spectroscopy and photochemistry - umd

Spectroscopy and Photochemistry

AOSC 620R. Dickerson

Copyright © 2013 R. R. Dickerson 1

Outline

• Additional details on spectroscopy and photochemistry as they relate to atmospheric chemistry.

• Direct measurements of photolysis rate coefficients (frequencies).

• Tropospheric ozone.


Copyright © 2010 R. R. Dickerson & Z.Q. Li 3

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

Recap from AOSC 620


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.

Copyright © R. R. Dickerson 12

So what are those funny symbols behind the O atoms and O2 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.


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.


Start 10/28/14 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/(82Ic), 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 = hvib( + ½) – hvib( + ½)2 + hvib( + ½)3 + …

Where vib is a constant dependent on the bond strength and length.


Term Symbols forAtoms and Molecules

SLj 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:• Lowest n• Highest l • Highest s (no two electrons in the same shell until they are all occupied by at least one electron).• Lowest j


Why don’t O(1D) atoms relax to O(3P) in the troposphere and stratosphere?

Where do O(1D) atoms relax to O(3P)?and O (1S0) relax to O(1D2) ?

17https://www.youtube.com/watch?v=fVsONlc3OUY


Transitions in oxygen atoms.


Selection Rules for Atomic Transitionsn = 0, 1, 2, … (no restrictions)l = ± 1j = 0, ± 1S = 0 This is the strongest rule: no multiplicity change.

S ≠ 0 is a “forbidden” transition.

Let us examine O atoms as an example.


Electrons in ground state Oxygen atoms: O(3P).

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Fraunhofer Lines in the solar spectrum


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. D1 & D2 are Na doublets, a is O2; C, F, G', and h are H-atoms.


Selection Rules for Molecular Transitions = 0, ±1J = ±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.

Ross already examined O2 molecules as an example.


For a purely rotational transition, the molecule must have a permanent dipole. N2 and O2 have no long-wave IR purely rotational spectra while CO, NO, HCl, and H2O do and are thus greenhouse gases. For a combination vibration/rotation, the molecule must have at least an induced dipole. CO2 and CH4 have easily induced dipoles. The stronger the dipole the greater the absorption coefficient.


Line shapes and Energy

Transition Wavelength (m)

Energy (kcal/mole)

NaturalLine shape

Pure rotation 30 1 Very sharp

Vibrations(with rotations)

1-30 1-10 Thin

Electronic 0.1 – 1 10-250 Broad

What causes these line shapes?


Line Shapes

Natural broadening, an inherent property of all atoms and molecules, is the result of the Heisenberg uncertainty Principle.

Et = h/2

N = 2/(2ct)

N = (2t)-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.


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.

DRT½

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 O3 adds to the greenhouse effect.

Copyright © 20110 R. R. Dickerson30

Spectroscopy of Simple MoleculesExample 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


Energy levels associated with theIR Spectrum of HCl Centered at 3.5 m

↑Big Gap

Selection rules:

J = ± 1, not 0 for diatomics

v = ± 1


Transmission spectrum of CO2

This is the bend near 15 m; there is a a Q-Branch because J = 0 is allowed. Strong absorption means CO2 is a greenhouse gas and NDIR spectroscopy is a great technique for detection. How are the wings related to temperature?


Energy levels in molecular oxygen, O2 Ground state is 3g


O2 + hv 2O(3P) Schumann-Runge bandsE ≥ 40,000 cm-1 or ≤ 250 nm.

Potential Energy Curves for O2

≥

O2 + hv O(3P) + O(1D) Herzberg bandE ≥ 57,000 cm-1 or ≤ 175 nm.

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


Absorption Spectrum of Ozone.


Atmospheric radiation absorption as a fnx of wavelength.

Chappuis


The Dobson Spectrometer.Courtesy of: Ulf Köhler, DWD Hohenpeissenberg


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https://www.youtube.com/watch?v=LKe5FdKInJs


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.


Spectroscopy and Photochemistry Take Home Messages, cont.

1.Oxygen: Schumann Runge Continuum <175 nm strong allowed.Schumann Runge Bands < 200 nmHerzberg 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.

