1. EM-Matter Interactions

2. Interaction of EM with Matter
Incident photons of a given energy can cause molecular, electronic, or atomic interactions in target…
Microscopic view e.g. molecular and atomic interactions
Mechanisms are largely energy (e.g. frequency or wavelength) dependent
If a photon of a given energy interacts with matter, there will be an exchange of energy
electrons, molecules, nuclei will be put into motion
e.g. decay, excitation, rotation, vibration, displacement

3. Absorption
Absorption is the process by which EMR is absorbed and converted into other forms of energy. The absorption of the incident radiant energy may take place in the atmosphere or on the terrain.
Absorption occurs when an atom or molecule has a same frequency (resonant frequency) as the incident energy. The incident energy is transformed into heat motion and is then reradiated (emission) at a longer wavelength.
An absorption band is a range of  in the EM spectrum within which radiant energy is absorbed by a substance.
Some wavelengths of radiation are affected far more by absorption than by scattering. Especially in infrared and ultra-violet.
Absorption plays a very important role in remote sensing, such as Chlorophyll in vegetation absorbs blue and red light for photosynthetic purposes; water is an excellent absorber of energy; many minerals have unique absorption characteristics.

4. EM-Matter Interactions

5. Absorption, Exitation, and Emission

6. Absorption, Exitation, and Emission

7. Interaction of EM with Matter
Atomic and molecular systems exist in certain stationary states with well-defined energy levels.
For isolated atoms, levels are related to orbitals and spins of electrons.
For molecules, levels are related to rotational and vibrational states
The distribution of energy levels depends on the atomic/molecular structure of the material.
Valence electrons are important for UV and VNIR spectra
For crystalline solids, their crystalline nature will be important
For metals and semi-conductors, electron bands will be important

12. Absorption, Exitation, and Emission

18. Hydrogen

19. Absorption, Exitation, and Emission

23. Restrahlen effect:
If wide spectrum light is incident on a polished surface…
…reflected light will contain a large portion of spectral energy around the absorption bands.

24. Particulate surface:
the incident energy is multiply scattered…
… some of the energy penetrates some of the particles…
… if material has absorption band, reflected energy is depleted of energy
in that band.

27. Electronic Processes Crystal Field Effect Charge Transfer
Conjugate Bonds
Energy Bands
Fluorescence

28. Electronic Processes
Electrons in atoms can only occupy specific quantized orbitals with specific energies.
For individual atoms, most of these transitions do not occur in VNIR-SWIR-TIR.
Instead occur at higher energy (UV, etc.)
Indirect information available due to interactions between crystal and/or molecular structure of a material and the energy levels of an individual atom.
Electronic energy levels can be modified by virtue of the effects of surrounding atoms and their arrangement.

29. Crystal Field Effect
In isolated atoms, valence electrons are unpaired.
Valence electrons in molecules are primary reason for color!
In molecular solids, valence electrons of adjacent atoms form pair (e.g. covalent and ionic bonds).
When bonds form, absorption bands of valence electrons displaced to the UV.
For transition metals (Fe, Mg, Cr, Cu, Ni, Co), atoms have inner shell electrons that remain unpaired.
Excited states of these electrons are in the visible.
These states are strongly affected by electrostatic field surrounding atom.
Field is determined by the molecular or crystal structure of material.
So same atom can have different energy levels in different materials.

30. Crystal Field Effect
Example of ruby (corundum, Al2O3) containing Cr3+ in place of Al.
Cr has 3 unpaired electrons in lowest energy state 4A2.
Cr has complicated spectrum of excited states.
3 are in the visible (2E, 4T1, 4T2)
Not allowed for 4A2 to go directly to 2E…but 4A2 to 4T1 and 4T2 is allowed.
Energies of these allowed transitions correspond to violet and yellow-green…
… so white light passing through ruby comes out deep red due to absorption.
Electrons can return from 4T to 4A2 only through 2E… emission of infrared (4T to 2E) and red wavelengths (2E to 4A2).

