinetic energy 2 h En = n n = 8mL,,, The energy of the particle is quantized

35 The Quantum Jump Quantum Energy States E n = 2 2 hn 8mL 2 Energy is Quantized! Only discrete energy states are allowed. Where is the electron between jumps?

36 EXAMPLE :Energy Levels and QUESTIONS: Quantum jumps

39 Finite Potential Wells The wave function in the classically forbidden region of a finite potential well is The wave function oscillates until it reaches the classical turning point at x = L, then it decays exponentially within the classically forbidden region. A similar analysis can be done for x 0. We can define a parameter η defined as the distance into the classically forbidden region at which the wave function has decreased to e 1 or 0.37 times its value at the edge:

41 Finite Potential Wells The quantum-mechanical solution for a particle in a finite potential well has some important properties: The particle s energy is quantized. There are only a finite number of bound states. There are no stationary states with E > U 0 because such a particle would not remain in the well. The wave functions are qualitatively similar to those of a particle in a rigid box, but the energies are somewhat lower because the wave functions are spread out which means lower kinetic energy. The wave functions extend into the classically forbidden regions.

42 Penetration distance of an electron

44 Quantum-Mechanical Tunneling The probability that a particle striking the barrier from the left will emerge on the right is found to be

45 Applications of Tunneling Alpha decay In order for the alpha particle to escape from the nucleus, it must penetrate a barrier whose energy is several times greater than the energy of the nucleus-alpha particle system Nuclear fusion Protons can tunnel through the barrier caused by their mutual electrostatic repulsion

More Applications of Tunneling Scanning Tunneling Microscope An electrically conducting probe with a very sharp edge is brought near the surface

46 More Applications of Tunneling Scanning Tunneling Microscope An electrically conducting probe with a very sharp edge is brought near the surface to be studied The empty space between the tip and the surface represents the barrier The tip and the surface are two walls of the potential well

47 Scanning Tunneling Microscope The STM allows highly detailed images of surfaces with resolutions comparable to the size of a single atom At right is the surface of graphite viewed with the STM

48 Quantum Tunneling Cosmology BIG BANG! The Universe Tunneled in from Nothing

49 The Quantum Harmonic Oscillator The potential-energy function of a harmonic oscillator: where we ll assume the equilibrium position is x e = 0. The Schrödinger equation for a quantum harmonic oscillator is then

50 The Quantum Harmonic Oscillator The wave functions of the first three states are Where ω = (k/m) ½ is the classical angular frequency, and n is the quantum number, b is classical turning point.

51 Energy Level Diagrams Simple Harmonic Oscillator The separation between adjacent levels are equal and equal to E = ω The energy levels are equally spaced The state n = 0 corresponds to the ground state The energy is E o = ½ ω Agrees with Planck s original equations!!

52 Light emission by an oscillating electron

53 Light emission by an oscillating electron

54 Molecular Vibrations Molecular bonds are modeled as quantum harmonic oscillators energies below the dissociation energy.

55 Casimir Effect: Model the Quantum Vacuum as a harmonic oscillator. The Zero Point Energy adds up! Eo = ½ hω

56 The Correspondence Principle Niels Bohr put forward the idea that the average behavior of a quantum system should begin to look like the classical solution in the limit that the quantum number becomes very large that is, as n. Because the radius of the Bohr hydrogen atom is r = n 2 a B, the atom becomes a macroscopic object as n becomes very large. Bohr s idea, that the quantum world should blend smoothly into the classical world for high quantum numbers, is today known as the correspondence principle.

57 The Correspondence Principle As n gets even bigger and the number of oscillations increases, the probability of finding the particle in an interval δx will be the same for both the quantum and the classical particles as long as δx is large enough to include several oscillations of the wave function. This is in agreement with Bohr s correspondence principle.

58 I think I can safely say that nobody understands quantum mechanics. Richard Feynman

59 Copenhagen Interpretation of the Wave Function Quantum mechanics is a model of the microscopic world. Like all models, it is created by people for people. It divides the world into two parts, commonly called the system and the observer. The system is the part of the world that is being modeled. The rest of the world is the observer. An interaction between the observer and the system is called a measurement. Properties of the system that can be measured are called observables. The initial information the observer has about the system comes from a set of measurements. The state of the system represents this information, which can be cast into different mathematical forms. It is often represented in terms of a wave function. Quantum mechanics predicts how the state of the system evolves and therefore how the information the observer has about the system evolves with time. Some information is retained, and some is lost. The evolution of the state is deterministic. Measurements at a later time provide new information, and therefore the state of the system, in general, changes after the measurements. The wave function of the system, in general, changes after a measurement. So quantum mechanics does not really describe the system, but the information that the rest of the world can possibly have about the system.

60 Measurement: Collapsing the Wave Function into an eigenstate In quantum mechanics, a measurement of an observable yields a value, called an eigenvalue of the observable. Many observables have quantized eigenvalues, i.e. the measurement can only yield one of a discrete set of values. Right after the measurement, the state of the system is an eigenstate of the observable, which means that the value of the observable is exactly known. A state can be a simultaneous eigenstate of several observable, which means that the observer can exactly know the value of several properties of the system at the same time and make exact predictions about the outcome of measurements of those properties. But there are also incompatible observables whose exact values cannot be known to the observer at the same time. A state cannot be a simultaneous eigenstate of incompatible observables. If it is in an eigenstate of one of the incompatible observables and the value of this observable is known, then quantum mechanics gives only the probabilities for measuring each of the different eigenvalues of the other incompatible observables. The eigenstate of the first observable is a superposition of eigenstates of any of the other incompatible observables. The outcome of a measurement any of the other incompatible observables is uncertain. A measurement of one of the other incompatible observables changes the state of the system to one of its eigenstates and destroys the information about the value of the first observable.

Richard Feynman: Electron waves are probability waves in the ocean of uncertainty.

Richard Feynman: Electron waves are probability waves in the ocean of uncertainty. Last Time We Solved some of the Problems with Classical Physics Discrete Spectra? Bohr Model but not complete. Blackbody