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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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

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

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

Rotational ProcessesPhotons 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.

Electronic ProcessesIsolated 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

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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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

Electronic transitions (Fe+2, Fe+3), charge transfer (Fe-O)

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Diopside

(Mg,Fe)2SiO4

Mg

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MgCaSi2O6

(Mg,Fe)SiO3

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

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Conduction band processes

http

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Vibrational ProcessesThe 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).

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

Vibration in water molecules

Vibrational - rotational modescombine to produce complexspectra with sharp bands

1

2

3

Vibrational modes

produce simple spectra

X-OH vibrations in minerals: band position in mica shifts with composition

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Band position in carbonate minerals shifts with composition

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CaCO3

MgCO3

QUARTZ

Si-O bond vibrational resonance

O

O

O

O

Si

Thermal infrared

www.pitt.edu/.../Quartz/QuartzCrystal.jpg

SiO2

Examples of mineral spectra

http://www.news.cornell.edu/photos/jarosite300.jpg

Fe2O3

α-FeO(OH)

KFe3(SO4)2(OH)6

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

Next lecture: more on spectroscopy