1. Interactions of Electromagnetic Radiation
Photons of elm wave arrive at antenna and induce synchronized electric current
Electromagnetic wave guiding (?) photons
Int Elm Rad
Receiver
Radio Tower
W. Udo Schröder, 20018

2. Interactions with Electric and Magnetic Fields
Since 1900’s: (Maxwell) classical unified theory of continuous elm fields describing most macroscopic observations. Features incompatible with classical picture:
Planck’s radiation laws  elm energy is granular (“quantized”)
Photoelectric/Compton effects  photons are corpuscular field quanta (mass-less bosons), spatial correlations with fields but straight trajectories
Macroscopic absorption/emission patterns for elm radiation have superimposed/embedded fine structure, threshold behavior (heat capacity)
Characteristic material dependence of absorption/emission  discrete internal structure of microscopic systems (molecules, atoms, nuclei,..)
Important spectroscopic tool
Int Elm Rad
Task: Develop quantitative quantal theory of elm interactions with matter
Method: 1. Review classical electric and magnetic interactions with charged particles
2. Derive interaction Hamiltonians and associated wave functions/vectors 3. Use Correspondence Principle to quantize classical elm multipolar fields ,
characterize field quanta 4. Apply to spectroscopy
W. Udo Schröder, 20018

3. Continuous & Discrete Electromagnetic Spectra
Spectrum = Intensity vs. color (wave length). (Intensity = number of photons.) Bound charges (electrons in atoms or molecules, ions or protons in nuclei) emit and absorb electromagnetic waves (photons) with discrete (characteristic) energies. Free moving charges emit a Maxwell-Boltzmann type elm. spectrum reflecting spectrum of particle velocities.
Blackbody Sun
Looking at Sun through gas
l 
molecules
Spectrometer
l 
l 
Looking directly at gas
Looking directly at Sun

4. Characteristic Emission and Absorption Spectra
Sun
Unbound (free) electric charges such as electrons in a macroscopic, hot mass (“blackbody”) of ionized gas (e.g., Sun) emit and absorb electromagnetic spectra with continuous wavelengths (frequencies, photon energies). Bound (spatially confined) electric charges (e.g., electrons in atoms, molecules) emit and absorb discrete (“line”) energy (wavelength) spectra.
Ionization Energy
Energy transfer by photons in bound systems: Absorption or emission of light quanta occurs in (reversible) transitions between discrete energy levels. Characteristic spacing  spectr. ID
Also: Excitation in chemical rxns. DE DE
-DE
Ground State
|DE| = hn = hc/l
h = × 10-34 m2 kg / s Planck’s constant

5. Characteristic Emission And Absorption Spectra
Reversible transitions between discrete energy levels
Sun
Ionization Energy
Unbound electric charges such as electrons in a hot body (“blackbody”) of ionized gas (e.g., Sun) emit and absorb electromagnetic spectra with continuous wavelengths (frequencies, photon energies). Bound electric charges (e.g., electrons in atoms, molecules) emit and absorb discrete (“line”) energy (wavelength) spectra. DE
Energy transfer by photons in bound systems: Absorption or emission of light occurs in transitions between discrete energy levels. Characteristic spacing  spectr. ID
Also: Excitation in chemical rxns. DE
Ground State
Ground State
-DE
Characteristic
Characteristic
|DE| = hn = hc/l
h = × 10-34 m2 kg / s Planck’s constant

6. Electric and Magnetic Fields
Example: RF power driven electric or magnetic dipoles broadcast t-dependent fields. AND: Irradiation of properly designed antennas with fields induce RF power in the antennas.
Task: Devise quantitative microscopic theory to describe/predict spontaneous and induced elm transitions between states of molecular, atomic, nuclear,.. systems coupled to elm field. Field character can be essentially electric or essentially magnetic. (Both components are always present in t-dependent fields.)
Int Elm Rad
W. Udo Schröder, 20018

7. Electric and Magnetic Fields
Strategy
Recall classical electric/magnetic multipoles 2ℓ
Quantize classical elm field for oscillating electric or magnetic 2ℓ poles (ℓ=1 dipole, ℓ=2 quadrupole, ℓ=3 octupole,….  Quantum wave function (non-rel.)
Derive interaction Hamiltonian 
Use perturbation theory to calculate rates for transitions between unperturbed system states.
2nd quantization of elm fields in terms of field quanta
Int Elm Rad
W. Udo Schröder, 20018

8. Symmetry Concept: Multipolar Electric and Magnetic Fields
Electric multipole moment
Magnetic multipole moment
Partial integration 
El. multipole moment unit = e×(length)ℓ , magn. multipole moment unit = µB×(length)ℓ−1,
e = elementary charge, µB = Bohr magneton. ∇ · B = 0  there are no magn. monopoles. Concept of magn. monopole useful in describing magn. features of composite objects
Int Elm Rad
Useful representation of spherical harmonics:
zero or positive integers p, q, and s, with p + q + s = ℓ and p − q = m.
W. Udo Schröder, 20018

9. Parity: A Quantal Symmetry
For spatially symmetric charge distribution
Hamiltonian is invariant against space inversion y x x
n even  p = +1 n odd  p = -1
Int Elm Rad
For “good” parity no permanent odd el moment
W. Udo Schröder, 20018