1. Lecture 8: Volume Interactions
Thursday, 31 January
Reading assignments
Major spectral features of minerals (p. xiii-xv), from Infrared ( mm) Spectra of Minerals, J W Salisbury et al., 1991 – pdf on the class website
LKC Ch 6, Satellites -- p. 392 – 431 p
Optional reference reading: Roger Clark’s tutorial on spectroscopy, on the class website

2. Spectral radiance (W/m2/nm/str)
Wavelength (nm)
Spectral radiance (W/m2/nm/str)
Spectral radiance (W/m2/nm/str)
Wavelength (nm)
Wavelength (nm)

3. Last lecture 1) reflection/refraction of light from surfaces
(surface interactions)
2) volume interactions
- resonance
- electronic interactions
- vibrational interactions
3) spectroscopy
- continuum vs. resonance bands
- spectral “mining”
- continuum analysis
4) spectra of common Earth-surface materials

4. Today 1) reflection/refraction of light from surfaces
(surface interactions)
2) volume interactions
- resonance
- electronic interactions
- vibrational interactions
3) spectroscopy
- continuum vs. resonance bands
- spectral “mining”
- continuum analysis
4) spectra of common Earth-surface materials

5. Interaction of Energy and Matter
What causes absorption features in spectra?
Three effects of radiant energy on matter:
Rotational absorption (gases)
Electronic absorption
Vibrational absorption

6. Rotational Processes
Photons striking free molecules can cause them to rotate. The rotational states are quantized, and therefore there are discrete photon energies that absorbed to cause the molecules to spin.
Rotational interactions are low-energy interactions and the absorption features are at long infrared wavelengths.

7. Electronic Processes Four types:
Isolated atoms and ions have discrete energy states. Absorption of photons of a specific wavelength causes a change from one energy state to a higher one. Emission of a photon occurs as a result of a change in an energy state to a lower one. When a photon is absorbed it is usually not emitted at the same wavelength. The difference is expressed as heat.
Four types:
Crystal Field Effects
Charge Transfer Absorptions
Conduction Bands
Color Centers

8. Electronic Processes Crystal Field Effects
The electronic energy levels of an isolated ion are usually split and displaced when located in a solid. Unfilled d orbitals are split by interaction with surrounding ions and assume new energy values. These new energy values (transitions between them and consequently their spectra) are primarily determined by the valence state of the ion (Fe 2+, Fe3+), coordination number, and site symmetry.

9. Electronic transitions, crystal field effects
Electronic transitions, crystal field effects

10. Electronic Processes Charge-Transfer Absorptions
Absorption bands can also be caused by charge transfers, or inter-element transitions where the absorption of a photon causes an electron to move between ions. The transition can also occur between the same metal in different valence states, such as between Fe2+ and Fe3+. Absorptions are typically strong. A common example is Fe-O band in the uv, causing iron oxides to be red.

11. Electronic transitions (Fe+2, Fe+3), charge transfer (Fe-O)Mg
Diopside Fe
(Mg,Fe)2SiO4
MgCaSi2O6
Electronic transitions
(Fe+2, Fe+3), charge transfer (Fe-O)
(Mg,Fe)SiO3

12. Electronic Processes Conduction Bands
In metals and some minerals, there are two energy levels in which electrons may reside: a higher level called the "conduction band," where electrons move freely throughout the lattice, and a lower energy region called the "valence band," where electrons are attached to individual atoms. The yellow color of gold and sulfur is caused by conduction-band absorption.
Gold
Sulfur
web.syr.edu/~iotz/Gallery.htm

13. Conduction band processes
HgS

14. Vibrational Processes
The bonds in a molecule or crystal lattice are like springs with attached weights: the whole system can vibrate. The frequency of vibration depends on the strength of each spring (the bond in a molecule) and their masses (the mass of each element in a molecule). For a molecule with N atoms, there are 3N-6 normal modes of vibrations called fundamentals. Each vibration can also occur at multiples of the original fundamental frequency (overtones) or involve different modes of vibrations (combinations).

15. vibrational interactions gases
Vibration-higher energy than rotation
Vibration - harmonic oscillators
stretching, bending
Molecular vibrations cause the multiple absorption bands
HCl
3.75 µm
3.26 µm
35Cl
37Cl

16. Vibrational - rotational modes combine to produce complex
spectra with sharp bands
Vibrational modes
produce simple spectra
Vibration in water molecules

17. X-OH vibrations in minerals: band position in mica shifts with composition
KAl2(AlSi3O10)(F,OH)2

18. Band position in carbonate minerals shifts with composition
MgCO3
CaCO3

19. Si-O bond vibrational resonance
QUARTZ O O
SiO2
Thermal infrared

20. Examples of mineral spectra
Fe2O3
α-FeO(OH)
KFe3(SO4)2(OH)6

21. Spectra of common Earth-surface materials

22. Next lecture: more on spectroscopy