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 |
(2.) can measure (A) wavelength or6. The Wave Character of Particles
(B) inner atomic spacing
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) |
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 lineAbsorption (dark) line:
photon kicks electron up from lower to higher unoccupied level photon is annihilated