Spectroscopy and Photochemistry AOSC 637 R. Dickerson Fall 2011 Copyright © 2011 R. R. Dickerson1.

Copyright © 2010 R. R. Dickerson & Z.Q. Li 2 Spectroscopy - The study of the interaction of substances with electromagnetic radiation. The energy can be very great such as that of gamma rays or relatively weak such as that of microwaves. Different substances have such differing spectra that spectroscopy is usually used for positive identification. For example when new elements were being discovered the visible emission spectra were used for confirmation. Finlayson - Pitts, Chapters 2 & 3 McEwan & Phillips, Chapter 1 Wayne, Chapter 2.6, 3.1 - 3.3 Seinfeld, Chapt. 4.1

Copyright © 2010 R. R. Dickerson & Z.Q. Li 3 Photochemistry - The study of chemical reactions caused by the absorption of light. Laws of Photochemistry 1. Only light absorbed by a molecule or atom can effect a chemical change. 2. Absorption of light is a one quantum process therefore the sum of the efficiencies of the primary processes must be unity. This law holds for atmospheric processes, but not for some laboratory processes in which the photon flux is so great that a second photon can be absorbed before the energy from first photon is expelled.

6 I/I 0 = 0.01 = exp(-1150 cl) or -ln(0.01)/ 1150 = cl  4x10  3 atm cm  2.0x10  2 cm at RTP Why do you think they call this region of the spectrum the vacuum ultraviolet? Later we will calculate the altitude of maximum absorption of various wavelengths radiation, and we will see that 150 nm radiation is absorbed pretty high up.

Copyright © 2010 R. R. Dickerson & Z.Q. Li 7 Example - Absorption Spectroscopy Life, as we know it, did not exist on the surface of the earth until ozone existed in the stratosphere. How much ozone is needed to protect life on the surface of the earth? Necessary Information 1. How much UV can a single celled organism withstand? 2. What is the solar UV flux? ln(I/I 0 )  cl Biomolecules, such as proteins of molecular weight ~1000, are destroyed by solar radiation at wavelengths around 280 nm. Assume that 1 g cm  2 yr  1 is the maximum allowable destruction rate.

Copyright © 2011 R. R. Dickerson8 The maximum allowable destruction rate of 1 g cm −2 yr −1 is the same as: 10 −3 moles cm −2 yr −1 or 6x10 20 molecules/cm 2 yr. If we also assume a quantum yield of unity (each photon absorbed causes a broken molecule) then the limit is: 6x10 20 UV photons/cm 2 yr. The lethal dosage is anything greater than about: 2x10 13 UV photons/cm 2 s

Copyright © R. R. Dickerson22 Start 3/3/11 So what are those funny symbols behind the O atoms and O 2 molecules? Term Symbols. Spectroscopy: A Quick Qualitative Description Term symbols show the energy state of atoms and molecules, as described by the quantum numbers. Atomic Quantum Numbers: n – principal quantum number. Value: 1, 2, 3,.... Tells which shell of an atom the e - resides. The farther from the nucleus the higher the n. l the azimuthal quantum number. Value: 0 to n-1. Describes the orbital angular momentum of the shape of the orbital. s – the spin quantum number. Value: ±½. j – the total (spin plus azimuthal) quantum number. Important for heavier atoms.

Copyright © R. R. Dickerson23 Spectroscopy: A Quick Qualitative Description, cont. Energy states of Molecules: Molecular Quantum Numbers  – the azimuthal quantum number. Value: 0 to n-1. Orbital angular momentum s – the spin quantum number. Value: ±½. Same as in atoms. J – rotational quantum number. Value: 1, 2, 3,.... Tells which shell of an atom the e - resides. The farther from the nucleus the higher the n. – vibrational quantum number. Value: 1, 2, 3,.... K – vertical component of the total angular momentum. This QN only exists for polyatomic molecules. g/u – gerade/ungerade; symmetry terms. Reflection through the center of symmetry of molecule. +/- – plus/minus; symmetry terms. Reflection through the plane of symmetry of molecule. Only for diatomics.

Copyright © R. R. Dickerson24 Internal Energy of Molecules E total = E rot + E vib + E elect The equipartition principle says that the total energy of a molecule will be the sum of the internal energy terms (rotational, vibrational, and electronic) and the external (translational) energy. Rotational energy can be expressed as: E rot = B J(J + 1) Where B = h / (8  2 Ic), often in units of cm -1, with I as the moment of inertia. Vibrational energy of an anharmonic oscillator can be expressed as: E vib = h vib ( + ½) – h vib ( + ½) 2 + h vib ( + ½) 3 + … Where vib is a constant dependent on the bond strength and length.

