2 Molecular absorption processes ~10-18 JElectronic transitionsUV and visible wavelengthsMolecular vibrationsThermal infrared wavelengthsMolecular rotationsMicrowave and far-IR wavelengthsEach of these processes is quantizedTranslational kinetic energy of molecules is unquantizedIncreasing energy~10-23 J
3 Atomic and molecular vibrations correspond to excited energy levels in quantum mechanics Energy levels are everything in quantum mechanics.Excited levelDE = hnEnergyGround levelThe atom is vibrating at frequency, ν.The atom is at least partially in an excited state.For a given frequency of radiation, there is only one value of quantum energy for the photons of that radiationTransitions between energy levels occur by absorption, emission and stimulated emission of photons
4 Excited atoms emit photons spontaneously When an atom in an excited state falls to a lower energy level, it emits a photon of light.Excited levelEnergyGround levelMolecules typically remain excited for no longer than a few nanoseconds. This is often also called fluorescence or, when it takes longer, phosphorescence.
5 Absorption spectra of molecules Hypothetical molecule with three allowed energy levelsNote relationship to emission!νij = ΔEij/hallowed transitionspositions of the absorption lines in the spectrum of the moleculeLine positions are determined by the energy changes of allowed transitionsLine strengths are determined by the fraction of molecules that are in a particular initial state required for a transitionMultiple degenerate transitions with the same energy may combine
6 FluorescenceFluorescent lighting exploits this phenomenon: certain phosphors emit visible light when bombarded with UV light. Much more efficient than incandescent lighting.Also whitening agents in detergents...
7 Atoms and molecules can also absorb photons, making a transition from a lower level to a more excited oneExcited levelThis photon has been absorbedEnergyGround level
8 In 1916, Einstein showed that another process, stimulated emission, can occur Before AfterSpontaneous emissionAbsorptionStimulated emission
10 Interaction of radiation with matter WavelengthIf there are no available quantized energy levels matching the quantum energy of the incident radiation, then the material will be transparent to that radiation
11 X-ray interactionsQuantum energies of x-ray photons are too high to be absorbed by electronic transitions in most atoms - only possible result is complete removal of an electron from an atomHence all x-rays are ionizing radiationIf all the x-ray energy is given to an electron, it is called photoionizationIf part of the energy is given to an electron and the remainder to a lower energy photon, it is called Compton scattering
12 Ultraviolet interactions Near UV radiation (just shorter than visible wavelengths) is absorbed very strongly in the surface layer of the skin by electron transitionsAt higher energies, ionization energies for many molecules are reached and the more dangerous photoionization processes occurSunburn is primarily an effect of UV radiation, and ionization produces the risk of skin cancer
13 UV SO2 and O3 absorption spectra Absorption cross-section represents the probability that a photon will be absorbed by a molecule of gas. TOMS Wavelengths not perfectly placed for SO2 as it is an Ozone instrument. Wavelengths are UV, Huggins bands. Spectral resolution affects our ability to resolve SO2 band structure and hence SO2 sensitivity and noise.
14 Ultraviolet interactions UV-A: nmUV-B: nmUV-C: nmThe ozone layer in the upper atmosphere absorbs most harmful UV radiation before it reaches the surfaceHigher UV-B frequencies are ionizing radiation and can produce harmful effects such as skin cancerThe ionosphere is a region of the upper atmosphere ionized by solar radiation
15 Visible light interactions Visible light is also absorbed by electron transitionsHigher energies at blue wavelengths relative to red wavelengths: hence red light is less strongly absorbed than blue lightAbsorption of visible light causes heating, but not ionizationCar windshields transmit visible light but absorb higher UV frequencies
16 Infrared (IR) interactions Quantum energy of IR photons ( eV) matches the ranges of energies separating quantum states of molecular vibrationsVibrations arise as molecular bonds are not rigid but behave like springs
17 Microwave interactions Quantum energy of microwave photons ( eV) matches the ranges of energies separating quantum states of molecular rotations and torsionNote that rotational motion of molecules is quantized, like electronic and vibrational transitions associated absorption/emission linesAbsorption of microwave radiation causes heating due to increased molecular rotational activityMost matter transparent to µ-waves, microwave ovens use high intensity µ-waves to heat material
18 Molecular dipole moments For a molecule to absorb IR radiation it must undergo a net change in dipole moment as a result of vibrational or rotational motion.The electric dipole moment for a pair of opposite charges of magnitude q is the magnitude of the charge times the distance between them, with direction towards the positive charge.The total charge on a molecule is zero, but the nature of chemical bonds is such that positive and negative charges do not completely overlap in most molecules. Such molecules are said to be polar because they possess a permanent electric dipole moment.Water is a good example of a polar molecule:Molecules with mirror symmetry like oxygen, nitrogen and carbon dioxide have no permanent dipole moments.
