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

2. 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

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

4. 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

5. 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

6. 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

7. 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

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

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

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

12. 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

13. 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

14. 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

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

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

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

18. 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

19. Next Class:
Satellites & orbits
Review for Midterm