In the spectral (signature) curves we have encountered throughout this Tutorial, the parameter given on the ordinate is usually reflectance. In much of the UV, the Visible, and Near IR spectral regions, some of the radiation is absorbed and some reflected. It is the latter that is represented by these curves. But absorption determines how much of the incoming photon energy, at different wavelengths, is subtracted from the outgoing portion of this energy, that we measure as reflected radiation.
The causes of absorption are multifold and different ones may manifest in the same material. We now examine briefly each of the more common processes, which depend on electron behavior in an atom's electronic shells and on bond characteristics during interactions with photons (see review of energy level diagrams on Page I-4 (in the Introduction)). For some of the discussion we will use minerals as examples, since they have been much studied in regards to their behavior under absorption conditions. As a visual guideline to several of these processes, scan the chart below, and return to it as needed during the ensuing treatment. Note that the dark, black bars represent principal absorption bands in various mineral species.
Electronic processes involve absorption of photons with specific energies (hence, wavelengths) that cause an electron jump from a lower energy state to an electron shell at a higher energy state. If the electron returns to a lower state (lower energy shell), it may emit a photon. For shared electrons between atoms, the energy jump spreads over a range of values, producing "energy bands".
A common electronic movement results from Crystal Field disturbance. A crystal field describes the effects of perturbing the "d" orbital shells of a transition metal (Fe, Cr, Ni, Co, Ti, V, etc.), distributed within the atomic lattice of a crystal held together by ionic bonding. This metal cation interacts with the electric field imposed by surrounding anions (negatively charged) or dipolar groups (ligands). The charge symmetry is thus distorted.
Five d orbital shells, with different spatial configurations, are available for occupancy by electrons. These orbital shells have different energy levels (i.e., are split). For a jump to occur, the amount of energy is quantized, that is, requires a photon of a particular energy that equals the difference between the specific levels of the higher and lower states. An electron that jumps may remain for a short period in the higher state (metastable), producing a characteristic absorption, or it may revert rapidly to a ground or lower level. The wavelength of the incident electromagnetic radiation that caused the jump correlates with the difference in the orbital-shell energies. Crystal field theory is important in determining the color and magnetic properties of minerals and other substances containing transition elements.
Iron-bearing minerals, such as Pyrite (FeS2) and Magnetite (Fe3 O4), are telling examples of how transition metals influence their spectra. Many minerals contain divalent iron (Fe2+) and trivalent iron (Fe3+). Thus, the valence state, the ionic coordination number, the site symmetry, the ligand geometry, and other factors determine the permissible energy increments.
The influence of iron is evident in this next spectral plot, through parts of the Visible-Near-IR and Short-Wave-IR ranges of two pyroxenes. Diopside (CaMgSi2O6) contains almost no iron. Bronzite ([Mg,Fe]SiO3) has Fe but no Ca. The presence of Fe2+ causes two absorption bands, near 1 and 2 µm , to deepen and shift notably towards lower wavelengths.
A variant of this iron influence is Charge Transfer Absorption. Here, photo absorption brings about a relocation of an ion (commonly, of the transition elements) to another position in the ionic or ligand crystal structure. The result is a series of deep absorption bands, many occurring in the ultraviolet wavelength region and extending into the blue and green. The reds of Hematite (see plot on the previous page) result from this type of absorption, in which the spectrum now shows a strong reflectance peak around 0.75 µm , set apart by absorption on either side.
A third process uses Conduction Bands, where electrons move freely through the crystal lattice at a higher energy level (the conduction level) but tend to stay attached to their individual atoms at lower levels (Valence Bands). Photon excitation raises the level through this Band Gap. Not typical of metal-ion behavior, this process is characteristic of semiconductors.
Some minerals, including Fluorite (CaF2) show a range of characteristic colors, controlled by absorption at Color Centers. These centers can develop from crystal structure defects, or commonly, from impurities. Such anomalies cause absorption at certain wavelengths, leaving one or more reflectance peaks that combine to give the observed colors. Fluorite is normally purple to blue, but green, yellow, and brown varieties are possible. The phenomenon contributes to this mineral's electron jumps, in response to ultraviolet radiation, in which the return to the ground state gives off a color glow or fluorescence.
Separate from electronic processes are the vibrational processes. These involve the bonds in a lattice or a molecular compound. At certain energy levels, the atomic units held by bonds (usually covalent) are set into motion, either as a back and forth vibration and/or as a rotation. The frequencies that these molecules absorb depend on the strengths of the bonds and the masses of atoms or ions participating in the movement. Solids experience smaller vibrations than liquids or gases. Given a molecule composed of N atoms, there are 3N - 6 normal modes of vibration. Each of these constitutes a fundamental vibration. There can be additional vibrations at lower frequencies (fixed multiples of the fundamentals), which comprise overtones. These overtones normally are weaker.
The majority of vibrational absorptions occur in non-metallic materials and appear throughout the infrared (short and mid-wavelength ranges). They tend to produce rather diverse and complex spectra. Some spectral troughs come from fundamental vibrations, while others come from overtones. In some instances, two or more vibrational modes are possible at the same frequency(ies).
The figure below shows mainly vibrational absorptions for several silicate and sulphate (Gypsum and Alunite) non-metallic minerals. Some of the absorption features are due to water (H2O) and others to hydroxyl (OH) molecules bonded to iron (Fe) or aluminum (Al). Not shown are Calcite and Dolomite spectra that show multiple absorption troughs in the 2.0 to 3.0 µm interval.