19 Molecular polarizability The polarizability of an atom or a molecule is a measure of the ease with which the electrons and nuclei can be displaced from their average positions (e.g., by an external electric field)When the electrons occupy a large volume of space, e.g., in an atom or molecule with many electrons, the polarizability of the substance is large. When an atom or molecule has large polarizability the magnitude of the instantaneous dipole can be large.The polarizability of molecules is important in Raman spectroscopy, based on Raman scattering.
20 Key atmospheric constituents Diatomic, homonuclear molecules (e.g., N2, O2) have no permanent electric dipole moment (also CO2)Molecular N2, the most abundant atmospheric constituent, has no rotational absorption spectrumOxygen (O2) has rotational absorption bands at 60 and 118 GHzLinear and spherical top molecules have the fewest distinct modes of rotation, and hence the simplest absorption spectraAsymmetric top molecules have the richest set of possible transitions, and the most complex spectraNote lack of permanent electric dipole moment in CO2 and CH4No
21 Vibration modes of simple molecules Fundamental or normal modesSymmetric stretchBend (Scissoring)Asymmetric stretchA normal mode is IR-active if the dipole moment changes during mode motion.Overtones, combinations and differences of fundamental vibrations are also possible (e.g., 2v1, v1+v3 etc.)A non-linear molecule of N atoms has 3N-6 normal modes of vibration; a linear molecule has 3N-5.
22 Absorption frequency for a diatomic molecule m1, m2 = atomic mass of vibrating atomsc = speed of light [3×108 m s-1]V = wavenumber [cm-1]Av = Avogadro’s number [6.023×1023 atoms mole-1]k = force constant (bond strength) [dynes cm-1]For a single bond, k = 5×105 dynes cm-1For a double bond, k = 10×105 dynes cm-1For a triple bond, k = 15×105 dynes cm-1
23 Infrared (IR) interactions Vibrational transitions are associated with larger energies than ‘pure’ rotational transitions.Vibrations can be subdivided into two classes, depending on whether the bond length or angle is changing:Stretching (symmetric and asymmetric)Bending (scissoring, rocking, wagging and twisting)Stretching frequencies are higher than corresponding bending frequencies (it is easier to bend a bond than to stretch or compress it) Bonds to hydrogen have higher stretching frequencies than those to heavier atoms. Triple bonds have higher stretching frequencies than corresponding double bonds, which in turn have higher frequencies than single bonds
25 Absorption spectra of molecules V = Vibrational quantum numberJ = Rotational quantum numberElectronic, vibrational and rotational energy levels are superimposedThe absorption spectrum of a molecule is determined by all allowed transitions between pairs of energy levels, and whether the molecule exhibits a sufficiently strong electric or magnetic dipole moment (permanent or otherwise) to interact with the radiation field
26 Vibrational-rotational transitions P branch (ΔJ = -1)Q branch (ΔJ = 0)(pure vibration)R branch (ΔJ = +1)Relative positions of transitions in the absorption spectrum of a molecule
27 Rotational absorption spectrum Photon frequency associated with a rotational transition
28 Hydrogen chloride (HCl) spectrum Q branch (ΔJ = 0)P branchR branchVibrational-rotational absorption spectrum of HCl: shows affect of two chlorine isotopes with slightly different mass
32 Absorption line shapes Doppler broadening: random translational motions of individual molecules in any gas leads to Doppler shift of absorption and emission wavelengths (important in upper atmosphere)Pressure broadening: collisions between molecules randomly disrupt natural transitions between energy states, so that absorption and emission occur at wavelengths that deviate from the natural line position (important in troposphere and lower stratosphere)Line broadening closes gaps between closely spaced absorption lines, so that the atmosphere becomes opaque over a continuous wavelength range.
33 Pressure broadeningAbsorption coefficient of O2 in the microwave band near 60 GHz at two different pressures. Pressure broadening at 1000 mb obliterates the absorption line structure.
