1. 1 The Diffraction Phenomena in Atoms and Molecules

2. 2 Electromagnetic Radiation  Diffraction sin  = n /d  The Compton Effect p photon = h/ p photon = h/  The Photoelectric Effect E photon = h E photon = h Wave Phenomena Particle Phenomena

3. 3 Matter: Electrons, Atoms, Molecules The Problem:  Momentum p = mv  Kinetic Energy ? E k = mv 2 /2 Wave Phenomena Particle Phenomena

4. 4 The Phenomenon Electron Diffraction from a Crystal Davison-Gremer 1927 When a beam of electrons, accelerated in low tensions, hits a Ni crystal, it creates a diffraction pattern

5. 5 Electron and Molecule Diffraction from a Crystal A beam of particles (He, H 2 ) hitting a LiF crystal splits into several rays 20 o 15 o 10 o 5 o 0 o 5 o 10 o 15 o 20 o He 100 o K He 300 o K The Phenomenon Stern 1930

6. 6 Diffraction of Atoms and Molecules from a Crystal The crystal is a grating for diffraction: the distance between the atoms is of the same order of magnitude as the wavelength. The Model sin  = n /d  1/mv

7. 7 When the beam of particles is passed through a “chopper”, as the angular velocity of the “chopper” increases, the angle of distraction decreases. The Phenomenon The Velocity Effect

8. 8 Time of Flight: Particles moving in various velocities arrive at various times, and therefore encounter different opening angles. The Model

9. 9 Wave Packets Needed: a model, capable of describing diffraction phenomena while maintaining locality in time and space. The Model