Copyright © R. R. Dickerson25 Term Symbols for Atoms and Molecules S L j S  ± g/u Where S = 2s + 1. When the value of S is 1, 2, 3, the spectra appear as singlets, doublets, triplets etc. L or  = 0 1 2 3 4 5 … Atoms = S P D F G H … Molecules =  Atoms and molecules tend toward the lowest energy levels. Finding the lowest levels for molecules is complicated, but for atoms: 1. Lowest n 2. Highest l 3. Highest s (no two electrons in the same shell until they are all occupied by at least one electron). 4. Lowest j

Copyright © R. R. Dickerson26 Selection Rules for Atomic Transitions  n = 0, 1, 2, … (no restrictions)  l = ± 1  j = 0, ± 1  S = 0 This is the strongest rule: no multiplicity change.  S ≠ 0 is a “forbidden” transition. Let us examine O atoms as an example.

29 Fraunhofer Lines in the solar spectrum

Copyright © 2010 R. R. Dickerson & Z.Q. Li 30 The Fraunhofer lines in the solar spectrum are a good example of absorption spectroscopy. Elements in the solar and terrestrial atmospheres absorb radiation. They have funny historical names not to be confused with spectroscopic designations. D 1 & D 2 are Na doublets, a is O 2 ; C, F, G', and h are H-atoms.

Copyright © R. R. Dickerson34 Selection Rules for Molecular Transitions   = 0, ±1  J = ±1 in monatomic molecules 0, ±1 in polyatomic molecules  ±1 for fundamental vibrations and rotations ±2, ±3… for overtones (Frank-Condon principle for vibronic transitions)  K = 0 for polyatomic molecules only. g and u, no change. +/- must change.  S = 0 This is the strongest rule: no multiplicity change.  S ≠ 0 is a “forbidden” transition. Shortly, we will examine O 2 molecules as an example.

Copyright © R. R. Dickerson35 For a purely rotational transition, the molecule must have a permanent dipole. N 2 and O 2 have no long-wave IR purely rotational spectra while CO, NO, HCl, and H 2 O do and are thus greenhouse gases. For a combination vibration/rotation, the molecule must have at least an induced dipole. CO 2 and CH 4 have easily induced dipoles. The stronger the dipole the greater the absorption coefficient.

Copyright © 2010 R. R. Dickerson & Z.Q. Li 36 Line shapes and Energy TransitionWavelength (  m) Energy (kcal/mole) Natural Line shape Pure rotation  301Very sharp Vibrations (with rotations) 1-301-10Thin Electronic0.1 – 110-250Broad What causes these line shapes?

Copyright © 2010 R. R. Dickerson37 Line Shapes Natural broadening, an inherent property of all atoms and molecules, is the result of the Heisenberg uncertainty Principle.  E  t = h/2   N = 2 /(2  ct)  N = (2  t) -1 The slowest transitions (rotations) must be accompanied by the least uncertainty in energy and are thus sharpest. For similar types of transitions the line width depends on the stability of the upper level. In the emission of light from an excited molecule, if the higher energy state is stable it will have a long lifetime and a small energy spread leading to sharp lines. Conversely if the excited state is unstable and the emission happens in a short time the line will be relatively broad.

Copyright © 2011 R. R. Dickerson38 Line Shapes Doppler broadening, caused by thermal motions toward or away from the observer, is the same as thermal broadening. an inherent property of all atoms and molecules, is the result of the Heisenberg uncertainty Principle.  D   RT  ½ Pressure broadening, caused by collisions between molecules, is also called Lorentz broadening. Collisions perturb the energy level of excited molecules and generally reduce their energy thus broadening and red shifting the lines. Ozone in the troposphere is subject to more pressure broadening than in the stratosphere and thus has broader (and asymmetric) absorptions lines, allowing ozone near the tropopause to absorb radiation that passes through the stratospheric ozone maximum. The 9.6  m band of O 3 adds to the greenhouse effect.

Copyright © 20110 R. R. Dickerson 39 Spectroscopy of Simple Molecules Example 1. HCl HCl has a strong dipole and strong transitions near 3.5 mm. There is only one degree of vibration freedom, and the observed transition corresponds to = 0  = 1. Rotations have such a low energy that they are already excited at room temperature with the maximum J = 3 and J = 12 common. In diatomics,  J = 0 is forbidden and there is no Q branch. R branch P branch

Copyright © 2010 R. R. Dickerson & Z.Q. Li 42 Transmission spectrum of CO 2 This is the bend; there is a a Q-Branch because  J = 0 is allowed. Strong absorption means CO 2 is a greenhouse gas and NDIR spectroscopy is a great technique for detection. How are the wings related to temperature?

Copyright © 2010 R. R. Dickerson & Z.Q. Li 44 O 2 + hv   2O( 3 P) Schumann-Runge bands  E ≥ 40,000 cm -1 or ≤ 250 nm. Potential Energy Curves for O 2 ≥ O 2 + hv   O( 3 P) + O( 1 D) Herzberg band  E ≥ 57,000 cm -1 or ≤ 175 nm.

Copyright © 2010 R. R. Dickerson & Z.Q. Li 45 Cross sections are greatest for allowed transitions. Continua form where there are dissociative states, and bands form where transitions between bound states occur.

