Spectroscopy & Photochemistry I

Transcription

1 Spectroscopy & Photochemistry I Required Reading: FP Chapter 3B, 3C, 4 Required Reading: Jacob Chapter 7 Atmospheric Chemistry CHEM-5151 / ATOC-5151 Spring 2013 Jose-Luis Jimenez Importance of Spectroscopy and Photochemistry I Most chemical processes in the atmosphere are initiated by photons Photolysis of O 3 generates OH the most important atmospheric oxidizer: O 3 + hv O 2 + O( 1 D) O( 1 D) + H 2 O 2 OH Solar photodissociation of many atmospheric molecules is often much faster than any other chemical reactions involving them: NO 2 + hv O + NO (source of O 3 in the troposphere) CF 2 Cl 2 + hv CF 2 Cl + Cl (photolysis of CFCs in the stratosphere) HONO + hv OH + NO (source of OH in the troposphere) NO 3 + hv O 2 + NO or O + NO 2 (removal of NO 3 generated at night) Cl 2 + hv Cl + Cl (source of Cl radicals) H 2 CO + hv H 2 + CO or H + HCO (important in hydrocarbon oxidation) etc. 1

2 Importance of Spectroscopy and Photochemistry II Absorption of solar and earth radiation by atmospheric molecules directly influences the energy balance of the planet Greenhouse effect (CO 2, H 2 O, N 2 O, CFCs) Stratospheric temperature inversion (O 3 photochemistry) Spectroscopy of atmospheric molecules is used to detect them in situ OH is detected via its electronic transition at 310 nm NH 3 is detected via its fundamental vibrational transition at 1065 cm -1, etc. Blackbody Radiation Linear Scale P/A= T 4 Log Scale T= b Interactive demo: From R.P. Turco, Earth Under Siege: From Air Pollution to Global Change, Oxford UP,

3 Solar & Earth Radiation Spectra Sun is a radiation source with an effective blackbody temperature of about 5800 K Earth receives circa 1368 W/m 2 of energy from solar radiation From Turco Clicker Question: are relative vertical scales correct in right plot? A. Yes B. Somewhat off C. Completely wrong D. I don t know Adapted from S. Nidkorodov Solar Radiation Spectrum UV C B A Photon Energy IR From Turco 3

4 Solar Radiation: Initiator of Atmos. Reactions Average thermal energy of collisions: Each degree of freedom ~ ½ kt (per molecule) Collision energy ~ RT = 8.3 J mol -1 K -1 x T (per mole) RT = 2.5 kj mol 300 K Energy of photons (E = hv): 300 nm photon = 380 kj mol nm photon = 190 kj mol -1 Typical bond strengths: D 0 (O 2 ) = 495 kj mol -1 D 0 (Cl 2 ) = 243 kj mol -1 C-H, O-H, C-O ~ 400 kj mol -1 Atmospheric chemistry on Earth is driven by photolysis, not by thermal excitation!!! Adapted from S. Nidkorodov From F-P&P What is light? Discuss in class: at a fundamental physical level, why are molecules capable of absorbing light? Dual nature Photon: as particle Energy but no mass As wave: electric and magnetic fields oscillating in space and time Wavelength, frequency c ~ 3 x 10 9 m/s 4

5 The Electromagnetic Spectrum Units used for photon energies and wavelengths: 1 ev = cm -1 = kj/mol = kcal/mol 1 Å = 0.1 nm = m; 1 micron ( m) = 10-6 m = 1000 nm Planck's constant = m 2 kg / s Clicker Q: the energy of a green photon ( = 530 nm) is: A. 1 kj/mol B. 230 kj/mol C. 230 kcal/mol D. 6 x 10-7 kj/mol E. I don t know c E h 1 v (wavenumber) Reminder of EM Spectrum 5

6 Types of radiation important in lower atmosphere Ultraviolet and visible radiation ( = nm) Excites bonding electrons in molecules Capable of breaking bonds in molecules photodissociation) Ultraviolet photons ( = nm) have most energy, can break more and stronger bonds. We will pay special attention to them. Infrared radiation ( = m) Excites vibrational motions in molecules With very few exceptions, infrared radiation is not energetic enough to break molecules or initiate photochemical processes Microwave radiation ( = mm) Excites rotational motions in molecules Spectroscopy & Photochemistry II Required Reading: FP Chapter 3B, 3C, 4 Required Reading: Jacob Chapter 7 Atmospheric Chemistry CHEM-5151 / ATOC-5151 Spring 2013 Jose-Luis Jimenez 6