32. Crystal Field Effect Emerald (beryl) also has Cr3+
Different crystal structure than ruby, so different crystal field affects Cr...
… result is lower energy for 4T states than in ruby…
… which shifts absorption…
… so emerald is green (and not red like ruby)!
Other minerals with similar behavior:
Aquamarine, jade, citrine quartz have Fe.
Azurite, turquoise, malachite have Cu.
Garnets have Fe.

33. Crystal Field Effect Fe2+ is very important in remote sensing…
Easily detected.
Present in a wide range of materials.
Fe2+ in perfectly octahedral site has one allowed transition in the near infrared…
… but if the octahedral site is distorted, result is splitting of energy level allowing additional transitions…
… and if Fe2+ is in different, non-equivalent sites (like in olivine), even more transitions can occur.

35. Crystal Field Effect Not limited to electrons in transition metals.
Electrons trapped in crystalographic defects (missing ions, raditation damage, impurities, etc.) can have similar transitions.
These are called color centers (F centers).
Example is fluorite (CaF2).
Calcium surrounded by 8 fluorines.
Can form F center when a fluorine is missing or displaced.
To keep neutrality, electron can be trapped in the fluorine site and bound by the crystal field.
This trapped charge can occupy ground and excited states…

36. Charge Transfer Electrons not always confined to a particular bond.
In some materials, electrons can travel longer distances, ranging throughout a molecule or even on a macroscopic scale.
Such electrons are not bound tightly, so the energy needed to put them in excited states is reduced.
Molecular orbitals (as opposed to atomic orbitals) such electrons can occupy are still quantized, etc.
Transfer of charge between molecular orbitals can lead to spectral features and intense colors.

37. Charge Transfer Fe2+ to Fe3+ charge transfer is an example
Give rise to dark colors (deep blue and black) in magnetite, etc.
Fe2+ to Ti4+ charge transfer in sapphire is another example
Electron transfer from Fe to Ti, giving both 3+ charge…
… 2 eV needed to do this…
… creates broad absorption band from yellow through red…
… creates deep blue color.

38. Conjugate Bonds
Biological pigments containing C, N joined by alternating single and double bonds (conjugate bonds or pi-orbitals).
Each such bond is a pair of shared electrons…
… electrons essentially move from the double bond to the single bond and back again.
Extended nature of electrons in these bonds diminishes energy needed to excite electrons…
… allows absorptions in visible.
Examples:
Red hemoglobin
Green chlorophyll
Organic dyes

39. Energy Bands
Electrons are least bound in metals and semi-conductors… not restricted to particular atoms or molecules, but can roam around a macroscopic volume in the material.
Metals
All valence electrons are equivalent…
… so energy levels that electrons can occupy are essentially continuous (e.g. nearly infinite number of states)…
… metals can absorb radiation at any wavelength.
Why, then, are metals not black?
Because energy levels are so close together in energy, as soon as photon is absorbed it can be re-emitted…
… gives metal shiny luster.
Variations in density of allowed states leads to different colored metals (gold, silver, titanium, cobalt, etc.)

40. Energy Bands
Electrons are least bound in metals and semi-conductors… not restricted to particular atoms or molecules, but can roam around a macroscopic volume in the material.
Semi-conductors
There is a splitting of energy levels into two broad bands with a forbidden gap in between.
Lower energy levels (valence band) are completely occupied.
Upper energy levels (conduction band) are relatively empty.
Width of gap determines spectral response of material (semiconductor cannot absorb photons with energy less than gap width).
Wider gap means more energy needed to make electron make the jump.
All photos with energy greater than gap are absorbed
Small gap…
… many VNIR wavelengths can be absorbed and remission happens easily (“metallic” Si).
Wide gap…
… fewer VNIR wavelengths can be absorbed and material may be colorless (Cdiamond; 5.4 eV, 0.23 microns).
Intermediate gap
… material with definite color (HgS, cinnebar; 2.1 eV, 0.59 microns).