34 Rovibrational EnergyVibrational and rotational transitions usually occur simultaneously splitting up vibrational absorption lines into a family of closely spaced linesRotational energy also dependent on direction of oscillation of dipole moment relative to axis of symmetryWhen oscillates parallel, ΔJ = 0 transition is forbidden and only P and R branches are seenWhen oscillates perpendicular, P, Q and R branches are all seenThe rotational constant is not the same in different vibrational states due to a slight change in bond-length, and so rotational lines are not evenly spaced in a vibrational bandRovibrational transitions in a CO2 moleculeDiagram taken from Patel (1968)
37 Water vapor (H2O) Most important IR absorber Asymmetric top → Nonlinear, triatomic molecule has complex line structure, no simple pattern3 vibrational fundamental modesHigher order vibrational transitions (Δv >1) give weak absorption bands at shorter wavelengths in the shortwave bands2H isotope (0.03% in atmosphere) and 18O (0.2%) adds new (weak) lines to vibrational spectrum3 rotational modes (J1, J2, J3)Overtones and combinations of rotational and vibrational transitions lead to several more weak absorption bands in the NIRooHHsymmetric stretchv1 = 2.74 μmbendv2 = 6.25 μmasymmetric stretchv3 = 2.66 μm
40 Absorption Spectrum of H2O v1=2.74 μmv2=6.25 μmv3=2.66 μm
41 Carbon dioxide (CO2)Linear → no permanent dipole moment, no pure rotational spectrumFundamental modes:The v3 vibration is a parallel band (dipole moment oscillates parallel to symmetric axis), transition ΔJ = 0 is forbidden, no Q branch, greater total intensity than v2 fundamentalThe v2 vibration is perpendicular band, has P, Q, and R branchThe v3 fundamental is the strongest vibrational band, but the v2 fundamental is most effective due to “matching” of vibrational frequencies with terrestrial Planck emission function13C isotope (1% of C in atmosphere) and 17/18O isotope (0.2%) cause a weak splitting of rotational and vibrational lines in the CO2 spectrumocosymmetric stretchv1 = 7.5 μm => IR inactiveasymmetric stretchv3 = 4.3 μmbendv2 = 15 μmbend v2
43 Which is the most potent greenhouse gas? Top: SF6 (sulfur hexafluoride); global warming potential ~23000 times that of CO2 over a 100 year period. SF6 is extremely long-lived as it is inert (lifetime of years)
44 Ozone (O3)Ozone is primarily present in the stratosphere except anthropogenic ozone pollution which exists in the troposphereAsymmetric top → similar absorption spectrum to H2O due to similar configuration (nonlinear, triatomic)Strong rotational spectrum of random spaced linesFundamental vibrational modes14.3 μm band masked by CO2 15 μm bandStrong v3 band and moderately strong v1 band are close in frequency, often seen as one band at 9.6 μm9.6 μm band sits in middle of 8-12 μm H2O window and near peak of terrestrial Planck functionStrong 4.7 μm band but near edge of Planck functionsoooosymmetric stretchv1 = 9.01 μmbendv2 = 14.3 μmasymmetric stretchv3 = 9.6 μm
46 Methane (CH4) Spherical top 5 atoms, 3(5) – 6 = 9 fundamental modes of vibrationDue to symmetry of molecule, 5 modes are degenerate, only v3 and v4 fundamentals are IR activeNo permanent dipole moment => No pure rotational spectrumFundamental modesHCCCCHHHv4 = 7.7 µmv1v2v3 = 3.3 µm
47 IR Absorption Spectrum of CH4 v3v47.6 µm band in otherwise largely transparent part of atmosphereMethane concentrations also directly/indirectly affected by human activities
48 Nitrous oxide (N2O)Linear, asymmetric molecule (has permanent dipole moment)Has rotational spectrum and 3 fundamentalsAbsorption band at 7.8 μm broadens and strengthens methane’s 7.6 μm band.4.5 μm band less significant as it is at the edge of the Planck function.Fundamental modes:ONNsymmetric stretchv1 = 7.8 μmasymmetric stretchv3 = 4.5 μmbendv2 = 17.0 μmbend v2
49 IR Absorption Spectrum of N2O v3=4.5 µmv1=7.8 µmv2=17 µm
51 Mineral and rock reflectance spectra Electronic transitions in solids; Fe2+ (iron) particularly important in remote sensing – minerals contain Fe2+ ionsFundamental vibrational modes of H2O: 2.74 µm, 6.25µm, 2.66 µmIn rock spectra, whenever water is present we see 2 absorption bands in near-IR spectra – one near 1.45 µm (2ν3 overtone) and one near 1.9 µm (v2+v3 combination). Sharpness of bands relates to sites in crystal structure occupied by the water molecules.Note that penetration depth into natural surfaces is usually restricted to the upper few microns. Consequences?