7 Fundamentals of Spectroscopy Molecules have energy in translation, vibration, rotation, and electronic state Translation (= T) cannot be changed directly with light We will focus on the other 3 energy types Molecule can absorb radiation efficiently if: The photon energy matches the energy spacing between molecule s quantum levels Optical transition between these quantum levels is allowed by selection rules Forbidden transitions can occur but are weaker E photon v', J', v", J", Vibrational Energy & Transitions Bonds can be viewed as springs Energy levels are quantized, E v = hv vib (v+1/2) v vib is constant dependent on molecule v = 0, 1, 2 is vibrational quantum number From F-P&P 7

8 Vibrational Energy Levels Ideally: Harmonic Oscillator Restoration force of spring follows Hooke s law: F= k x E v = hv vib (v+1/2), v = 0, 1, 2 Energy levels are equally spaced Really: Anharmonic oscillator Restauration force rises sharply at small r, bond breaks at large r E vib hν 2 3 v 1 h x v 1 h y v e Vibrational quantum levels are more closely spaced as v increases 2 e 2 From F-P&P /energy-skate-park Example: Ground Electronic State of HF E E total rot E E E BJ( J 1) vib rot hνv vib Rotational level manifolds for different vibrational quanta overlap with each other HF molecular constants B v=0 = cm -1 (rotational constant) = cm -1 (harmonic frequency) x e = cm-1 (anharmonicity) Possible rovibrational transition: v=0 v=1 J=14 J=15 From S. Nidkorodov 8

9 Vibration-rotation of HCl Molecules vibrate and rotate simultaneously From F-P&P Electronic Transitions (ETs) Molecules can undergo an ET upon absorption of an appropriate photon Simultaneous vibrational and rotational transitions No restriction on v, many vib. trans. can occur J = -1, 0, +1 P, Q, and R branches Frank-Condon principle Time for ET so short (10-15 s) that internuclear distance cannot change vertical transitions From F-P&P 9

10 Pathways for Loss of e - Excitation Photophysical processes Lead to emission of radiation Energy converted to heat Photochemical processes Dissociation, ionization, reaction, isomerization From Wayne Piazza details Business Items Student s answer, instructor answer, comments alerts digest 10

11 No minima in PE vs r curves Dissociation occurs immediately after absorption of light Repulsive States From F-P&P Molecules & Light Simulation 11

12 Quantum Yields ( ) Relative efficiency of various photophysical and photochemical processes: Number of excited molecules proceeding by process i i Total number of photons absorbed E.g.: NO 3 + hv NO 3 * (3) 4a NO 3* NO 2 + O (4a) NO + O 2 (4b) NO 3 + hv Number of NO2 molecules formed Total number of photons absorbed (4c) and so on i Are wavelength dependent, above all important at different Quantum Yields II From F-P&P 12

13 Spectroscopy and Photochemistry III Required Reading: FP Chapter 3B, 3C, 4 Required Reading: Jacob Chapter 7 Atmospheric Chemistry ATOC-5151 / CHEM-5151 Spring 2013 Prof. Jose-Luis Jimenez On Beer s Law The taller the glass, the darker the brew, The less the amount of light that comes through Clicker prediction: If I set up (1) 200 M concentration & 1 cm path (2) 100 M concentration and 2 cm path The transmittance will be: A. Same B. More C. Less D. A lot less E. I don t know 13

14 Gas Absorption: Beer-Lambert Law I I I0 exp( L N) Allows the calculation of the decay in intensity of a light beam due to absorption by the molecules in a medium Solve in class: Show that in the small absorption limit the relative change in light intensity is approximately equal to absorbance. Definitions: A = ln(i 0 /I) = Absorbance = L N (also optical depth ) absorption cross section [cm 2 /molec] L absorption path length [cm] n density of the absorber [molec/cm 3 ] From F-P&P & S. Nidkorodov Strength of the bands Quantify w/ absorption cross-section for one molecule: cm 2 What is a large (but still realistic) value of for molecules? a cm 2 b. 10 cm 2 c cm 2 d cm 2 e cm 2 From Tolbert 14

