Download Lecture #8

Spectroscopy and
Photochemistry
AOSC 637
R. Dickerson
Fall 2011
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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
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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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I/I0 = 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.
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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/I0) = -scl
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.
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The maximum allowable destruction rate of 1 g cm−2 yr−1 is the
same as:
10−3 moles cm−2 yr−1 or 6x1020 molecules/cm2 yr.
If we also assume a quantum yield of unity (each photon
absorbed causes a broken molecule) then the limit is: 6x1020
UV photons/cm2 yr.
The lethal dosage is anything greater than about:
2x1013 UV photons/cm2 s
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Extraterrestrial solar flux
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Example: Photolysis of molecular oxygen.
This problem left for students.
f 5l
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Start 3/3/11 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.
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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.
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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.
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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
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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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Fraunhofer Lines in the solar spectrum
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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.
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Transitions in oxygen atoms.
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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.
Shortly, we will examine O2 molecules as an example.
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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.
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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?
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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.
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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.
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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
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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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Transmission spectrum of CO2
This is the bend; 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?
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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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Cross sections are greatest
for allowed transitions.
Continua form where there
are dissociative states, and
bands form where transitions
between bound states occur.
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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.
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Atmospheric radiation absorption as a fnx of wavelength.
Chappuis
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The Dobson Spectrometer.
Courtesy of: Ulf Köhler, DWD Hohenpeissenberg
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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.
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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.
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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
DHo (kJ/mole)
NO + NO2 + H2O ↔ 2HONO
-41
NO + OH + M → HONO + M†
-209
HONO + hn → HO + NO
+202
O + HONO → HO + NO2
-97
O3 + HONO → HO + NO2
-198
OH + HONO → H2O + NO2
-169
O2 + HONO → O + HNO3
+194
2NO2 + H2O (het) → HONO↑ + HNO3 (aq) -1.75
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(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 DHo
have a prohibitively low rate constant. Students should calculate kmax to prove
that this is an irrelevant reaction. In general oxidation by molecular oxygen is
too slow to be important in the atmosphere.
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300 nm
400 nm
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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.
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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.
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Stutz at al., Atmos.
Environ., 2009.
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From Stutz et al., (JGR, 2004)
d[HONO]/dt = gNO2 →HONO (RH) x S/V x vNO2/4 x [NO2]
- gHONO (RH) x S/V x vHONO/4 x [HONO]
Where g 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.
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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.
2.
3.
4.
Gas phase diffusion to surface of droplet
Transport across air-water interface
Diffusion of solvated species into bulk phase of droplet
Reaction of species in aqueous phase or at interface
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Uptake and Reaction of Gases in Liquids
2
1
• Diffusion of gases
fast relative to in
aqueous phase
3
4
• Dg ~ 0.1-1 cm2 s-1
• Daq ~ 10-5 cm2 s-1
• Most cases gas
phase diffusion is
not slowest step
From Finlayson-Pitts and Pitts
4
D
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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.
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