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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