Presentation on theme: "Absorption of EM radiation. Molecular absorption processes Electronic transitions UV and visible wavelengths Molecular vibrations Thermal infrared wavelengths."— Presentation transcript:
Absorption of EM radiation
Molecular absorption processes Electronic transitions UV and visible wavelengths Molecular vibrations Thermal infrared wavelengths Molecular rotations Microwave and far-IR wavelengths Each of these processes is quantized Translational kinetic energy of molecules is unquantized Increasing energy ~ J ~ J
Atomic and molecular vibrations correspond to excited energy levels in quantum mechanics Energy Ground level Excited level E = h The atom is at least partially in an excited state. The atom is vibrating at frequency, ν. Energy levels are everything in quantum mechanics. For a given frequency of radiation, there is only one value of quantum energy for the photons of that radiation Transitions between energy levels occur by absorption, emission and stimulated emission of photons
Excited atoms emit photons spontaneously When an atom in an excited state falls to a lower energy level, it emits a photon of light. Molecules typically remain excited for no longer than a few nanoseconds. This is often also called fluorescence or, when it takes longer, phosphorescence. Energy Ground level Excited level
Absorption spectra of molecules (a)allowed transitions (b)positions of the absorption lines in the spectrum of the molecule Line positions are determined by the energy changes of allowed transitions Line strengths are determined by the fraction of molecules that are in a particular initial state required for a transition Multiple degenerate transitions with the same energy may combine ν ij = ΔE ij /h Hypothetical molecule with three allowed energy levels Note relationship to emission!
Fluorescence Fluorescent 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...
Atoms and molecules can also absorb photons, making a transition from a lower level to a more excited one This photon has been absorbed Energy Ground level Excited level
In 1916, Einstein showed that another process, stimulated emission, can occur Before After Absorption Stimulated emission Spontaneous emission
Interaction of radiation with matter If there are no available quantized energy levels matching the quantum energy of the incident radiation, then the material will be transparent to that radiation Wavelength
X-ray interactions Quantum 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 atom Hence all x-rays are ionizing radiation If all the x-ray energy is given to an electron, it is called photoionization If part of the energy is given to an electron and the remainder to a lower energy photon, it is called Compton scattering
Ultraviolet interactions Near UV radiation (just shorter than visible wavelengths) is absorbed very strongly in the surface layer of the skin by electron transitions At higher energies, ionization energies for many molecules are reached and the more dangerous photoionization processes occur Sunburn is primarily an effect of UV radiation, and ionization produces the risk of skin cancer
UV SO 2 and O 3 absorption spectra
Ultraviolet interactions The ozone layer in the upper atmosphere absorbs most harmful UV radiation before it reaches the surface Higher UV-B frequencies are ionizing radiation and can produce harmful effects such as skin cancer The ionosphere is a region of the upper atmosphere ionized by solar radiation UV-A: nm UV-B: nm UV-C: nm
Visible light interactions Visible light is also absorbed by electron transitions Higher energies at blue wavelengths relative to red wavelengths: hence red light is less strongly absorbed than blue light Absorption of visible light causes heating, but not ionization Car windshields transmit visible light but absorb higher UV frequencies
Infrared (IR) interactions Quantum energy of IR photons ( eV) matches the ranges of energies separating quantum states of molecular vibrations Vibrations arise as molecular bonds are not rigid but behave like springs
Microwave interactions Quantum energy of microwave photons ( eV) matches the ranges of energies separating quantum states of molecular rotations and torsion Note that rotational motion of molecules is quantized, like electronic and vibrational transitions associated absorption/emission lines Absorption of microwave radiation causes heating due to increased molecular rotational activity Most matter transparent to µ-waves, microwave ovens use high intensity µ-waves to heat material
Molecular dipole moments 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. For a molecule to absorb IR radiation it must undergo a net change in dipole moment as a result of vibrational or rotational motion.
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.
