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Required Reading: FP Chapter 3Suggested Reading: SP Chapter 3
Atmospheric ChemistryCHEM-5151 / ATOC-5151
Spring 2005Maggie Tolbert & Jose-Luis Jimenez
Lecture 5: Spectroscopy and Photochemistry I
Outline of Next Two Lectures• Today
– Importance of spectroscopy & photochemistry– Nature of light, EM spectrum– Molecular spectroscopy
• Thursday– The Sun as a radiation source– Light absorption– Atmospheric photochemistry
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Importance of Spectroscopy and Photochemistry I
• Most chemical processes in the atmosphere are initiated by photons
– Photolysis of O3 generates OH – the most important atmospheric oxidizer:O3 + hv → O2 + O(1D) O(1D) + H2O → 2 OH
– Solar photodissociation of many atmospheric molecules is often much faster than any other chemical reactions involving them:
CF2Cl2 + hv → CF2Cl + Cl (photolysis of CFCs in the stratosphere)HONO + hv → OH + NO (source of OH in the troposphere)NO2 + hv → O + NO (source of O3 in the troposphere)NO3 + hv → O2 + NO or O + NO2 (removal of NO3 generated at night)Cl2 + hv → Cl + Cl (source of Cl atoms)H2CO + hv → H2 + CO or H + HCO (important step of hydrocarbon
oxidation)etc.
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 (CO2, H2O, N2O, CFCs)– Stratospheric temperature inversion (O3 photochemistry)
• Spectroscopy of atmospheric molecules is used to detect them in situ
– OH is detected via its electronic transition at 310 nm– NH3 is detected via its fundamental vibrational transition at
1065 cm-1, etc.
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Solar Radiation: Initiator of Atmos. ReactionsAverage thermal energy of collisions:
~ RT = 8.3 J mol-1 K-1 x TRT = 2.5 kJ mol-1 @ 300 K
Energy of photons (E = hv):300 nm photon = 380 kJ mol-1
600 nm photon = 190 kJ mol-1
Typical bond strengths:D0(O2) = 495 kJ mol-1
D0(Cl2) = 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!!!
From S. Nidkorodov
What is 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 109 m/s
From F-P&P
Discuss in class: at a fundamental physical level, why are molecules capable of absorbing light?
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The Electromagnetic Spectrum
• Units used for photon energies and wavelengths:– 1 eV = 8065.54 cm-1 = 96.4853 kJ/mol = 23.0605 kcal/mol =
11604.4 K– 1 Å = 0.1 nm = 10-10 m; micron = 10-6 m = 1000 nm
• Solve in class: Calculate the energy, frequency, andwavenumber of a green photon (λ = 530 nm).
λ
νν
λ
1
c
=
=
=
v
hE
(wavenumber)
Types of radiation important in lower atmosphere• Ultraviolet and visible radiation (λ = 100-800 nm)
– Excites bonding electrons in molecules– Capable of breaking bonds in molecules (⇒
photodissociation)– Ultraviolet photons (λ = 100-300 nm) have most energy, can
break more and stronger bonds. We will pay special attention to them.
• Infrared radiation (λ = 0.8 - 300 µm)– Excites vibrational motions in molecules– With a very few exceptions, infrared radiation is not energetic
enough to break molecules or initiate photochemical processes
• Microwave radiation (λ = 0.5 - 300 mm)– Excites rotational motions in molecules
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v', J', …
v", J", …
E pho
ton
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
Vibrational Energy & Transitions• Bonds can be
viewed as “springs”
• Energy levels are quantized, – Ev = hvvib(v+1/2)– vvib is constant
dependent on molecule
– v = 0, 1, 2… is vibrational quantum number
From F-P&P
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Vibrational Energy Levels• Ideally: Harmonic Oscillator
– Restoration force of “spring” follows Hooke’s law: F= k ∆x
– Ev = hvvib(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
– Vibrational quantum levels are more closely spaced as vincreases
( ) ( ) ( ) ...yhxhhνE eevib ++++−+=32
21
21
21 vvv νν
From F-P&P
Vibrational Selection Rules• For ideal harmonic oscillator
– ∆v = ±1• For anharmonic oscillator
– ∆v = ±2, ±3 weaker “overtone” transitions can occur• At room T most molecules at v = 0
– Energy spacing of levels is large (~1000 cm-1)– v'‘ = 0 → v‘ = 1 is by far strongest
• For purely vibrational transition– Absorption of light can occur if dipole moment
changes during vibration. E.g. HCl, CO, NO– Homonuclear diatomics, e.g. O2, N2 don’t have v.t.
