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Lecture 9 Quantum Mechanics (made fun and easy) Slide 2 Why the world needs quantum mechanics Slide 3 Slide 4 Slide 5 Slide 6 Slide 7 Slide 8 Quantum Mechanics in Action 2 nm CdS (cadmium yellow) CdS nanocrystal Slide 9 Slide 10 Slide 11 Slide 12 Slide 13 Slide 14 Quantum Weirdness: The Zeno Effect Slide 15 Quantum Weirdness: superposition of states Slide 16 Slide 17 Slide 18 Light as a particle (Newton, 1643-1727) Slide 19 The classical view of light as an electromagnetic wave. An electromagnetic wave is a traveling wave with time-varying electric and magnetic fields that are perpendicular to each other and to the direction of propagation. Light as a Wave (1861: Maxwell) Slide 20 From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap ( McGraw-Hill, 2005) Light as a wave Intensity of light wave = energy flowing per unit area per second Traveling wave description k=wavevector c= /k = Slide 21 Schematic illustration of Youngs double-slit experiment. Constructive interference occurs when n L=xd Youngs Double Slit Experiment http://www.youtube.com/watch?v=DfPeprQ7oGc L d x Slide 22 X-ray diffraction involves constructive interference of waves being "reflected" by various atomic planes in the crystal. X-ray Diffraction Slide 23 From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap ( McGraw-Hill, 2005) Braggs Law Bragg diffraction condition The equation is referred to as Braggs law, and arises from the constructive interference of scattered waves. Slide 24 Diffraction patterns obtained by passing X-rays through crystals can only be explained by using ideas based on the interference of waves. (a) Diffraction of X-rays from a single crystal gives a diffraction pattern of bright spots on a photographic film. (b) Diffraction of X-rays from a powdered crystalline material or a polycrystalline material gives a diffraction pattern of bright rings on a photographic film. X-ray Diffraction Slide 25 The Photoelectric Effect (1921 Nobel Prize) Illuminate cathode and monitor generated current as a function of applied voltage Slide 26 Photoelectric current vs. voltage when the cathode is illuminated with light of identical wavelength but different intensities ( I ). The saturation current is proportional to the light intensity Results: Photocurrent versus voltage & intensity Photocurrent Slide 27 The stopping voltage and therefore the maximum kinetic energy of the emitted electron increases with the frequency of light . Results: Photocurrent versus voltage & wavelength Photocurrent Slide 28 When an electron traverses a voltage difference V, its potential energy changed by eV. When a negative voltage is applied to the anode, the electron has to do work to get to this electrode This work comes from the electrons kinetic energy just after photoemission When the negative anode voltage V is equal to Vo, which just extinguishes the current I, the potential energy gained by the electron balances the kinetic energy lost by the electron eV o = 1 / 2 mv 2 =KE m Interpretation I: Slide 29 Since the magnitude of the saturation photocurrent depends on the light intensity, only the number of ejected electrons depends on the light intensity. Interpretation II: Photocurrent Slide 30 The effect of varying the frequency of light and the cathode material in the photoelectric experiment. The lines for the different materials have the same slope h but different intercepts Results: Kinetic energy & light frequency Slide 31 Photoelectric Effect Photoemitted electrons maximum KE is KE m Work function, F 0 The constant h is called Plancks constant. Slide 32 The PE of an electron inside the metal is lower than outside by an energy called the workfunction of the metal. Work must be done to remove the electron from the metal. First full interpretation: 1905, Einstein =hc/e o, where o is the longest wavelength for photoemission Slide 33 Light Intensity (Irradiance) Classical light intensity Light Intensity Photon flux (# photons crossing a unit area per unit time) Slide 34 X-ray image of an American one-cent coin captured using an x-ray a-Se HARP camera. The first image at the top left is obtained under extremely low exposure and the subsequent images are obtained with increasing exposure of approximately one order of magnitude between each image. The slight attenuation of the X-ray photons by Lincoln provides the image. The image sequence clearly shows the discrete nature of x-rays, and hence their description in terms of photons. SOURCE: Courtesy of Dylan Hunt and John Rowlands, Sunnybrook Hospital, University of Toronto. X-rays are photons Slide 35 Quantum Weirdness II: Youngs Double Slit Experiment, Revisited http://www.youtube.com/watch?v=DfPeprQ7oGc L d x What happens when we observe which slit the photon goes through?