lecture 4 photochemistry

What is Photochemistry?

• Photochemistry: a chemical interaction involving radiation

• Why mention photochemistry?

Photochemistry plays an important role in atmospheric processes.

• The wavelengths in important atmospheric photochemical reactions are shortwave – radiation from the sun

lecture 4 photochemistry

Nuclear Energy = neutrons + protons minus binding energy

Atomic Weight/(Neutrons+Protons) does not equal 1 because of the binding energy. More binding energy, lower ratio AW/(N+P), lower total energy.

If two light nuclei join to form a heavier nucleus with higher binding energy (lower total energy), the extra energy is released as radiation – fusion.

What is the Sun’s Source of Energy?

lecture 4 photochemistry

Fusion is a thermonuclear reaction. It releases energy but needs a high enough temperature to bring the two nuclei together.

2H+2H 4He+E

2x2.0144.0026+0.0254(as energy)

Requirement, high temperature for activation ~25,000,000oC

This is the temperature in the sun’s interior, but not the temperature at the sun’s surface (the temperature at which the sun emits).

Can calculate Tsurface based on a steady state assumption (heat flow from interior balances heat loss from surface):

dT/dt=C1MsTinterior-C2(S.A.)sTsurface=0

4/3R3C’1Tinterior=4R2C’2Tsurface

Tsurface=C”RTinterior=~6000oK

Solar surface temperature is determined from the ratio of surface to volume.

Fusion

lecture 4 photochemistry

Energy per photon:E=h=hc/ h = 6.626 x 10-34 J s c = 3 x 108 m s-1

Energy of Radiation

lecture 4 photochemistry

Energy Per One Mole of Photons (Einstein)

Energy per one mole of photons at:

100nm = 290 kcal/mole

400nm = 72.5 kcal/mole

700nm = 41.5 kcal/mole

1000nm = 29 kcal/mole

Comparison to chemical bond strength:

N-N = 225 kcal/mole Very strong

O-O = 120 kcal/mole Strong

C-Cl =75 kcal/mole Intermediate

O-O2 = 35 kcal/mole Weak

HO…..H=5 kcal/mole Very weak

mol

kcal12390.0

mol

kJ 1

mol

kJ

nm

101.19625

hc 106.02

hcN hN moleper

523

00

E

lecture 4 photochemistry

What Happens When Radiation Hits a Molecule?

We learned in radiative transfer that two possible outcomes are:

1. scattering (no chemical interaction)

2. absorption:

Following absorption, there are a number of possibilities.

*ABh AB

lecture 4 photochemistry

Pathways Following Absorption

lecture 4 photochemistry

i. Dissociation/photolysis: breaking a chemical bond in the molecule

Energy of radiation must be greater than bond energy.

=100-1000nm is sufficient to break any chemical bond.

ii. Ionization: removing an electron from the molecule

In general ionization energy is greater than chemical bond strength:

He = 552 kcal/mole =52.6 nm}

N2 = 398 kcal/mole = 79.6 nm}

Na = 120 kcal/mole = 250 nm}

Pathways Following Absorption cont.

*)B(A* AB

eAB* AB

lecture 4 photochemistry

Table of Ionization Energies

lecture 4 photochemistry

Pathways Following Absorption cont.iii. Luminescence: re-emission of photon

hAB* AB

In atoms, re-emited photon is of same energy as excitation: em=ex. In molecules, it can be less: em>ex.

Flourescence: visible wavelengths

Phosphorescence: non-visible wavelengths

lecture 4 photochemistry

Pathways Following Absorption cont.

iv. Intramolecular energy transfer: conversion of the absorbed energy to several forms of lower energy (vibration, rotation and eventually to heat – typical for large molecules).

§

v. Intermolecular energy transfer

vi. Quenching

vii. Reaction: conversion to more active state and undergo selective chemical reactions

AB* AB

*CDABCD)( * AB

K.E.)( MABM* AB

speciesdifferent chemicallyEF* AB

lecture 4 photochemistry

What Determines the Pathway?

1. wavelength – whether or not its possible

2. population of excited states – whether or not its probable

3. conservation of orbital angular momentum and spin – whether or not its probable

lecture 4 photochemistry

rate of formation of AB* =

J for a photochemical reaction is the equivalent of a rate constant for a chemical reaction

J can be treated as a first order rate constant (units of time-1) but it depends on light intensity and spectral distribution.

Rate of a Photochemical Reaction

*ABh AB

]AB[ [AB*]

ABJdt

d

lecture 4 photochemistry

How is J Calculated?

For a given wavelength:

J{}=PF{} {} Y{}

PF{}: PhotoFlux

{} – Absorption cross section (population of excited states)

Y{} – Quantum yield (conservation of orbital angular momentum and spin)

Y{} =

The quantum yield is sometimes also symbolized

For a range of wavelengths (solar range):

J = PF{} {} Y{} d

absorbed photons ofnumber

process undergoing molecules ofnumber

1}{Y0

lecture 4 photochemistry

NO2+h NO + O(3P) J{NO2}

The rate of O(3P) formation

d[O(3P)]/dt = J{NO2} [NO2]

The rate of O formation will change diurnally even at constant NO2.

Photolysis Rate Example

lecture 4 photochemistry

J=Jmax cos() cos(

Jmax SRI/R2 SRI: solar radiation intensity

txx – Geographical Latitude – Seasonal motion of the earth cos(2{JD} /365)

JD: Julian Day

How Does J Vary With Latitude and Season?

lecture 4 photochemistry

The Solar Spectrum