Chapter 38. Photons and Matter Waves

Transcription

1 Chapter 38 Photons and Matter Waves The sub-atomic world behaves very differently from the world of our ordinary experiences. Quantum physics deals with this strange world and has successfully answered many questions in the sub-atomic world, such as: Why do stars shine? Why do elements order into a periodic table? How do we manipulate charges in semiconductors and metals to make transistors and other microelectronic devices? Why does copper conduct electricity but glass does not? In this chapter we explore the strange reality of quantum mechanics. Although many topics in quantum mechanics conflict with our common sense world view, the theory provides a well-tested framework to describe the subatomic world. 38-1

2 Quantum physics: Study of the microscopic world The Photon, the Quantum of Light Many physical quantities found only in certain minimum (elementary) amounts, or integer multiples of those elementary amounts These quantities are "quantized" Elementary amount associated with this a quantity is called a "quantum" (quanta plural) Analogy example: 1 cent or $0.01 is the quantum of U.S. currency Electromagnetic radiation (light) is also quantized, with quanta called photons. This means that light is divided into integer number of elementary packets (photons). 38-2

3 The Photon, the Quantum of Light, cont'd So what aspect of light is quantized? Frequency and wavelength still can be any value and are continuously variable, not quantized f = c λ where c is the speed of light 3x10 8 m/s However, given light of a particular frequency, the total energy of that radiation is quantized with an elementary amount (quantum) of energy E given by: E = hf (photon energy) Where the Planck constant h has a value: h = = J s ev s The energy of light with frequency f must be an integer multiple of hf. In the previous chapters we dealt with such large quantities of light, that individual photons were not distinguishable. Modern experiments can be performed with single photons. 38-3

4 The Photoelectric Effect When short wavelength light illuminates a clean metal surface, electrons are ejected from the metal. These photoelectrons produce a photocurrent. First Photoelectric Experiment: Photoelectrons stopped by stopping voltage V stop. The kinetic energy of the most energetic photoelectrons is K max = ev stop K max does not depend on the intensity of the light! single photon ejects each electron Fig

5 The Photoelectric Effect, cont d Second Photoelectric Experiment: slope = ab bc Photoelectric effect does not occur if the frequency is below the cutoff frequency f o, no matter how bright the light! single photon with energy greater than work function Φ ejects each electron Fig

6 The Photoelectric Effect, cont d Photoelectric Equation The previous two experiments can be summarized by the following equation, which also expresses energy conservation hf = K max + Φ (photoelectric equation) Using h Kmax = evstop Vstop = f e Φ e equation for a straight with slope h/e and intercept Φ/e h ab 2.35 V 0.72 V = = = = e bc Hz -15 slope V s Multiplying this result by e ( ) ( )( ) h = = V s C J s 38-6

7 p = hf c Photons Have Momentum h = (photon momentum) λ Fig Fig

8 Fig Photons Have Momentum, Compton shift Conservation of energy 2 K mc γ ( 1) = 1 γ = 1 vc hf = hf ' + K Since electron may recoil at speed approaching c we must use the relativistic expression for K ( ) 2 where γ is the Lorentz factor Substituting K in the energy conservation equation f = c λ 2 h h hf = hf ' + mc ( γ 1 ) = + mc( γ 1) p = γ mv λ λ' h h Conservation of momentum along x = cosφ + γmv cosθ λ λ ' h Conservation of momentum along y 0 = sinφ γmvsinθ 38-8 λ '

9 Photons Have Momentum, Compton shift cont d Want to find wavelength shift λ = λ' λ Conservation of energy and momentum provide 3 equations for 5 unknowns (λ, λ, v, φ, and θ ), which allows us to eliminate 2 unknowns v and θ. λ = h mc ( φ) 1-cos (Compton shift) λ, λ, and φ, can be readily measured in the Compton experiment h mc is the Compton wavelength and depends on 1/m of the scattering particle Loose end: Compton effect can be due to scattering from electrons bound loosely to atoms (m=m e peak at θ 0) or electrons bound tightly to atoms (m m atom >>m e peak at θ 0) 38-9

10 Light as a Probability Wave How can light act both as a wave and as a particle (photon)? Standard Version: photons sent through double slit. Photons detected (1 click at a time) more often where the classical intensity: I 2 Erms cµ = is maximum. 0 Fig The probability per unit time interval that a photon will be detected in any small volume centered on a given point is proportional to E 2 at that point. Light is not only an electromagnetic wave but also a probability wave for detecting photons 38-10

11 Light as a Probability Wave, cont'd Single Photon Version: photons sent through double slit one at a time. First experiment by Taylor in We cannot predict where the photon will arrive on the screen. 2. Unless we place detectors at the slits, which changes the experiment (and the results), we cannot say which slit(s) the photon went through. 3. We can predict the probability of the photon hitting different parts of the screen. This probability pattern is just the two slit interference pattern that we discussed in Ch. 35. The wave traveling from the source to the screen is a probability wave, which produces a pattern of "probability fringes" at the screen

12 Light as a Probability Wave, cont'd Single Photon, Wide-Angle Version: More recent experiments (Lai and Diels in 1992) show that photons are not small packets of classical waves. 1. Source S contains molecules that emit photons at well separated times. 2. Mirrors M 1 and M 2 reflect light that was emitted along two distinct paths, close to 180 o apart. 3. A beam splitter (B) reflects half and transmits half of the beam from Path 1, and does the same with the beam from Path At detector D the reflected part of beam Fig combines (and interferes) with the transmitted part of beam The detector is moved horizontally, changing the path length difference between Paths 1 and 2. Single photons are detected, but the rate at which they arrive at the detector follows the typical two slit interference pattern. Unlike the standard two slit experiment, here the photons are emitted in nearly opposite directions! Not a small classical wavepacket! 38-12