48 According to the Spin Conservation Rule (Wayne 1991, p. 86-94) the products of ozone ( 1 A) must both be singlets or both triplets. This is also critical for O( 1 D) and OH production. Spin angular momentum sums vectorially: Products |S O2 + S O |, |S O2 + S O – 1|,…|S O2 – S O | = 2, 1, 0 for 3 P + 3 S, but can only be zero for 1 D + 1 , so the latter is favored.

Copyright © 2010 R. R. Dickerson & Z.Q. Li 52 Spectroscopy and Photochemistry Take Home Messages 1.The spectra of atoms and molecules are related to their ability to interact with electromagnetic radiation, and to their shape and structure. 2.We use the observed spectra to determine the energy levels and geometry of atoms and molecules. 3.Extraterrestrial radiation is absorbed by the atmosphere except in window regions such as the visible and IR near 10  m. 4.Transitions and reactions are influenced by selection rules, esp. spin conservation. 5.The energy and lifetime set the natural line shape: a.Rotations are slow, low energy, and very sharp. b.Vibrations are intermediate. c.Electronic transitions are very fast, high energy, and broad.

Copyright © 2010 R. R. Dickerson & Z.Q. Li 53 Spectroscopy and Photochemistry Take Home Messages, cont. 1.Oxygen:Schumann Runge Continuum <175 nm strong allowed. Schumann Runge Bands < 200 nm Herzberg Continuum < 242 nm forbidden weak. 2.Ozone:Hartley ~250 nm, allowed, strong. Huggins < forbidden, weaker ~330 nm Chappuis ~ 600 nm Forbidden, weak. 3. The production of OH and thus all of atmospheric chemistry depends strongly on the wavelength dependent absorption of UV radiation.

Let ’ s look at an important photochemically active molecule in detail from the ground up, considering all possible reactions. Example: Budget for Nitrous Acid, HONO Reaction  H o (kJ/mole) NO + NO 2 + H 2 O ↔ 2HONO-41(1) NO + OH + M → HONO + M † -209(2) HONO + h → HO + NO +202(3) O + HONO → HO + NO 2 -97 (4) O 3 + HONO → HO + NO 2 -198(5) OH + HONO → H 2 O + NO 2 -169(6) O 2 + HONO → O + HNO 3 +194(7) 2NO 2 + H 2 O (het) → HONO↑ + HNO 3 (aq) -1.75(8) We can examine each reaction in terms of thermodynamics and kinetics. Reaction 8 involves surfaces – it is a multiphase (heterogeneous) reaction and must be treated differently. Reactions such as R7 with a large positive  H o have a prohibitively low rate constant. Students should calculate k max to prove that this is an irrelevant reaction. In general oxidation by molecular oxygen is too slow to be important in the atmosphere. Copyright © 2010 R. R. Dickerson54

Copyright © 2010 R. R. Dickerson & Z.Q. Li 61 Are there other important HONO sinks? How do they compare to j(HONO)? Consider attack by O atoms. We’ll compare effective first order rate constants or lifetimes with respect to each loss.

Copyright © 2010 R. R. Dickerson & Z.Q. Li 76 Stutz et al. (2004; 2009) measured a lot of HONO during the morning. They observed HONO/NO2 ratios of 2 to 9%. Concentrations were in the range of 1 ppb for NOx of 20 ppb. The homogeneous chemistry alone will not explain HONO.

Copyright © 2010 R. R. Dickerson & Z.Q. Li 78 From Stutz et al., (JGR, 2004) d[HONO]/dt =  NO2 →HONO (RH) x S/V x v NO2 /4 x [NO 2 ]  HONO (RH) x S/V x v HONO /4 x [HONO] Where  is the accommodation coefficient  S/V stands for Surface area to Volume ratio, related to the 1/PBL height; RH is relative humidity; v stands for the mean molecular velocities. This is due to just the multiphase reactions.

Copyright © 2011 R. R. Dickerson79 Reactions in Solution (Also called multiphase or heterogeneous reactions) Atmosphere contains aqueous phase material: Clouds, fogs, rain, particulate matter Aqueous solutions or film of water surrounding insoluble core More on this stuff later in course How do gases interact with these particles: 1.Gas phase diffusion to surface of droplet 2.Transport across air-water interface 3.Diffusion of solvated species into bulk phase of droplet 4.Reaction of species in aqueous phase or at interface

Copyright © 2010 R. R. Dickerson & Z.Q. Li 81 Take home messages: We can do a steady state analysis and learn a lot about the atmosphere. Sometimes multiphase reactions dominate. HONO photolysis leads to OH production and smog formation. For HONO surfaces have to be wet. The air, waters, and land are intimately linked.

Spectroscopy and Photochemistry AOSC 637 R. Dickerson Fall 2011 Copyright © 2011 R. R. Dickerson1.

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Spectroscopy and Photochemistry AOSC 637 R. Dickerson Fall 2011 Copyright © 2011 R. R. Dickerson1.

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