Lecture 9: Spectroscopy Tuesday, 2 February 2010 Lecture 9: Spectroscopy Reading Ch 5.14 Ch 6.1-3 Ch 4.5

Discussion: 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

Spectra vary with composition Minerals Ices CaCO3 MgCO3 Be3Al2(SiO3)6 CaSO42(H2O) KAl(SO4)212H2O KFe+33(OH)6(SO4)2

Fig 2.21, Siegal & Gillespie For silica in TIR Thermal infrared Molecular vibration modes in silicates affect the thermal infrared Fig 2.21, Siegal & Gillespie For silica in TIR Thermal infrared silicates

Reflectance spectrum of SiO2 in the TIR QUARTZ: SiO2 The doubled peak is due to crystallographic asymmetry (hexagonal) in quartz The silica tetrahedron is distorted in quartz: the Si-O bond down the c-axis has a different length than it does across it

Phase affects spectra Ice – liquid transition for water Bands don’t broaden much as ice turns to water Band centers shift subtly Amount of absorption increases with optical length z in Beer’s law (e-kz) – there are no grain interfaces in water. This is a particle size affect Low water content Ice – liquid transition for water High water content

Particle size affects spectra Coarse particles – spectra dominated by absorption inside grains Fine particles – spectra dominated by surface reflection Low surface/volume ratio Average optical path is long High surface/volume ratio Path is shorter

Particle size affects spectra H2O Pyroxene XY(Si,Al)2O6

Spectral resolution: multispectral remote sensing vs. imaging spectroscopy KAl(SO4)212H2O Imaging spectroscopy is more likely to resolve absorption bands

Spatial resolution also affects spectra (by mixing) KAl(SO4)212H2O KFe+33(OH)6(SO4)2 Areal (checkerboard) mixing: additive Intimate mixing: “subtractive”

Intimate mixing can be highly non-linear Adding highly absorptive charcoal greatly reduces the optical path length (“z” in Beer’s Law: e-kz) A small amount has a large effect Larger amounts have diminishing effect

Spectroscopy considerations - continuum vs. resonance bands Absorption bands are measured relative to the “continuum” – the value of the spectrum if the absorption band was not present

Discussion: 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

Spectra of common Earth-surface materials SOIL Path length Clay H2O Fe-O Water absorption

Spectra of common Earth-surface materials Cellular scattering Green Vegetation Water absorption Chlorophyll absorption

Spectra of common Earth-surface materials Dry Vegetation Cellulose Cellular scattering Water absorption Chlorophyll absorption

Leaf structure and its relation to spectra Absorption band in red: chlorophyll pigment Reflective NIR: scattering in the prismatic leaf cells SWIR absorption: absorption by leaf water

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