15 Physical interpretation of, absorption cross section (cm 2 / molecule) Effective area of the molecule that photon needs to traverse in order to be absorbed. The larger the absorption cross section, the easier it is to photoexcite the molecule. E.g., pernitric acid HNO 4 Collisions cm 2 /molec Light absorption cm 2 /molec From S. Nidkorodov Measurement of Absorption Cross Sections Measurement of absorption cross sections is, in principle, trivial. We need a light source, such as a lamp (UV), a cell to contain the molecule of interest, a spectral filter (such as a monochromator) and a detector that is sensitive and responds linearly to the frequency of radiation of interest: Filter I 0 Gas cell n [#/cm 3 ] I Detector L Measurements are repeated for a number of concentrations at each wavelength of interest. From S. Nidkorodov 15

16 Clicker Q A sample contains 1 mbar of molecules of interest (A) with =2x10-21 cm 2 /molec and 1 bar of impurity (I) with =2x10-18 cm 2 /molec, both at =530 nm. Which contributes the most to the total absorbance in a 50 cm cell with photons cm -2 s -1 at the wavelength of interest? A) Molecules A contribute most B) Impurity I contributes most C) They contribute equally D) They are both negligible E) I don t know Oxygen Spectrum From Brasseur and Solomon Hole in the spectrum coincident with Lyman line of H-atom A. 2.3 cm B. 23 m C. 23 km D. 230 m E. I don t know O 2 photolysis in the nm range is the major source of O 3 in the stratosphere O 2 can absorb nearly all radiation with = nm high up in the atmosphere Clicker Q: Estimate the length of air column at P = 0.01 mbar and T= 200 K (characteristic of 80 km altitude) required to reduce the radiation flux at 150 nm by 10 orders of magnitude. Neglect the fact that a substantial portion of oxygen is atomized at this altitude. 16

17 Calculation of Photolysis Rates I Generic reaction: A + hν B + C d[ A] J dt [ A] First-order process What does J A for a given molecule depend on? A. Light intensity from above B. Light intensity from all directions C. Absorption cross section ( ) & Path length (L) D. Quantum yield for fluorescence ( f ) E. I don t know A Calculation of Photolysis Rates II Generic reaction: A + hν B + C d[ A] J A[ A] A( ) A( ) F( ) d [ A] dt J A first order photolysis rate of A (s -1 ) σ A ( ) wavelength dependent cross section of A (cm 2 /#) A ( ) wavelength dependent quantum yield for photolysis F( ) spectral actinic flux density (#/cm 2 /s) 17

18 Group Problem The graphs below show the approximate solar flux at the Earth surface, absorption cross section of NO 2 molecule, and photodissociation quantum yield of NO 2. What is the photodissociation lifetime of NO 2 ( NO2 )? A. 16 s B. 6 s -1 C s D. 116 s -1 E. I don t know Part of 2005 Final exam Corollary: What are the smallest cross sections that matter in the atmosphere? Group Problem The graphs below show the approximate solar flux at the Earth surface, absorption cross section of NO 2 molecule, and photodissociation quantum yield of NO 2. What is the photodissociation lifetime of NO 2 ( NO2 )? A. 16 s B. 6 s -1 C s D. 116 s -1 E. I don t know Part of 2005 Final exam 18

19 TUV Model from NCAR (as run for previous slides) Actinic surface & 20 km Output of TUV model for clean atmosphere 19

20 Nitrogen Dioxide (NO 2 ) NO 2 is one of a very few atmospheric molecules that absorb & photolyze in the visible range Photolysis of NO 2 generates ozone in the troposphere: NO 2 + hv NO + O( 3 P) O( 3 P) + O 2 + M NO 2 Absorption cross sections are structured, and have a non-trivial dependence on T,P. NO 2 contributes to the brown color of air in very polluted cities (but most due to aerosols!). 8x10-19 Cross Section (cm 2 molec -1, base e) From S. Nidkorodov Wavelength, nm Photochemistry of NO 2 Photolysis occurs with nearly 100% yield below nm. O-atom immediately makes O 3 NO 2 + hv NO + O( 3 P) O( 3 P) + O 2 (+ M) O 3 Between 398 nm and 415 nm, room temperature NO 2 still partially photodissociates because of contributions of internal energy to the process, but the quantum yield declines rapidly with wavelength Above 410 nm, electronic excitation of NO 2 can result in the following processes: NO 2* NO 2 + hv Fluorescence NO 2* + O 2 NO 2 + O 2 (a 1 g ) Electronic energy transfer NO 2* + N 2 NO 2 + N 2 (v) Electronic-to-vibrational energy transfer NO 2* + NO 2 NO + NO 3 Disproportionation (lab conditions only) From F-P&P 20

