Major Concepts in Physics Lecture 19.

1. Major Concepts in Physics Lecture 19. Prof Simon Catterall Office 309 Physics, x 5978 smc@physics.syr.edu http://physics/courses/PHY102.08Spring PHY102

2. Announcements • Exam 3 – Monday April 14 in class • Material: • everything since exam 2. eg temperature, heat, work, laws of thermodynamics. • Photon. Photoelectric effect. Wave-particle duality. Electron volts. Bohr model of H atom – basic ideas. • Electron as a wave. Diffraction, interference. Uncertainty principle. Pauli exclusion principle Lasers, chemical bonding PHY102

3. Recap • At atomic level classical distinctions of wave and particle blur • Light as photons E=hf, electron as wave l=h/p • Uncertainty principle DxDp>h important for reconciling these differing concepts • Understand discrete energy levels/spectra using ideas of wave theory • Wave function tells us where the particle is most likely to be found – quantum mechanics predicts not definite outcomes but only probabilties …. PHY102

4. Iron atoms confine electron (waves) PHY102

5. Java simulations of quantum systems • Electron in potential well. Represented by spread out wave packet which oscillates back and forth • Motion of wave packet determined by the Schrodinger equation • www.falstad.com/mathphysics.html PHY102

6. Atomic structure • For a H atom at low temperature electron occupies lowest allowed energy state – ground state • To completely remove the electron from the atom requires at least 13.6 eV energy – called ionization energy • What is the ground state structure of next lightest atom helium – 2 electrons ? PHY102

7. Helium – energy level diagram E=0 energy electron n=1 quantum number ground state PHY102

8. Pauli exclusion principle • Experiment tells us that the lowest energy state of a multi-electron atom does not consist of all electrons inhabiting the lowest energy level • In fact each new electron must occupy a new energy level – no two electrons can exist in the same state Pauli Exclusion Principle PHY102

9. General atomic structure • Fill up energy levels one electron at a time • Typically levels with larger E correspond to electron states that are further from nucleus • These outer electrons can easily be excited or transferred to other atoms • Responsible for chemical properties of that particular atom • Allow us to understand the periodic table PHY102

10. Absorption / Emission of Photons • Photon energy Ephoton = Efinal - Einitial • The frequency f of the emitted photon is determined by Ephoton = hf PHY102

11. Fig. 28.23 PHY102

12. Three types of photon-electron interaction • Absorption – photon with correct energy is absorbed • Spontaneous emission. Electron drops to lower available energy state emitting photon whose energy is difference • Stimulated emission. Presence of a photon can encourage electron to drop – producing an identical photon PHY102

13. Lasers • If cascade of stimulated emissions can take place – obtain a large number of photons with same energy, direction of propagation and are in phase • Chain reaction – each new emission increases the number of photons which in turn stimulates further photon emission • Basis of laser … PHY102

14. How it works • Need stimulated emission to be more likely than absorption: more of the atoms must be in an excited state not the ground state. • Called population inversion • Need a long lived excited state – metastable state. • If atoms can be ``pumped’’ up to metastable state fast enough a population inversion can occur. PHY102

15. Ruby laser • Uses optical pumping: incident light of correct wavelength is absorbed causing atoms to make transitions to short-lived excited state. • This state then decays to a metastable state. PHY102

16. Fig. 28.24 PHY102

17. How it works II • Ruby rod has ends polished and silvered to become mirrors • High intensity flash lamp wraps rod. • Spontaneous emission from metastable state Leads to cascade of stimulated emissions • Only photons bouncing back and forth along ends participate • Some escape from one end – laser beam PHY102

18. Helium-neon laser • Gas discharge tube contains low pressure mixture of helium and neon. • Electrically pumped – discharge excites helium atoms to metastable state. • Collisions of helium with neon populate a similar state in neon – decays by stimulated emission yielding a 2 eV photon. PHY102

19. Fig. 28.25 PHY102

20. Semiconductor lasers • Small, inexpensive and efficient. • Use in CD/DVD players, bar code readers and laser pointers • Electrically pumped – electrons are promoted to a higher state through passage of electrical current. Decay back to ground state (band) via stimulated emission. • Wavelength depends on energy (band) gap. PHY102

21. Problems. An electron and a neutron have the same de Broglie wavelength. Which is true ? • A:The electron has more kinetic energy and a higher speed • B: The electron has less kinetic energy but a higher speed. • C: electron and neutron have same kinetic energy but electron has higher speed. • D: electron has less kinetic energy and less speed. PHY102

22. A bullet is fired from a rifle. The end of barrel is a circular aperture. Is diffraction a measureable effect? • A: No, because only charged particles have a de Brogle wavelength • B: No, because a circular aperture never causes diffraction • C: No, because the de Broglie wavelength is too large • D: No, because the de Brogle wavelength is too small PHY102

23. What is the minimum kinetic energy for an electron confined to a region the size of an atomic nucleus (1.0 fm) ? PHY102

24. What is the wavelength of the light usually emitted by a helium-neon laser. PHY102