spectroscopy and photochemistry aosc 637 r. dickerson fall 2011 copyright © 2011 r. r. dickerson1

Spectroscopy and Photochemistry

AOSC 637R. Dickerson

Fall 2011

Copyright © 2011 R. R. Dickerson 1

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

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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 4x103 atm cm 2.0x102 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) cl

Biomolecules, such as proteins of molecular weight ~1000, are destroyed by solar radiation at wavelengths around 280 nm. Assume that 1 g cm2 yr1 is the maximum allowable destruction rate.


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.

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Copyright © R. R. Dickerson 22

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.


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.


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


Selection Rules for Atomic Transitions

n = 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


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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.


Transitions in oxygen atoms.


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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.

Shortly, we will examine 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.


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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.

D RT½

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. Dickerson39

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


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Transmission spectrum of CO2

This is the bend; 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?


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


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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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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 + 1, 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


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 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.


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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.

Let’s look at an important photochemically active molecule in detail from the ground up, considering all possible reactions.

Example: Budget for Nitrous Acid, HONOReaction Ho (kJ/mole)

NO + NO2 + H2O ↔ 2HONO -41 (1)NO + OH + M → HONO + M† -209 (2)HONO + h → HO + NO +202 (3)

O + HONO → HO + NO2 -97 (4)

O3 + HONO → HO + NO2 -198 (5)

OH + HONO → H2O + NO2 -169 (6)

O2 + HONO → O + HNO3 +194 (7)

2NO2 + H2O (het) → HONO↑ + HNO3 (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 Ho 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. Copyright © 2010 R. R. Dickerson 54


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300 nm400 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 = NO2 →HONO (RH) x S/V x vNO2/4 x [NO2]HONO (RH) x S/V x vHONO/4 x [HONO]

Where is the accommodation coefficientS/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.


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 courseHow do gases interact with these particles:1. Gas phase diffusion to surface of droplet2. Transport across air-water interface3. Diffusion of solvated species into bulk phase of droplet4. Reaction of species in aqueous phase or at interface


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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.

spectroscopy and photochemistry aosc 637 r. dickerson fall 2011 copyright © 2011 r. r. dickerson1

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