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MTU GE 4250 - Absorption of EM radiation

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4/14/10 1 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 ~10-18 J ~10-23 J4/14/10 2 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 νij = ΔEij/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...4/14/10 3 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 scattering4/14/10 4 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 SO2 and O3 absorption spectra4/14/10 5 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 (0.001-1.7 eV) matches the ranges of energies separating quantum states of molecular vibrations • Vibrations arise as molecular bonds are not rigid but behave like springs4/14/10 6 Microwave interactions • Quantum energy of microwave photons (0.00001-0.001 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.4/14/10 7 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 spectrum • Oxygen (O2) 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 CO2 and CH4 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., 2v1, v1+v3 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 modes4/14/10 8 Absorption frequency for a diatomic molecule m1, m2 = atomic mass of vibrating atoms c = 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-1 For a double bond, k = 10×105 dynes cm-1 For a triple bond, k = 15×105 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 bonds4/14/10 9 Infrared (IR) interactions Region Wavelength [µm] Energy [meV] Wavenumber [cm-1] Type of excitation Far IR 50 - 1000 1.2 - 25 10 – 200 Lattice vibrations, Molecular rotations Mid IR 2.5 -


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