21 Clicker Q What is the order of magnitude of the shortest possible species lifetime due to photolysis at the 210 nm radiation 20 km altitude? A. 50 years B. 50 months C. 50 days D. 50 hrs E. 50 min 20 km Corollary: what about the surface? Surface Examples of Photolysis Rates From F-P&P 21

22 General Remarks Photodissociation is the most important class of photochemical process in the atmosphere: AB + hv A + B In order to photodissociate a molecule it must be excited above its dissociation energy (D 0 ). In the lower troposphere, only molecules with D 0 corresponding to > 290 nm are photochemically active. Most common atmospheric molecules, including N 2, CO, O 2, CO 2, CH 4, NO, etc. are stable against photodissociation in the troposphere. In addition, the molecule should have bright electronic transitions above D 0. For example, HNO 3 has a low dissociation energy (D 0 = 2.15 ev) but it needs UV for its photolysis because it does not have appropriate electronic transitions in the visible. In general, both the absorption cross sections and photodissociation quantum yields are wavelength dependent. Photoionization processes are generally not important in the lower atmosphere (ionization potentials for most regular molecules > 9 ev). From S. Nidkorodov O 3 Absorption Spectrum Hartley bands Infrared absorption Huggins bands Chappuis bands From Yung & DeMore 22

23 O 2 Photochemistry Schumann continuum very efficient screening radiation below 200 nm Solar radiation more intense towards longer Dissociation of O 2 in Herzberg continuum ( nm) is very important for O 3 in the stratosphere O 2 + hv O( 3 P) + O( 3 P) O( 3 P) + O 2 (+ M) O 3 Troposphere > 290 nm Not enough energy for O 2 dissociation O 3 from NO 2 + hv From Brasseur and Solomon From F-P&P Importance of O 3 Central role in atmospheric chemistry Highly reactive Highly toxic => health effects in humans Crop degradation => billions of $ in losses Absorbs UV Shield surface from hard UV Its photolysis produces O( 1 D), which yields OH OH is most important tropospheric oxidant Photolysis to O( 3 P) regenerates O 3, not important! Absorbs IR Greenhouse gas 23

24 O 3 Photochemistry Most important aspect is production of O( 1 D) (and thus OH) O 3 + hv O 2 + O( 1 D) ( 1 D) 90% below 305 nm O 3 + hv O 2 + O( 3 P) ( 3 P) 10% below 305 nm O( 1 D) + H 2 O 2 OH OH yield 10% (at the surface) O( 1 D) + M O( 3 P) the rest of O( 1 D) atoms are quenched Energy Threshold From F-P&P UV absorption by O 2 and O

25 From F-P&P Solar Radiation Spectrum IV O 2 O 3 Solar spectrum is strongly modulated by atmospheric absorptions Remember that UV photons have most energy O 2 absorbs extreme UV in mesosphere; O 3 absorbs most UV in stratosphere Chemistry of those regions partially driven by those absorptions Only light with >290 nm penetrates into the lower troposphere Biomolecules have bonds that can break with UV absorption => damage to life Importance of protection provided by O 3 layer Solar Radiation Spectrum vs. altitude From F-P&P Very high energy photons are depleted high up in the atmosphere Some photochemistry is possible in stratosphere but not in troposphere Only > 290 nm in trop. 25

26 Photoionization Clicker Q: is photoionization important in the troposphere? A. Yes for many species B. Yes for Na C. Yes for Na & Mg D. No E. I don t know Chlorofluorocarbons (CFCs) Photolysis Rates Absorption Spectra No other CFC sinks than photolysis Known that Cl would destroy O Nobel Prize (M&R) is the idea in this slide: CFCs will provide large source of Cl in stratosphere and lead to O 3 destruction From Brasseur and Solomon 26