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Infrared Active and Inactive Modes• Only vibrational modes
that change the dipole moment can interact with light and lead to absorption
• CO2 is infrared active, but not all of its modes are
Rotational Energy and Transitions• If molecule has permanent dipole
– Rotation in space produces oscillating electric field– Can interact with light’s fields and result in absorption– Only heteronuclear molecules
• Rigid rotor– No simultaneous vibration– Allowed energy levels:
• Nonrigid rotor
• Spacing increases with J• Spacing between levels small, many levels are populated
...)1()1( 22 ++−+= JDJJBJErot
2
21
212
2
1
8
)1(
Rmm
mmII
hB
cmJBJE -rot
+==
+=
where
π
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Rotational level manifolds for different vibrational quanta
overlap with each other
Example: Ground Electronic State of HF
HF molecular constantsBv=0 = 20.557 cm-1 (rotational constant)ν = 4138.32 cm-1 (harmonic frequency)νxe = 89.88 cm-1 (anharmonicity)
v)1(
hνEJBJE
EEE
vib
rot
vibrottotal
≈+≈
+=
Possible rovibrational
transition:v=0 → v=1
J=14 → J=15
From S. Nidkorodov
Vibration-rotation of HCl• Molecules vibrate and rotate simultaneously
From F-P&P
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Electronic Energy and Transitions• Several additional quantum numbers
– Λ: related to electronic angular momentum– S: spin number
• Multiplicity = (2S + 1)• Mult = 1, 2, 3 are referred to as singlet, doublet, triplet• Most stable molecules have singlet ground states• O2 has triplet ground state, important exception
– Ω = | Λ+ Σ|• Σ = +S, S-1, …. , -S
– “g” or “u” states– “+” or “-” states of Σ
• More complex selection rules involving these numbers:
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” transitionsFrom F-P&P
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Potential Energy Curves for an ET• At room T, v''=0• Prob of transition
proportional to product of vib. wavefucntions– Transition to v'=4
in upper electronic state most intense
From F-P&P
Repulsive States• No minima in PE
vs r curves• Dissociation
occurs immediately after absorption of light
From F-P&P
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More complex case & Predissociation• Some repulsive and some
non-repulsive upper elec. states
• Example– Trans. to R causes
immediate dissociation– Trans. to E can lead to
dissociation if cross over to state R occurs
• “Predissociation”
– If high enough energy, trans. to E can yield A + B*
From F-P&P
1. Number of vibrations increases to s = 3N-6 (s = 3N-5 for linear molecules), where N is the number of atoms in the molecule:
H2O: N = 3 ⇒ s = 3C6H6: N = 12 ⇒ s = 30C60: N = 60 ⇒ s = 174
2. Three independent axes of rotation, each characterized by its own rotational constant (A, B, C):
3. Complexity of the absorption spectrum increases very quickly with N. New types of bands become possible:
Asymmetric tops A ≠ B ≠ C H2O molecule, meat grinder
Prolate symmetric tops A < B = C CH3F molecule; a pencil
Oblate symmetric tops A = B < C CH3 radical, planet Earth
ab
c ab
cbac
Sequence bands: one vibration excited while maintaining excitation in another vibration (allowed)
Combination bands: two different vibrations excited simultaneously (forbidden in harmonic approximation)
Overtone bands are also possible, just like for diatomic molecules (forbidden in harmonic approximation)
Polyatomic Molecules From S. Nidkorodov
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• Water has s = 3 vibrations:
v1 = 1595 cm-1
v2 = 3652 cm-1
v3 = 3756 cm-1
• It is a strongly asymmetric top:
A = 27.9 cm-1
B = 14.5 cm-1
C = 9.3 cm-1
• Overtone and combination bands are relatively intense (only selected bands shown in the graph)
Example: Vibrational Spectrum of H2O
From S. Nidkorodov
• v1+v3 combination band shown – a pure vibrational transition.
• No obvious pattern in the spectrum (this is very typical for asymmetric tops).
Sample Near-IR Spectrum of H2O
From S. Nidkorodov
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From Wayne
Pathways for Loss of e- Excitation• Photophysical
processes– Lead to emission
of radiation– Energy converted
to heat– Read details in
book• Photochemical
processes– Dissociation,
ionization, reaction, isomerization
Photochemical processes• Can produce new chemical species• Photodissociation
– most important by far– E.g. sole source of O3 in troposphere:NO2(X2A1) + hv (290 < λ < 430 nm) →
NO(X2P) + O(3P)• Others: intramolecular rearrangments,
photoisomerization, photodimerization, H-atom abstraction, and photosensitized reactions
• Reminder: photochemistry drives the chemistry of the atmosphere
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Quantum Yields (φ)• Relative efficiency of various photophysical and
photochemical processes:
• E.g.: NO3 + hv → NO3* (3)
NO3* → NO2 + O (4a)→ NO + O2 (4b)→ NO3 + hv (4c)
and so on
• φi Are wavelength dependent, all important at different λ
absorbed photons ofnumber Totali processby proceeding molecules excited ofNumber
=iφ
absorbed photons ofnumber Totalformed molecules NO ofNumber 2
4 =aφ
Quantum Yields IIFrom F-P&P