41. Energy Bands
Semiconductors have a sharp transition “edge” in their spectra due to the sharp gap between valence and conduction bands.
Sharpness is function of purity, crystallinity, and grainsize…
… particulate materials have more of a slope.
Semiconductors can be engineered by doping them with impurities…
… allows intermediate energy levels to exist…
… shifts transition edge to other wavelengths…
… can create materials with specific spectral responses for use in detectors.
Example: Si doped with As
Doping makes gap with lower transition energy…
… longer wavelengths (into infrared, beyond normal cut-off of 1.1 microns for undoped Si) can be detected.

44. Fluorescence
Phenomenon of absorption at one wavelength and remission at a different wavelength (like in ruby and emerald).
Reason is that excited electrons sometimes MUST cascade down steps to ground state rather than transition directly nor via the same pathway that excitation took.
Problem is that in VNIR and SWIR (unlike the TIR), illumination source should swamp out any emitted light.
Can make use of Fraunhofer lines in solar spectrum…
Due to absorption in solar atmosphere.
0.01 to 0.1 microns wide.
Can be 90% darker than surrounding spectrum.
Method checks amount that these lines are “filled in” by fluorescence from the target.

47. Vibrational Processes
Small displacements of atoms from equilibrium positions.
Molecule with N atoms has 3N modes of motion because each atom has 3 degrees of freedom…
3 modes are translation
3 modes are rotation (2 only for linear molecule)
3N-6 modes are independent vibrations (3N-5 for linear molecules).

48. Vibrational Processes
Each mode of motion leads to a vibrational frequency i
Energy levels given by:
(ni +1/2)hi are the energy levels of a linear harmonic oscillator
i are vibrational quantum numbers
number and value of energy levels depends on molecular structure.

49. Vibrational Processes
Ground state is all ni = 0
Transition to state where only one ni = 1 is a fundamental tone.
Corresponding frequencies are 1, 2, …, i
Occur in far to mid-infrared (>3 microns)

50. Vibrational Processes
Transitions between ground and state where only one ni = 2 (or some multiple integer) are called overtone tones.
Corresponding frequencies are 21, 22, etc..
There are increasingly higher order overtones e.g. 31, etc.

51. Vibrational Processes
Other transitions are possible… combination tones.
Linear combinations of fundamental and overtone transitions
Corresponding frequencies are li + mj, where l and m are integers.
Features due to overtones and combination tones appear between 1 and 5 microns.

53. Vibrational Processes
Water in rocks and minerals appears in two bands
1.45 microns (due to 23)
1.9 microns (due to  + 3).
These are diagnostic of water.
Sharpness indicates where water is located:
Sharp feature… well-defined, ordered sites
Broad feature… unordered sites or several unequivalent sites.

54. Vibrational Processes
Fundamental modes for Si, Mg, Al and oxygen do not occur in VNIR-SWIR wavelengths.
They occur in TIR at 10 microns or longer.
Also do not see first overtones at 5 microns or higher order overtones in VNIR-SWIR.
VNIR and SWIR features that ARE seen are typically overtones and combinations involving very high fundamental frequencies such as OH- stretching.

56. Vibrational Processes
OH- is hydroxyl ion.
One stretching fundamental at about 2.77 microns.
Can vary depending on site where OH- is and what it is attached to.
Al-OH bending is at 2.2 microns
Mg-OH bending is at 2.3 microns
Sometimes stretching (and other) features are doubled….
Means OH- is in two different locations or attached to two different things.
First overtone of OH- (1.4 microns) is very common in VNIR-SWIR.

58. Vibrational Processes
Carbonate minerals
Similar features to water and hydroxl (but different fundamentals, of course)
microns
Combinations and overtones of internal vibrations of CO32- ion

60. Surface interactions depend on material properties
Spectra
Surface interactions depend on material properties
VNIR-SWIR: composition
TIR: temperature & composition
Microwave: electromagnetic properties & structure (not much compositional information)
Given sufficient resolution and satisfactory models, we can remotely measure reflectance spectra in the VNIR-SWIR and emissivity spectra in the TIR and compare them to known materials.