27 Photolysis Rates for O 2, NO 2, and O 3 Typical values for photodissociation From Yung & DeMore coefficients, J, for O 3, O 2, & NO 2 as a function of altitude. Photolysis rate for O 2 is strongly altitude dependent because the lower you go the less UV radiation capable of breaking O 2 is available (self-shielding). Photolysis rate for O 3 becomes altitude dependent below 40 km for similar self-shielding reasons. On the contrary, visible NO 2 photolysis occurs with about the same rate throughout the atmosphere because there is not enough of it for self-shielding. From Warneck Solve in class: Based on the figures shown here, estimate the lifetime of NO 2, O 2 and O 3 against photodissociation at 20 km and 50 km. Solar Radiation Spectrum II From F-P&P From Turco Solar spectrum is strongly modulated by atmospheric scattering and absorption 27

28 From Turco Scattering by Gases t sg Purely physical process, not absorption Approximation: ( n0 1) / Strongly increases as decreases Reason why sky is blue during the day Spectroscopy and Photochemistry IV Required Reading: FP Chapter 3B, 3C, 4 Required Reading: Jacob Chapter 7 Atmospheric Chemistry ATOC-5151 / CHEM-5151 Spring 2013 Prof. Jose-Luis Jimenez 28

29 To calculate solar spectral distribution in any given volume of air at any given time and location one must know the following: Solar spectral distribution outside the atmosphere Path length through the atmosphere Wavelength dependent attenuation by atmospheric molecules Amount of radiation indirectly scattered by the earth surface, clouds, aerosols, and other volumes of air Solar Radiation Intensity From F-P&P Solar Radiation: Further Details Output of TUV model for clean atmosphere 29

30 Solar Zenith Angle Aside form the altitude, the path length through the atmosphere critically depends on the time of day and geographical location. Path length can be calculated using the flat atmosphere approximation for zenith angles under 80º. Beyond that, Earth curvature and atmospheric refraction start to matter Solar Flux Solar Flux [Photons cm -2 s -1 nm -1 ] SZA = 0º SZA = 86º At large SZA very little UV-B radiation reaches the troposphere Actual pathlength L " Air Mass" m sec Vertical pathlength h Wavelength [nm] 1000 Radiation vs. time of day, location, season Q: summer/winter solstices intensity at noon? From F-P&P 30

31 Wavelength dependent! Question: for the same incident solar flux, will you tan faster over snow or over a desert? Surface Albedo Albedo( ) Reflected Radiation( ) Incident Radiation( ) From F-P&P Direct Attenuation of Radiation t t I I sg t I radiation intensity (e.g., F) I 0 radiation intensity above atmosphere m air mass t attenuation coefficient due to absorption by gases (ag) scattering by gases (sg) scattering by particles (sp) absorption by particles (ap) 0 ag e t -t m sp t ap From F-P&P & S. Nidkorodov t sp -n much more complex Rayleigh scattering t sg -4 Deep UV O, N 2, O 2 Mid UV & visible O 3 Near IR H 2 O Infrared CO 2, H 2 O, others t ag 31

32 Scattering & Absorption by Particles Particles can scatter and absorb radiation Scattering efficiency is very strong function of particle size For a given wavelength Visible: ~ 0.5 m Particles m are most efficient scatterers! Will discuss in more detail in aerosol lectures From Jacob Spectroscopy and Photochemistry V (and last!) Required Reading: FP Chapter 3B, 3C, 4 Required Reading: Jacob Chapter 7 Atmospheric Chemistry ATOC-5151 / CHEM-5151 Spring 2013 Prof. Jose-Luis Jimenez 32

33 Review Clicker Q Jose said during last lecture that: ozone is a greenhouse gas the absorption spectrum of ozone to the right is centered around 1000 cm -1 = 1 m Is that correct? A. Yes B. Only approximately C. No D. I don t know O 3 Infrared absorption Database of Absorption Spectra 33

34 Example of Spectra from Database Limitations of Available Information Spectra in database: Quantum yields in database: Much less data for quantum yields, much harder to measure than absorption spectra. (Why?) 34

35 Structure of Important N-Species From Jacobson (1999) Table B Many other species there, useful when you don t know detailed structure From Warneck Absorption Spectra and Photolysis Rates of Selected N-Species Note sensitivity of photolysis rates to shape of absorption spectra Solve in class: Estimate the photodissociation lifetimes of N 2 O and HONO at 20 km. Compare those with characteristic times for vertical transport in the stratosphere ( 2 years). From S. Nidkorodov 35