53 Why are most plants green and then red or yellow in the fall? Chlorophyll absorbs in the red and blue, and hence reflects in the green.Its absorption spectrum is due to electronic transitionsDuring spring and summer, leaves get their green cast from chlorophyll, the pigment that plays a major role in capturing sunlight. But the leaves also contain other pigments whose colors are masked during the growing season. In autumn, trees break down their chlorophyll and draw some of the components back into their tissues. Conventional wisdom regards autumn colors as the product of the remaining pigments, which were finally unmasked. In other words, autumn leaves were a tree's gray hair.But in recent years, scientists have recognized that autumn colors probably play an important role in the life of many trees. Yellow leaves get their color from a class of pigments called carotenoids. Another group of molecules, anthocyanins, produce oranges and reds. Trees need energy to make carotenoids and anthocyanins, but they cannot reclaim that energy because the pigments stay in a leaf when it dies. If the pigments did not help the tree survive, they would be a waste. What's more, leaves actually start producing a lot of new anthocyanin when autumn arrives."The reds are not unmasked-they are made in autumn," said Dr. David Lee, a botanist at Florida International University.Evolutionary biologists and plant physiologists offer two different explanations for why natural selection has made autumn colors so widespread, despite their cost. Dr. William Hamilton, an evolutionary biologist at Oxford University, proposed that bright autumn leaves contain a message: they warn insects to leave them alone.Dr. Hamilton's "leaf signal" hypothesis grew out of earlier work he had done on the extravagant plumage of birds. He proposed it served as an advertisement from males to females, indicating they had desirable genes. As females evolved a preference for those displays, males evolved more extravagant feathers as they competed for mates.In the case of trees, Dr. Hamilton proposed that the visual message was sent to insects. In the fall, aphids and other insects choose trees where they will lay their eggs. When the eggs hatch the next spring, the larvae feed on the tree, often with devastating results. A tree can ward off these pests with poisons.Photo and notes text borrowed from the NY Times.Plot borrowed from Peter v. Sengbusch -Comment from Ron Fox regarding the missing green absorption of chlorophyll:First off for the visual system, rod cells have an absorption max at 510 nm, i.e in the green. The three cone cells have maxima at 445, 545 and 585. All peaks are broad but the one at 545 is called the green cone. Thus the visual system has evolved to use what the plants don't use. Blue-green algae and red algae have pigments that do use the green ( ) missed by chlorophyll. The solar spectrum today at sea level is monotonically decreasing from a high at 700 to a low at 400, as far as the visible is concerned. The green part is less than the red part but by only a bit. A younger sun that was cooler would still be strong in the red but less so in the green. However, there is debate about the young sun's surface temperature. Chlorophyll a and chlorophyll b have complementary absorptions in separate portions of the red, suggesting that evolution tried to do as well as possible with the red.In the fall, trees produce carotenoids, which reflect yellow, and anthocyanins, which reflect orange and red.
54 Why Mars looks redIron oxides prevalent in Martian soil show increased reflectance at the red end of the visible spectrum.
55 WTC dust spectra Chrysotile standard Spectra of World Trade Center dustGypsum (water-bearing) = pulverized wallboardChrysotile = asbestos (fireproof coatings?)OH- (hydroxyl ion) – has first overtone at ~1.4 µm – most common feature present in near-IR spectra of terrestrial materials
56 Spectral response of vegetation Reflectance of vegetation in Visible – SWIR region
57 Light excites atoms, which emit light that adds (or subtracts) with the input light. When light of frequency w excites an atom with resonant frequency w0:Electric field at atomElectron cloudEmitted field+=Incident lightEmitted lightTransmitted lightOn resonance (w = w0)An excited atom vibrates at the frequency of the light that excited it and re-emits the energy as light of that frequency.The crucial issue is the relative phase of the incident light and this re-emitted light. For example, if these two waves are ~180° out of phase, the beam will be attenuated. We call this absorption.
58 Refractive Index vs. Wavelength Since resonance frequencies exist in many spectral ranges, the refractive index varies in a complex manner.Electronic resonances usually occur in the UV; vibrational androtational resonances occur in the IR; and inner-shell electronicresonances occur in the x-ray region.n increases with frequency, except in anomalous dispersion regions.