Dear CC104 students:
All transparancies used in Lecture 9 are on the web at http://www.bu.edu/CORE. (Lectures 10, 13, and 14, in last year's versions, are currently under reconstruction for this year.) Please browse the website and give us feedback on how to improve it.... and what questions remain unclear.
Sincerely,
Larry Sulak, sulak@bu.edu

CC104 An Exploration of Particle Waves and Atoms
Outline of Lecture 2

Lecture 2: Atoms to Quarks: An Exploration of Particle Waves and Atoms

1. Wave/Particle Duality of Quanta (Electrons or Photons)

"Wave packets"-   localized waves whose amplitudes decrease from the central location
Solutions to Schroedinger's quantum mechanical wave equation

2. About Wave Diffraction

 most interesting property of waves: when 2 waves pass through each other effects add...interference

wave properties:
key to understanding "particle" behavior

3. Double Slit Experiment

 light source blocked by sheet of metal with 2 slits on screen behind 2 light waves formed (one from each hole) both light waves travel different distances, interfering with each other... creating an interference pattern repeat experiment using particle (electron) beam a similar interference pattern appears!!! ...all particles have wave properties.

4. Electrons diffracting through Double Slits

. . . electron diffraction experiment through multiple slits

a carbon crystal

5. Diffraction of Electrons by a Crystal

 atomic layers = slits for waves of comparable wavelength electron wave packets diffracted from atomic lattice 2 rings produced on scintillating screen (like 2 rainbows from diffraction grating) double the energy (E) of the electrons by doubling the accelerating voltage wavelength of particles halved l= h/E ring spacing (measuring l of electron) halved

1. electron particles behave like waves
(2.) can measure (A) wavelength or
(B) inner atomic spacing
6. The Wave Character of Particles

a primitive television picture tube: electrons bioled off of a hot filament a plate at positive 2500 volts attracts

and accelerates - electrons , some pass through a hole in the plate, coast to a fluorescent screen, making a bright spot where they hit

In hole place a thin window of polycrystal carbon: graphite with rows of atoms (multiple slits) at 3 A spacing

(1 Angstron=0.1 nanometers ~ size of an atom) . . . see model of graphite

7. Electron Waves Interfering from atomic gold "slits"

 real interference pattern from electrons scattering off gold foil:

solid matter particles are wave-like,

matter particles have frequency, wavelength,
they interfere with each other
 particle's wavelength not due to "jiggling" wave function phasing in and out, e.g. like electron(+ and -) magnetic (North & South) field of photon phasing from one sign to other as it propagates

8. Excitation and Deexcitation of Atom

(b) Photon arrives from right with proper energy
is absorbed by atom, "exciting" out the electron

(c) Excited electron in level 3 jumps to level 2
emitting photon of energy E3 - E2

9.  Hydrogen Atom Energy Levels

ionization of atom
first excited space
ground state energy

Energy   Quantum Level

Energies measured in electron volts,
the amount gained by one electron
traversing one volt potential drop

Bound states have negative energies... the binding energies

10. DeBroglie's Vision of Bohr's Atom

 electron wave packet... standing wave vibrating in "orbital" around a nucleus (4 wavelengths in above picture)
only integral numbers of wavelengths allowed
deBroglie's wavelength of a particle

l = h/p, or h/E for an electron

11. ... but photons are bosons... Stimulated Emission by many Excited Atoms ... the Laser

Excite most atoms of Helium-Neon gas mixture
with photons from a flash lamp

First atom emits a photon
stimulates billions of bosons to emit
simultaneously into same quantum state
...same direction, energy, etc.

laser = light amplication by stimulated emission of radiation

12. Periodic Chart Reflects Number of Outer Electrons Greatly Distorted Images!!!

The Shell model of Atoms
... see scanning tunneling microscope picture of atoms

13. Particle Spin and Statistics

Pauli Exclusion Principle:
no two identical fermions in the same state,
(same spin, color charge, angular momentum ...)
can exist in the same place at the same time

Fermions...The Matter Particles
electrons, nucleons...leptons and quarks
particles with internal angular momentum,
intrinsic spin = 1/2 integer
subject to Pauli exclusion...only 1 in each state

Bosons...The Force Particles
photons, gluons, gravitons...
integer intrinsic spin = 0, 1, 2
not subject to Pauli exclusion, can occupy same state

14.    ... but Atoms bind together... how?

NaCl Bond

... ionic bonds... so tight you can see through them \

look through NaCl crystalls

15 ... nature likes filled levels for stablity... why not share?

Covalent Bonds
the hydrogen molecule

graphite...linked hexagons in a 2-D structure
macroscopic slip planes

contrast with most tightly packed carbon structure (diamond)

see models

16. Bohr Hydrogen Atom (1913)
(not to scale!)

n = 1 lowest energy level; higher levels more closely spaced

each energy level specific electron energy

bright line emission of photon:

electron jumps down, from upper to lower unoccupied level
e.g. n = 3 to n = 2, the Hydrogen a famous red line
Absorption (dark) line:
photon kicks electron up from lower to higher unoccupied level photon is annihilated