36 Nitrous Acid (HONO) HONO + hv OH + NO 1 below 400 nm HONO, like NO 2, has strong absorption in visible and a highly structured spectrum. Its photochemical lifetime in the atmosphere is ~ a few minutes. From F-P&P Very important as source of OH radicals in the morning Formaldehyde (HCHO) Two competing photodissociation channels: HCHO + hν H + HCO (a) HCHO + hν H 2 + CO (b) HCHO can be a dominant Ho x source (e.g. Mexico City) Channel (a) leads to HO 2 radical production via: H + O 2 + M HO 2 (19) HCO + O 2 HO 2 + CO (20) Sources of HO 2 are effectively sources of OH because: HO 2 + NO OH + NO 2 (17) All aldehydes have relatively weak transitions at around 300 nm. Photoexcitation of aldehydes normally results in a release of HCO, which is quickly converted into HO 2 and CO in the atmosphere: RCHO + hν R + HCO (slow) R + O 2 RO 2 (very fast) HCO + O 2 HO 2 + CO (very fast) 36

37 Sources of HO x = OH + HO 2 in Mexico City HO x drive smog and secondary aerosol chemistry HONO photolyzes at long, very important in early morning HCHO (formaldehyde) is dominant source O 3 source needs to wait for O 3 to be produced! (depends on others) From R. Volkamer & W. Brune (MIT & PSU) HNO 3 + hv OH + NO 2 HNO 3 + hv O + HONO HNO 3 + hv H + NO 3 Overall relatively long lifetime against photolysis Nitric Acid (HNO 3 ) 1 between 200 and 315 nm requires vacuum UV; is only 0.03 at 266 nm requires vacuum UV; is only at 266 nm From F-P&P 37

38 Nitrate Radical (NO 3 ) From F-P&P NO 3 is important in nighttime chemistry. It has unusually large cross sections in the red photodissociates in seconds in the morning. NO 3 + hv NO 2 + O( 3 P) is dependent; important towards the blue NO 3 + hv NO + O 2 is dependent; important in the red; competes with fluorescence; this process is nearly thermoneutral but it is inhibited by a tall energy barrier. Nitrous Oxide (N 2 O) N 2 O is extremely long-lived because it is unreactive, and it does not absorb much above 200 nm. Below 200 nm: N 2 O + hv N 2 + O( 1 D) - Note: this is a popular laboratory method of generating O( 1 D); 1 Subsequent reactions of O( 1 D) with N 2 O lead to production of other nitrogen oxides in the stratosphere: N 2 O + O( 1 D) NO + NO major source of NO in the stratosphere N 2 + O 2 competing step Because of its stability, N 2 O is used a "tracer". Concentrations of other molecules are often compared to that of N 2 O to see is they are well mixed. From Calvert & Pitts Solve in class: Find a conversion between absorption coefficient in cm -1 atm -1 an cross sections in cm 2 /# at room temperature. Evaluate N 2 O cross sections at 193 nm using the data shown here. 38

39 Atm. Profiles of Important N-Species Altitude (km) From NASA Total Density Important N- containing molecules in lower atmosphere: extremely photo-stable N 2 O easily-degradable NO 2, NO 3, and N 2 O 5, HO 2 NO 2 multitude of poorly quantified organic nitrates, RONO 2 (most nitrates are relatively stable towards UV) Generally, concentrations are inversely proportional to photodissociation / reaction rates in the atmosphere HW 5 discussion How to load data from databases into Igor Interpolation in HW5.2 Diagram for HW5.3 NO 2 + hv NO + O (1) O + O 2 + M O 3 (2) NO + O 3 NO 2 + O 2 (3) O 3 + hv O( 1 D) + O 2 (4) O( 1 D) + H 2 O 2 OH (5) O( 1 D) + O 2 O( 3 P) (6) O( 1 D) + N 2 O( 3 P) (7) OH + NO 2 HNO 3 (8) H + O 2 HO 2 (9) HO 2 + NO OH + NO 2 (10) HO 2 + HO 2 H 2 O 2 + O 2 (11) 39

40 Space for Diagram for HW5.3 40