Lecture 02

# Lecture 02 - EEE 434/591Quantum Mechanics L2:1 David K....

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EEE 434 Quantum Mechanics http://www.eas.asu.edu/~ferry/EEE434.htm L2:1 EEE 434/591—Quantum Mechanics David K. Ferry Regents’ Professor Arizona State University Munich, Germany

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EEE 434 Quantum Mechanics http://www.eas.asu.edu/~ferry/EEE434.htm L2:2 Last time, we got to a point just over 25 years after Planck gave us the idea a quantum nature for light. In this time, the particle nature of light has been confirmed, and the wave picture of electrons has been proposed and confirmed. If this isn’t confusing enough, then an electron making a transition in the Bohr atom, really needs to know what shell it wants to jump to know how much energy to emit; what are these jumps and is this a requirement for cognition? Some thoughts from future players: “Physics at the moment is again very muddled; in any case, for me it is too complicated, and I wish I were a film comedian or something of that sort and had never heard about physics.” Wolfgang Pauli “If all this damned jumping were here to stay, I should be sorry I ever got involved with quantum theory.” Erwin Schrödinger
EEE 434 Quantum Mechanics http://www.eas.asu.edu/~ferry/EEE434.htm L2:3 In the Bohr model of the atom, each shell held a fixed number of electrons. In the lowest shell, there was only 1 state designated by n = 1. But, it held two electrons ! In the second shell, n = 2, one could have a state of no angular momentum ( k = 0 in Sommerfeld’s notation), or you could have a state with angular momentum ( k = 1), in which this angular momentum could point in any of three directions (roughly speaking). Hence, there were 4 states, but this shell held 8 electrons . Where did this factor of 2 come from?

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EEE 434 Quantum Mechanics http://www.eas.asu.edu/~ferry/EEE434.htm L2:4 Wolfgang Pauli was a brilliant young Austrian, who studied in Munich with Sommerfeld. In 1925, he was an assistant in Copenhagen with Bohr, and insisted that there must be another quantum number for the atom. Not just 3 ( n , k , and one designated in which which direction the angular momentum pointed). This 4 th quantum number would designate the handedness (left or right). Then, he gave us the exclusion principle , which said that each quantum state, designated by its 4 quantum numbers could hold only 1 electron, and this explained how the shells were filled. Pauli received the Nobel prize for the exclusion principle only in 1945 (twenty years later). But, what was this handedness ?
EEE 434 Quantum Mechanics http://www.eas.asu.edu/~ferry/EEE434.htm L2:5 Ralph Kronig was born in Dresden to American parents. In 1925, while finishing his doctorate at Columbia, he listened to Pauli give a lecture, and suggested that this handedness could be the electron spinning upon its own axis. Pauli dismissed the idea, telling Kronig that "it is indeed very clever but of course has nothing to do with reality.” Kronig quit working on the idea and did not publish this suggestion!

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## This note was uploaded on 10/03/2010 for the course EEE 434 taught by Professor Roedel during the Fall '08 term at ASU.

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Lecture 02 - EEE 434/591Quantum Mechanics L2:1 David K....

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