Lecture 8: Volume Interactions
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Transcript of Lecture 8: Volume Interactions
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Lecture 8: Volume Interactions Thursday, 28 January 2010
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)
Reading
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500 1500 2000 25001000
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Wavelength (nm)
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Wavelength (nm)
Spe
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ianc
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/m2 /
nm/s
tr)
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Wavelength (nm)S
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adia
nce
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Spe
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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
Last lecture
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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
Today
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Interaction of Energy and Matter
What causes absorption features in spectra?
Three effects of radiant energy on matter:
1) Rotational absorption (gases)
2) Electronic absorption
3) Vibrational absorption
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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.
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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 EffectsCharge Transfer AbsorptionsConduction BandsColor Centers
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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.
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Ele
ctro
nic
tra
nsi
tion
s, c
ryst
al f
ield
eff
ects
http
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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
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Electronic transitions (Fe+2, Fe+3), charge transfer (Fe-O)
http
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Diopside
(Mg,Fe)2SiO4
Mg
Fe
MgCaSi2O6
(Mg,Fe)SiO3
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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.
www.egyptcollections.com web.syr.edu/~iotz/Gallery.htm
Gol
d
Sul
fur
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Conduction band processes
http
://w
ww
.mii
.org
/Min
eral
s/M
inpi
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Cin
naba
r.jp
g
HgS
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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).
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gases
Vibration-higher energy than rotationVibration - harmonic oscillators
stretching, bending
HCl
3.75 µm 3.26 µm
35Cl 37Cl
Molecular vibrations cause the multiple absorption bands
vibrational interactions
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Vibration in water molecules
Vibrational - rotational modescombine to produce complexspectra with sharp bands
1
2
3
Vibrational modes
produce simple spectra
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X-OH vibrations in minerals: band position in mica shifts with composition
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KAl2(AlSi3O10)(F,OH)2
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Band position in carbonate minerals shifts with composition
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CaCO3
MgCO3
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QUARTZ
Si-O bond vibrational resonance
O
O
O
O
Si
Thermal infrared
www.pitt.edu/.../Quartz/QuartzCrystal.jpg
SiO2
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Examples of mineral spectra
http://www.news.cornell.edu/photos/jarosite300.jpg
Fe2O3
α-FeO(OH)
KFe3(SO4)2(OH)6
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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
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Next lecture: more on spectroscopy