Lecture 14b, Spectroscopy and Photochemistry II

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Spectroscopy and
Photochemistry
AOSC 620
R. Dickerson
Copyright © 2013 R. R. Dickerson
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Outline
• Additional details on spectroscopy and
photochemistry as they relate to atmospheric
chemistry.
• Direct measurements of photolysis rate
coefficients (frequencies).
• Tropospheric ozone.
Copyright © 2013 R. R. Dickerson
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Recap from AOSC 620
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
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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.
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Z.Q. Li
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Z.Q. Li
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Z.Q. Li
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Copyright © 2010 R. R. Dickerson &
Z.Q. Li
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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.
Copyright © R. R. Dickerson
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Spectroscopy: A Quick Qualitative Description, cont.
Energy states of Molecules: Molecular Quantum Numbers
L – 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.
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. Dickerson
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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/(8p2Ic), 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 = hnvib(n + ½) – hnvib(n + ½)2 + hnvib(n + ½)3 + …
Where nvib is a constant dependent on the bond strength and length.
Copyright © R. R. Dickerson
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Term Symbols for
Atoms and Molecules
SL
j
SL±
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 L = 0 1 2 3 4 5 …
Atoms = S P D F G H …
Molecules = S P D F G H ...
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. Dickerson
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Dr. Salawitch asked:
Why don’t O(1D) atoms relax to O(3P) in the
troposphere and stratosphere?
(wait for answer)
Where do O(1D) atoms relax to O(3P)?
and O (1S0) relax to O(1D2) ?
Copyright © R. R. Dickerson
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Z.Q. Li
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Transitions in oxygen atoms.
Copyright © 2011 R. R. Dickerson
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Selection Rules for Atomic Transitions
Dn = 0, 1, 2, … (no restrictions)
Dl = ± 1
Dj = 0, ± 1
DS = 0 This is the strongest rule: no multiplicity change.
DS ≠ 0 is a “forbidden” transition.
Let us examine O atoms as an example.
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Electrons in ground state Oxygen atoms: O(3P).
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Z.Q. Li
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Fraunhofer Lines in the solar spectrum
23
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.
Copyright © 2013 R. R. Dickerson &
Z.Q. Li
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Selection Rules for Molecular Transitions
DL = 0, ±1
DJ = ±1 in monatomic molecules
0, ±1 in polyatomic molecules
D n = ±1 for fundamental vibrations and rotations
±2, ±3… for overtones
(Frank-Condon principle for vibronic transitions)
DK = 0 for polyatomic molecules only.
g and u, no change.
+/- must change.
DS = 0 This is the strongest rule: no multiplicity change.
DS ≠ 0 is a “forbidden” transition.
Ross already examined O2 molecules as an example.
Copyright © R. R. Dickerson
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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.
Copyright © R. R. Dickerson
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Line shapes and Energy
Transition
Wavelength
(mm)
Energy
(kcal/mole)
Natural
Line 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?
Copyright © 2013 R. R. Dickerson &
Z.Q. Li
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Line Shapes
Natural broadening, an inherent property of all atoms and molecules,
is the result of the Heisenberg uncertainty Principle.
DEDt = h/2p
DlN = l2/(2pct)
DnN = (2pt)-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 © 2013 R. R. Dickerson
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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.
DlD  l(2RT/M)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 mm band of O3 adds to the greenhouse effect.
Copyright © 2011 R. R. Dickerson
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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 n = 0  n = 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, DJ = 0 is forbidden and there is
no Q branch.
R branch
P branch
Copyright © 20110 R. R. Dickerson
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Energy levels associated with the
IR Spectrum of HCl Centered at 3.5 mm
↑
Big Gap

Selection rules:
DJ = ± 1, not 0
for diatomics
Dv = ± 1
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Copyright © 2013 R. R. Dickerson &
Z.Q. Li
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Transmission spectrum of CO2
This is the bend near 15 mm; there is a a Q-Branch because DJ = 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?
Copyright © 2013 R. R. Dickerson &
Z.Q. Li
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Energy levels in molecular oxygen, O2
Ground state is 3Sg
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Potential Energy Curves for O2
O2 + hv  O(3P) + O(1D)
Herzberg band
DE ≥ 57,000 cm-1 or l ≤ 175 nm.
O2 + hv  2O(3P)
Schumann-Runge bands
DE ≥ 40,000 cm-1 or l ≤ 250 nm.
≥
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Z.Q. Li
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Z.Q. Li
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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 + 1D, so the latter is favored.
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Absorption Spectrum of Ozone.
Copyright © 2013 R. R. Dickerson &
Z.Q. Li
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Atmospheric radiation absorption as a fnx of wavelength.
Chappuis
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Z.Q. Li
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The Dobson Spectrometer.
Courtesy of: Ulf Köhler, DWD Hohenpeissenberg
Copyright © 2013 R. R. Dickerson
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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 mm.
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 © 2013 R. R. Dickerson &
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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.
Copyright © 2013 R. R. Dickerson &
Z.Q. Li
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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),
UMD jNO2
Actinometer
Schematic
NO2 + hn  NO +
O
j NO 2 
D NO
[NO 2 ] 0 D t
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:
j NO 2 ~
D [ NO ]
D t [ NO 2 ] 0
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
Copyright © R. R. Dickerson 2013
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Trailer
UMD Actinometer
UMD Actinometer
on top
inside
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 + hn
 NO + O
(7') O + O2 + M  O3 + M
------------------------------------------------(3'-7') CO + 2 O2  CO2 + O3
NET
Copyright © 2009 R. R. Dickerson
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Smog Machine
NOx, VOCs
Smog
O3
PAN
etc
CO2, H2O, HNO3
NO
NO2
NO2
NO
EKMA.
Empirical Kinetic
Modeling
Approach, or
EKMA. See
Finlayson & Pitts
page 892.
Copyright © 2009 R. R. Dickerson
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How fast do precursor pollutants make ozone (ppb/hr)?
Rural~1990
Urban ~1990
Where is the
Balt/Wash area?
(boundary layer )
Where is
Western MD?
VOC’s (reactivity)
Smog chamber and modeling results on O3 formation rates.
Copyright © 2012 R.R. Dickerson based on Chameides et al., 1992
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Reactions in Solution
(Also called multiphase or heterogeneous reactions)
Dr. Salawitch also showed 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 gasphase configuration that puts the two molecules into a
favorable configuration.
Copyright © 2013 R. R. Dickerson
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Reactions Rate Guidelines
Breaking 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 → O 3 + 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
Copyright © 2013 R. R. Dickerson
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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 DG 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 © 2013 R. R. Dickerson
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Lecture Summary
• Changes in enthalpy and entropy, DH and DS, 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), t, 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.
Copyright © R. R. Dickerson 2013
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