61. Reflectance

62. Multispectral Data

63. Reflectance Spectra

64. 3 bands make a false color image

65. Spectra
Spectrum: Distribution of all photon energies over the range of observed wavelengths

67. Vegetation, Water, and Soil
Water complications…
Sediment: brighter, slightly redder
Water depth: brighter
Algae: greener
Water surface topography…

71. Spectra and Spectral Vectors

72. Spectra

73. Spectra

75. Signatures of Geologic Materials in VNIR-SWIR
Result of electronic and vibrational transitions (see reading)
Absorption bands strongly affected by
Crystalline structure
Distribution of species in a material
Impurities
Water, hydroxyl, transition metals, carbonate, sulfur are important species

78. MAJOR MINERAL ABSORPTION FEATURE GROUPS
1 - SPECTRAL REFLECTANCE (VNIR + SWIR)
- FERRIC AND FERROUS IRON
- REE
- Al-OH
- Fe,Mg-OH
- HOH
2 - SPECTRAL EMITTANCE (TIR)
- SILICATE BENDING AND STRETCHING MODES
- CARBONATE
- SULFATE

79. COMPOSITIONAL INFORMATION
ASTER
SPECTRAL
REGION/
SPATIAL
RESOLUTION
BAND CENTER,
MICROMETERS
COMPOSITIONAL INFORMATION
VNIR /
15 m
SWIR /
30 m
TIR /
90 m B B B B B B B B B B B B B B
FERRIC AND FERROUS IRON
AND REE ABSORPTION
- AL-O-H IN CLAYS, MICAS,
SULFATE MINERALS
- CO 3
IN CARBONATES
- Mg-O-H IN AMPHIBOLES,
MICAS
- H-O-H IN EVAPORITES, CLAYS
- SILICATE MINERALS,
ESPECIALLY SHIFT TO
SHORTER WAVELENGTHS
- SULFATE MINERALS
- CARBONATE MINERALS

80. IMPORTANT SPECTRAL VARIATIONS WITHIN
CERTAIN ABSORPTION FEATURE GROUPS
1 - I/S WAVELENGTH SHIFT OF Al-OH FEATURES
- I/S ABSORPTION INTENSITY VARIATION
2 - CARBONATE WAVELENGTH SHIFT
3 - CHLORITE WAVELENGTH SHIFT OF Fe, Mg-OH
FEATURES
4 - ALUNITE WAVELENGTH SHIFT OF Al-OH FEATURES

81. Geologic Materials
Wide variety so difficult to uniquely identify a mineral or a rock with just a few spectral measurements (multispectral vs. hyperspectral)
Without hyperspectral data, multispectral analysis relies on
Band ratios and indicies (focus on particular features)
Classification/feature extraction
“Pretty picture” semiquantitative mapping (DCS)

84. Signatures of Biological Materials in VNIR-SWIR
Chlorophyll in vegetation leads to strong absorption in visible.
Water in the plant cells leads to strong reflection (index of refration effect) at microns (the “red edge”).
Reflectance between 1.3 and 2.5 microns is essentially that of water.

85. Biological Materials How do you study dynamic behavior of vegetation?
Look for variations in spectral signatures as function of
Health
Season
Moisture Content
Soil Contaminants
Biomass

87. Compare 0.8, 1.6, and 2.2 micron reflectance to gauge
moisture content. Why not look at 1.4 and 1.9 microns?

88. As vegetation grows, spectra become dominated by its
signature. Can gauge amount of biomass by looking
at microns and comparing it to 0.4 microns (NDVI).

89. Changes in growing cycle reflected in chlorophyll content lead
to changes in slope and position of the “red edge” at 0.7 microns

92. Depth of Penetration
Reflectivity of surfaces in VNIR-SWIR is governed by just the top few microns!
So weathering rinds, desert varnish, snow, vegetation, are all problematic.

94. Effect of Vegetation
Cover: We Need A
Digital Defoliant!