photo chemistry spectroscopy

7/29/2019 photo chemistry spectroscopy

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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

photo chemistry spectroscopy

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