International Photolysis Frequency Measurement and

Modeling Intercomparison (IPMMI)

NCAR Marshall Field Site, 39°N 105°W, elevation: 1.8 km; June 15–19, 1998

Objectives: j [NO2 NO + O], j [O3 O2 + O(1D)], spectral actinic flux.Measurements by 21 researchers from around the world. Photolysis Frequency of NO2: Measurement and Modeling During the International Photolysis Frequency Measurement and Modeling Intercomparison (IPMMI), R. E. Shetter, W. Swartz, et al., J. Geophys. Res., 108(D16), 10.1029/2002JD002932, 2003.

UMD jNO2 Actinometer Schematic

NO2 + h NO + O

tj

02NO NO

NO2 ][

Copyright © R. R. Dickerson 2013 4949

Problem for the student:

Show that for 1.00 ppm NO2, 1.00 atm pressure, exposure times of 1.00 s, and j(NO2) values of 10-2 s-1 the errors to:

from complicating reactions are less than 1%.

1. O + O2 + M → O3 + M k1 = 6.0 x10-34 cm6 s-1

2. O3 + NO → NO2 + O2 k2 = 1.9×10–14 cm3 s-1

3. O + NO2 → NO + O2 k3 = 1.04×10–11 cm3 s-1

02 ]NO[

]NO[~2

NO tj

Trailer UMD Actinometer

UMD Actinometer

inside

on top

quartz photolysis tube

UMD jNO2 Actinometer Data


DIRTY AIR (3') OH + CO H + CO2

(4') H + O2 + M HO2 + M

(5') HO2 + NO NO2 + OH

(6') NO2 + h NO + O

(7') O + O2 + M O3 + M

------------------------------------------------- (3'-7') CO + 2 O2 CO2 + O3 NET

Smog Machine

SmogO3

PANetc

NO2

NO

NO

NO2

CO2, H2O, HNO3

NOx, VOCs


EKMA. Empirical Kinetic

Modeling Approach, or EKMA. See Finlayson & Pitts page 892.

Copyright © 2012 R.R. Dickerson based on Chameides et al., 1992 56

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How fast do precursor pollutants make ozone (ppb/hr)?

Where is theBalt/Wash area?(boundary layer )

Smog chamber and modeling results on O3 formation rates.VOC’s (reactivity)

Where is Western MD?


Reactions in Solution (Also called multiphase or heterogeneous

reactions) Dr. Salawitch will show that the reaction

N2O5 + H2O → 2HNO3

Has favorable enthalpy and Gibbs Free Energy, but proceeds only in the condensed (aqueous) phase. Why does it not go in the gas phase?

This reaction involves breaking two bonds and forming two new bonds. That is too many. There is no gas-phase configuration that puts the two molecules into a favorable configuration.


Reactions Rate GuidelinesBreaking bonds requires energy.Forming bonds releases energy.Both processes require proximity.

Reactions where only one bond is broken (such as thermal dissociation or photolysis) proceed quickly if there is enough energy.

HO2NO2 → HO2 + NO2

Reactions where only one bond if formed (such as ozone formation) have negative activation energy and proceed more quickly at high pressures and low temperatures.

O + O2 + M → O3 + M

Reactions where only one bond is broken and one other is formed (such as thermal dissociation or photolysis) proceed quickly if there is enough energy released.

OH + VOC → R + H2O


Reactions Rate Guidelines, continued

Reactions where there are two bonds broken or two bonds formed proceed more slowly.

NO + NO + O2 → 2NO2

Reactions where the total of bonds broken and formed exceeds three proceed slowly or not at all in the gas phase. Here G is a ‘false friend’.

O3 + H2S → H2O + SO2

NH3 + O3 → H2O + HNO2

SO2 + H2O2 → H2SO4

(Count the bonds broken and formed.) None of these reactions proceed quickly in the gas phase. The final reaction is important in cloud water however.

Copyright © R. R. Dickerson 2013 60

Lecture Summary• Changes in enthalpy andentropy,H and S, are powerful indicators of

reaction probabilities and rates and are nearly independent of temperature.• Gibbs free energy provides the criterion of feasibility, but does not dictate

rates.• The residence time (lifetime), , is the inverse of the first order rate constant,

k.• If second or third order reactions can be approximated as first order then

lifetimes can be estimated.• For reversible reactions, kf/kr = Keq

• Photolysis rates can be both calculated and measured directly. • Tropospheric ozone production depends on the rate of formation of NO2 from

NO + RO2 and on UV flux, j(NO2). • The concentration of NO an UV flux are usually limiting.

spectroscopy and photochemistry - umd

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