Post on 04-Apr-2018
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Required Reading: FP Chapter 3
Suggested 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 byphotons
Photolysis of O3 generates OH the most important atmospheric oxidizer:O3 + hv o O2 + O(1D)
O(1D) + H2O o 2 OH
Solar photodissociation of many atmospheric molecules is often much fasterthan any other chemical reactions involving them:
CF2Cl2 + hv o CF2Cl + Cl (photolysis of CFCs in the stratosphere)
HONO + hv o OH + NO (source of OH in the troposphere)
NO2 + hv o O + NO (source of O3 in the troposphere)
NO3 + hv o O2 + NO or O + NO2 (removal of NO3 generated at night)
Cl2 + hv o Cl + Cl (source of Cl atoms)
H2CO + hv o H2 + CO or H + HCO (important step of hydrocarbonoxidation)
etc.
Importance of Spectroscopy and Photochemistry II
Absorption of solar and earth radiation byatmospheric 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 todetect them in situ
OH is detected via its electronic transition at 310 nm
NH3 is detected via its fundamental vibrational transition at1065 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: electricand magneticfields oscillating inspace 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 = 0.1 nm = 10-10 m; micron = 10-6 m = 1000 nm
Solve in class: Calculate the energy, frequency, andwavenumber of a green photon (O = 530 nm).
O
Q
QO
1
c
v
hE
(wavenumber)
Types of radiation important in lower atmosphere
Ultraviolet and visible radiation (O = 100-800 nm) Excites bonding electrons in molecules Capable of breaking bonds in molecules (
photodissociation) Ultraviolet photons (O = 100-300 nm) have most energy, can
break more and stronger bonds. We will pay special attentionto them.
Infrared radiation (O = 0.8 - 300 Pm) Excites vibrational motions in molecules
With a very few exceptions, infrared radiation is not energeticenough to break molecules or initiate photochemicalprocesses
Microwave radiation (O = 0.5 - 300 mm) Excites rotational motions in molecules
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v',J',
v",J", E
photon
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 efficientlyif: The photon energy matches the energy
spacing between molecules quantum levels
Optical transition between these quantumlevels is allowed by selection rules
Forbidden transitions can occur but areweaker
Vibrational Energy & Transitions Bonds can be
viewed assprings
Energy levels arequantized, Ev = hvvib(v+1/2)
vvib is constantdependent on
molecule v = 0, 1, 2 isvibrationalquantum number
From F-P&P
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Vibrational Energy Levels Ideally: Harmonic Oscillator
Restoration force of springfollows Hookes 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 aremore closely spaced as vincreases
...yhxhhE eevib 32
21
21
21 vvv QQ
From F-P&P
Vibrational Selection Rules For ideal harmonic oscillator
'v = r1
For anharmonic oscillator 'v = r2, r3 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 momentchanges during vibration. E.g. HCl, CO, NO
Homonuclear diatomics, e.g. O2, N2 dont have v.t.
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Infrared Active and Inactive Modes
Only vibrational modesthat change the dipolemoment can interactwith light and lead toabsorption
CO2 is infrared active,but not all of its modesare
Rotational Energy and Transitions If molecule has permanent dipole
Rotation in space produces oscillating electric field
Can interact with lights fields and result in absorption
Only heteronuclear molecules
Rigid rotor No simultaneous vibration Allowed energy levels:
Nonrigid rotor
Spacing increases withJ
Spacing between levels small, many levels are populated
...)1()1( 22 JDJJBJErot
2
21
212
2
1
8
)1(
Rmm
mmI
I
hB
cmJBJE-
rot
whereS
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Rotational level manifolds for
different vibrational quanta
overlap with each other
Example: Ground Electronic State of HF
HF molecular constants
9 Bv=0 = 20.557 cm-1 (rotational constant)
9 Q = 4138.32 cm-1 (harmonic frequency)
9 Qxe = 89.88 cm-1 (anharmonicity)
v
)1(
hE
JBJE
EEE
vib
rot
vibrottotal
|
|
Possible
rovibrational
transition:
v=0 o v=1
J=14 o J=15
From S. Nidkorodov
Vibration-rotation of HCl Molecules vibrate and rotatesimultaneously
From F-P&P
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Electronic Energy and Transitions Several additional quantum numbers
/: 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
: = | /+ 6| 6 = +S, S-1, . , -S
g or u states
+ or - states of6 More complex selection
rules involving these numbers:
From F-P&P
Electronic Transitions (ETs) Molecules can undergo an
ET upon absorption of anappropriate 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 (10-15s) that internuclear distancecannot change
vertical transitions
From F-P&P
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Potential Energy Curves for an ET
At room T, v''=0 Prob of transitionproportional toproduct of vib.wavefucntions Transition to v'=4
in upperelectronic statemost intense
From F-P&P
Repulsive States No minima in PE
vs r curves
Dissociationoccursimmediately afterabsorption 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 todissociation if cross over tostate 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 Nis the number of atoms in the molecule:
H2O: N= 3 s= 3
C6H6: N= 12 s= 30
C60: N= 60 s= 174
2. Three independent axes of rotation, each characterized by its own rotationalconstant (A, B, C):
3. Complexity of the absorption spectrum increases very quickly with N. Newtypes of bands become possible:
Asymmetric topsA B C
H2O molecule, meat grinder
Prolate symmetric topsA
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Water has s = 3
vibrations:v1 = 1595 cm-1
v2 = 3652 cm-1
v3 = 3756 cm-1
It is a stronglyasymmetric top:
A = 27.9 cm-1
B = 14.5 cm-1
C= 9.3 cm-1
Overtone andcombinationbands are
relatively intense(only selectedbands shown inthe graph)
Example: Vibrational Spectrum of H2O
From S. Nidkorodov
v1+v3 combination bandshown a pure
vibrational transition. No obvious pattern in the
spectrum (this is verytypical for asymmetrictops).
Sample Near-IR Spectrum of H2
O
From S. Nidkorodov
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From Wayne
Pathways for Loss of e- Excitation
Photophysicalprocesses Lead to emission
of radiation Energy converted
to heat Read details in
book
Photochemicalprocesses
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 < O< 430 nm) o
NO(X2P) + O(3P)
Others: intramolecular rearrangments,
photoisomerization, photodimerization, H-atomabstraction, and photosensitized reactions Reminder: photochemistry drives the chemistry
of the atmosphere
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Quantum Yields (I) Relative efficiency of various photophysical and
photochemical processes:
E.g.: NO3 + hv oNO3* (3)NO3* oNO2 + O (4a)
oNO + O2 (4b)
oNO3 + hv (4c)
and so on
Ii Are wavelength dependent, all important at different O
absorbedphotonsofnumberTotaliprocessbyproceedingmoleculesexcitedofNumber
iI
absorbedphotonsofnumberTotal
formedmoleculesNOofNumber 24 aI
Quantum Yields IIFrom F-P&P