Key atmospheric constituents Diatomic, homonuclear molecules (e.g., N 2, O 2 ) have no permanent electric dipole moment (also CO 2 ) Molecular N 2, the most abundant atmospheric constituent, has no rotational absorption spectrum Oxygen (O 2 ) has rotational absorption bands at 60 and 118 GHz Linear and spherical top molecules have the fewest distinct modes of rotation, and hence the simplest absorption spectra Asymmetric top molecules have the richest set of possible transitions, and the most complex spectra Note lack of permanent electric dipole moment in CO 2 and CH 4 No
Vibration modes of simple molecules A 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., 2v 1, v 1 +v 3 etc.) Symmetric stretch Bend (Scissoring) Asymmetric stretch A non-linear molecule of N atoms has 3N-6 normal modes of vibration; a linear molecule has 3N-5. Fundamental or normal modes
Absorption frequency for a diatomic molecule m 1, m 2 = atomic mass of vibrating atoms c = speed of light [3×10 8 m s -1 ] V = wavenumber [cm -1 ] A v = Avogadro’s number [6.023×10 23 atoms mole -1 ] k = force constant (bond strength) [dynes cm -1 ] For a single bond, k = 5×10 5 dynes cm -1 For a double bond, k = 10×10 5 dynes cm -1 For a triple bond, k = 15×10 5 dynes cm -1
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
Infrared (IR) interactions RegionWavelength [µm] Energy [meV] Wavenumber [cm -1 ] Type of excitation Far IR – 200Lattice vibrations, Molecular rotations Mid IR Molecular vibrations Near IR Overtones
Absorption spectra of molecules Electronic, vibrational and rotational energy levels are superimposed The 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 V = Vibrational quantum number J = Rotational quantum number
Vibrational-rotational transitions Relative positions of transitions in the absorption spectrum of a molecule Q branch (ΔJ = 0) (pure vibration) R branch (ΔJ = +1) P branch (ΔJ = -1)
Rotational absorption spectrum Photon frequency associated with a rotational transition
Hydrogen chloride (HCl) spectrum Vibrational-rotational absorption spectrum of HCl: shows affect of two chlorine isotopes with slightly different mass P branchR branch Q branch (ΔJ = 0)
Transmittance spectrum for ozone (O 3 )
Transmittance spectrum for CO 2
Transmittance spectrum for H 2 O
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.
Pressure broadening Absorption coefficient of O 2 in the microwave band near 60 GHz at two different pressures. Pressure broadening at 1000 mb obliterates the absorption line structure.
Rovibrational Energy Vibrational and rotational transitions usually occur simultaneously splitting up vibrational absorption lines into a family of closely spaced lines Rotational energy also dependent on direction of oscillation of dipole moment relative to axis of symmetry – When oscillates parallel, ΔJ = 0 transition is forbidden and only P and R branches are seen – When oscillates perpendicular, P, Q and R branches are all seen The 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 band Diagram taken from Patel (1968) Rovibrational transitions in a CO2 molecule
Sulfur dioxide (SO 2 ) ν 1 : 1151 cm -1, 8.6 µm ν 3 : 1361 cm -1, 7.3 µm ν 2 : 519 cm -1, 19.2 µm
Water vapor (H 2 O) Most important IR absorber Asymmetric top → Nonlinear, triatomic molecule has complex line structure, no simple pattern 3 vibrational fundamental modes Higher order vibrational transitions (Δv >1) give weak absorption bands at shorter wavelengths in the shortwave bands 2 H isotope (0.03% in atmosphere) and 18 O (0.2%) adds new (weak) lines to vibrational spectrum 3 rotational modes (J 1, J 2, J 3 ) Overtones and combinations of rotational and vibrational transitions lead to several more weak absorption bands in the NIR symmetric stretch v 1 = 2.74 μm asymmetric stretch v 3 = 2.66 μm bend v 2 = 6.25 μm H o H o
Transmission spectrum of H 2 O
Explain the peaks in n i ….