13 Light as a Probability Wave, cont'd Conclusions from the previous three versions/experiments: 1. Light generated at source as photons. 2. Light absorbed at detector as photons. 3. Light travels between source and detector as a probability wave

14 Electrons and Matter Waves If electromagnetic waves (light) can behave like particles (photons), can particles behave like waves? λ = h p (de Broglie wavelength) where p is the momentum of the particle Electrons θ Fig

15 Waves and Particles Consider a particle that is detected at Point I and then is detected at Point F. Can we think of this as the propagation of a wave. Imagine any non-straight path connecting I and F. This path will have a neighboring path that will destructively interfere with it. For the straight (direct) path connecting I and F the neighboring paths will constructively interfere to reinforce the probability of traveling in a straight line. Of course, if the particle is charged and in the presence of an electric or magnetic field, the interference conditions on neighboring paths will change and the path may become curved. Fig

16 Schrödinger s Equation For light E(x, y, z, t) characterizes its wavelike nature, for matter the wave function Ψ(x, y, z, t) characterizes its wavelike nature. Like any wave, Ψ(x, y, z, t) has an amplitude and a phase (it can be shifted in time and or position), which can be conveniently represented using a complex number a+ib where a and b are real numbers and i 2 = -1. On the situations that we will discuss, the space and time variables can be grouped separately: ( xyzt,,, ) ψ (,,, ) Ψ = i t xyzte ω where ω=2πf is the angular frequency of the matter wave

17 Schrödinger s Equation, cont d What does the wave function mean? If the matter wave reaches a particle detector that is small, the probability that that a particle will be detected there in a specified period of time is proportional to Iψ 2, where Iψ is the absolute value (amplitude) of the wave function at the detector s location. The probability per unit time interval of detecting a particle in a small volume centered on a given point in a matter wave is proportional to the value Iψ 2 at that point. Since ψ is typically complex, we obtain Iψ 2 by multiplying ψ by its complex conjugate ψ*. To find ψ* we replace the imaginary number i in ψ with i wherever it occurs. 2 ( )( ) ψ = ψψ* = a+ ib a ib 38-17

18 Schrödinger s Equation, cont d How do we find (calculate) the wave function? Matter waves are described by Schrödinger s equation. Light waves a described Maxwell s equations, matter waves are described by Schrödinger s equation. For a particle traveling in the x direction through a region in which forces on the particle cause it to have a potential energy U(x), Schrödinger s equation reduces to: 2 2 d ψ 8π m E U ( x ) ψ =0 (Schrodinger's eq. in 1D) dx h & where E is the total energy of the particle. If U(x) = 0, this equation describes a free particle. In that case the total energy of the particle is simple its kinetic energy (1/2)mv 2 and the equation becomes: + ψ=0 + 2 π ψ=0 dx h 2 dx h d ψ 8π m mv d ψ p

19 Schrödinger s Equation, cont d In the previous equation, since λ=h/p, we can replace the p/h with 1/λ, which in turn is related to the angular wave number k=2π/λ. 2 d ψ dx & + 2 k ψ =0 (Schrodinger's eq., free particle) 2 The most general solution is: ψ ( x)= Ae + Be ikx ikx Leading to: ω (, ) ψ ( ) = ( ) Ψ x t = x e Ae + Be e i t ikx ikx iωt ( ω ) ( + ω ) ikx t ikx t = Ae + Be 38-19

20 Finding the Probability Density Iψ 2 iθ iθ e = cosθ + isin θ and e = cosθ isin θ Choose arbitrary constant B=0 and let A= ψ 0 ψ ( x) = ψ0 e ikx ( ) 2 ikx 2 2 ikx = 2 0e = 0 e ψ ψ ψ ikx 2 ikx ikx ikx ikx ikx ikx ( )( ) e = e e * = e e = e = 1 ( ) 2 ikx e = 0 = 0 ψ = ψ ψ 1 ψ (a constant) Fig

21 Heisenberg s Uncertainty Principle In the previous example, the momentum (p or k) in the x-direction was exactly defined, but the particle s position along the x-direction was completely unknown. This is an example of an important principle formulated by Heisenberg: Measured values cannot be assigned to the position r and the momentum p of a particle simultaneously with unlimited precision. x p h where h= h 2π y p z p x y z h (Heisenberg's uncertainty principle) h Note that if p = p = p = 0 x y z Then x, y, and z 38-21

22 Barrier Tunneling As puck slides up hill, kinetic energy K is converted to gravitational potential energy U. If the puck reaches the top its potential energy is U b. The puck can only pass over the top if its initial mechanical energy E> U b. Otherwise the puck eventually stops its climb up left side of hill and slides back top left. For example, if U b =20 J and E=10 J, the puck will not make pass over the hill, which acts as a potential barrier. Fig

23 Barrier Tunneling, cont d What about an electron approaching an electrostatic potential barrier? Fig Due to the nature of quantum mechanics, even if E< U b there is a non-zero transmission probability (transmission coefficient T) that the electron will get through (tunnel) to the other side of the electrostatic potential barrier! ( ) 2 2bL 8π mub E where 2 T e b= h Fig Fig

24 The Scanning Tunneling Microscope (STM) As tip is scanned laterally across the surface, the tip is moved up or down to keep the tunneling current (tip to surface distance L) constant. As a result the tip maps out the contours of the surface with resolution on the scale of 1 nm instead of >300 nm for optical microscopes! Fig