Lecture 8: Volume Interactions

1. Thursday, 28 January 2010 Lecture 8: Volume Interactions Reading Ch 1.8 http://speclib.jpl.nasa.gov/ Major spectral features of minerals (p. xiii-xv), from Infrared (2.1-25 mm) Spectra of Minerals, J W Salisbury et al., 1991 – ( class website) Optional reference reading: Roger Clark’s tutorial on spectroscopy (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 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. http://img.alibaba.com/photo/50502613/Europium_Oxide.jpg 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. http://en.wikipedia.org/wiki/Image:Hematite.jpg http://www.galleries.com/minerals/silicate/olivine/olivine.jpg

11. Mg Diopside Fe (Mg,Fe)2SiO4 http://www.gemstone.org/images/01/Stones_Diopside.jpg 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. Sulfur Gold www.egyptcollections.com web.syr.edu/~iotz/Gallery.htm

13. Conduction band processes http://www.mii.org/Minerals/Minpics1/Cinnabar.jpg 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. 35Cl 37Cl HCl 3.75 µm 3.26 µm vibrational interactions gases Vibration-higher energy than rotation Vibration - harmonic oscillators stretching, bending Molecular vibrations cause the multiple absorption bands

16. n1 n3 n2 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 www.pitt.edu/.../1IgneousMineralz/Micas.html KAl2(AlSi3O10)(F,OH)2

18. Band position in carbonate minerals shifts with composition MgCO3 CaCO3 www.galleries.com/.../calcite/calcite.htm

19. Si-O bond vibrational resonance O O Si QUARTZ O O SiO2 www.pitt.edu/.../Quartz/QuartzCrystal.jpg Thermal infrared

20. Examples of mineral spectra Fe2O3 α-FeO(OH) KFe3(SO4)2(OH)6 http://www.news.cornell.edu/photos/jarosite300.jpg

21. Spectra of common Earth-surface materials www.gfmer.ch/.../Papaver_somniferum.htm www.oznet.ksu.edu/fieldday/kids/soil_pit/soil.htm www.bigwhiteguy.com/photos/images/814.jpg

22. Next lecture: more on spectroscopy