Absorption Spectrum of H 2 O v 3 =2.66 μm v 1 =2.74 μm v 2 =6.25 μm
Carbon dioxide (CO 2 ) Linear → no permanent dipole moment, no pure rotational spectrum Fundamental modes: The v 3 vibration is a parallel band (dipole moment oscillates parallel to symmetric axis), transition ΔJ = 0 is forbidden, no Q branch, greater total intensity than v 2 fundamental The v 2 vibration is perpendicular band, has P, Q, and R branch The v 3 fundamental is the strongest vibrational band, but the v 2 fundamental is most effective due to “matching” of vibrational frequencies with terrestrial Planck emission function 13 C isotope (1% of C in atmosphere) and 17/18 O isotope (0.2%) cause a weak splitting of rotational and vibrational lines in the CO 2 spectrum symmetric stretch v 1 = 7.5 μm => IR inactive asymmetric stretch v 3 = 4.3 μm bend v 2 = 15 μm bend v 2 oco
IR Absorption Spectrum of CO 2 v3v3 v2v2
Which is the most potent greenhouse gas?
Ozone (O 3 ) Ozone is primarily present in the stratosphere except anthropogenic ozone pollution which exists in the troposphere Asymmetric top → similar absorption spectrum to H 2 O due to similar configuration (nonlinear, triatomic) Strong rotational spectrum of random spaced lines Fundamental vibrational modes –14.3 μm band masked by CO 2 15 μm band –Strong v 3 band and moderately strong v 1 band are close in frequency, often seen as one band at 9.6 μm –9.6 μm band sits in middle of 8-12 μm H 2 O window and near peak of terrestrial Planck function –Strong 4.7 μm band but near edge of Planck functions symmetric stretch v 1 = 9.01 μm asymmetric stretch v 3 = 9.6 μm bend v 2 = 14.3 μm o o oo
IR Absorption Spectrum of O 3 v2v2 v 1 /v 3
Methane (CH 4 ) Spherical top 5 atoms, 3(5) – 6 = 9 fundamental modes of vibration Due to symmetry of molecule, 5 modes are degenerate, only v 3 and v 4 fundamentals are IR active No permanent dipole moment => No pure rotational spectrum Fundamental modes C H H H H CC C v1v1 v2v2 v 3 = 3.3 µm v 4 = 7.7 µm
IR Absorption Spectrum of CH 4 v3v3 v4v4 7.6 µm band in otherwise largely transparent part of atmosphere Methane concentrations also directly/indirectly affected by human activities
Nitrous oxide (N 2 O) Linear, asymmetric molecule (has permanent dipole moment) Has rotational spectrum and 3 fundamentals Absorption 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: symmetric stretch v 1 = 7.8 μm asymmetric stretch v 3 = 4.5 μm bend v 2 = 17.0 μm bend v 2 NN O
IR Absorption Spectrum of N 2 O v 3 =4.5 µm v 1 =7.8 µm v 2 =17 µm
Mineral and rock reflectance spectra Electronic transitions in solids; Fe 2+ (iron) particularly important in remote sensing – minerals contain Fe 2+ ions Fundamental vibrational modes of H 2 O: 2.74 µm, 6.25µm, 2.66 µm In rock spectra, whenever water is present we see 2 absorption bands in near-IR spectra – one near 1.45 µm (2ν 3 overtone) and one near 1.9 µm (v 2 +v 3 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?
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 transitions In the fall, trees produce carotenoids, which reflect yellow, and anthocyanins, which reflect orange and red.
Why Mars looks red Iron oxides prevalent in Martian soil show increased reflectance at the red end of the visible spectrum.
WTC dust spectra Spectra of World Trade Center dust Gypsum (water-bearing) = pulverized wallboard Chrysotile = asbestos (fireproof coatings?) OH - (hydroxyl ion) – has first overtone at ~1.4 µm – most common feature present in near-IR spectra of terrestrial materials Chrysotile standard
Spectral response of vegetation Reflectance of vegetation in Visible – SWIR region
Light excites atoms, which emit light that adds (or subtracts) with the input light. When light of frequency excites an atom with resonant frequency 0 : 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. Electric field at atom Electron cloud Emitted field On resonance ( = 0 ) + = Incident light Emitted light Transmitted light
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 and rotational resonances occur in the IR; and inner-shell electronic resonances occur in the x-ray region. n increases with frequency, except in anomalous dispersion regions.