Chapter 1

Chapter 1
Photochemical principles
Dr. Suzan A. Khayyat 1

Photochemistry
• Photochemistry is concerned with the absorption,

excitation and emission of photons by atoms,
atomic ions, molecules, molecular ions, etc.
• Photochemistry is the study of the interactions
between atoms, molecules, and light.
• The simplest photochemical process is seen with
the absorption and subsequent emission of a
photon by a gas phase atom such as sodium.

• When the sodium atom absorbs a photon it is
said to be excited. After a short period of
time, the excited state sodium atom emits a
photon of 589 nm light and falls back to the
ground state:

Photosynthesis

THE NATURE OF LIGHT
Light is an ELECTROMAGNETIC WAVE

Light is also a PARTICLE: the PHOTON
This is principle of
Wave- Particle Duality
theory.

• Einstein:
Radiation behaves as though it consists
of a stream of particles, or photons. Each
of these photons has a fixed energy
depends on the frequency or wavelength.

Quantum Theory
• Max Blanck:
The chemical changes wrought by
electromagnetic radiation in its interaction
with matter would be very difficult to
interpret without assuming that the
radiation can behave as particles whose
energy is quantized.
E=hʋ
1- Light as a Waves
The Seven Bands of the EM Spectrum
 Microwave or millimeter
Dr.between Radio and IR
Suzan A. Khayyat 11
Visible light is that portion of the
electromagnetic spectrum which stimulates
the retina of the
human eye.
Visible spectrum
wavelengths range
from about 400 nm (violet) to 760 nm (red).
Light travels at about 3 x 108 m/s through
empty space and slightly slower through air.
Remember that for all waves, v = f.

The Wave Nature of light
• There are two types of waves:

1- Transverse waves: light waves.
(electromagnetic waves, water waves )
2- Longitudinal waves: sound waves.
Electromagnetic field= (electric +magnetic) field
• Electric (E) fields oscillate perpendicular to

Magnetic (B) fields.

James Clerk Maxwell
Transverse of Electric and magnetic fields (EM) in planes
perpendicular to each other

Electric and Magnetic Fields
• Charged particles
(protons or ions +,
electrons -) attract
or repel each other.
• Electric fields
accelerate charged
particles along the
lines.
• Charged particles
orbit around
magnetic field lines.
Characteristics of All Waves
• Frequency (f or  [nu]): oscillations per sec (Hz)

• Speed (v): depends on medium (m/s)
• Wavelength ( [lambda]): distance between crests (nm)
• Amplitude (A): strength of oscillation

Wavelength and Frequency for
Light
wavelength x frequency = speed of light = constant

v = f. Dr. Suzan A. Khayyat 17
Regions of the
electromagnetic spectrum
• Visible Light: 400–700 nanometer (nm)
wavelength range
• Ultraviolet: 100–400 nm wavelength range
• Near Infrared: 700–1000 nm wavelength
range
• Far infrared: 15–1000 micrometer (µm)
wavelength range

How are Electromagnetic Waves Made?
• Most come from
ATOMIC,
MOLECULAR or
NUCLEAR TRANSITIONS.
I.e., electrons or protons changing quantum
states.
• BUT FUNDAMENTALLY, EM RADIATION IS
PRODUCED BY AN ACCELERATED CHARGED
PARTICLE.
• Since ELECTRONS have the LOWEST MASSES
they are MOST EASILY ACCELERATED,
therefore, electrons produce most EM waves.

Shortest Wavelengths
• ULTRAVIOLET (UV): 380 nm >  > 300Å = 30nm
Mostly absorbed in atmosphere: ozone (O3)
Good thing, since UV radiation causes skin
cancer.
• X-RAY: 300 Å >  > 0.1 Å = 0.01nm,
Absorbed in atmosphere: by any atom (N, O)
A good thing too: X-rays can penetrate the body
and cause cancer in many organs.
• GAMMA-RAY (-ray):  < 0.1 Å =0.01 nm,
The most energetic form of EM radiation.
Absorbed high in atmosphere: by any atomic
nucleus.
A VERY good thing: gamma-rays quickly cause
severe burns and cancer.
Colors of Light
• White light is made up of many different colors

How do Waves Interact with Matter?
• EMIT (light is sent out when a bulb is turned on)
• REFLECT (angle of incidence = angle of reflection) or
Scatter (spread out reflection)
• TRANSMIT (low opacity)
• ABSORB (high opacity)
• REFRACT (bend towards normal when entering a
medium with a slower propagation speed)
• INTERFERE (only a WAVE can do this) Either:
CONSTRUCTIVE (waves add when in phase)
DESTRUCTIVE (waves cancel when out of phase)
• DIFFRACT (only a WAVE can do this)
Waves spread out when passing through a hole or slit.
This is important only if the size of the hole or slit is
comparable to the Dr. wavelength.
Suzan A. Khayyat 24
Reflection and Scattering
Mirror reflects Movie screen scatters light

light in a particular in all directions
direction
Interactions of Light with Matter
Interactions between light and matter determine the appearance of

everything around us: objects reflect some wavelengths, absorb others and
emit others.
Blue light is (compared to red light),
1. Shorter wavelength
2. Longer wavelength
3. Higher energy photons
4. 1 and 3
5. None of the above

We can’t see infrared, but we can
perceive it as:
1. Heat
2. Radar
3. Sound
4. AM
5. FM

When light approaches matter, it
can
1. Be absorbed by the atoms in the matter

2. Go through the matter, and be transmitted
3. Bounce off the matter, and be reflected
4. Any of the above
5. Only 2 or 3

When light approaches matter, it
can
1. Be absorbed by the atoms in the matter
2. Go through the matter, and be transmitted
3. Bounce off the matter, and be reflected
4. Any of the above
5. Only 2 or 3

Thought Question
Why is a rose red?
a) The rose absorbs red light.

b) The rose transmits red light.
c) The rose emits red light.
d) The rose reflects red light.

Thought Question
Why is a rose red?
a) The rose absorbs red light.

b) The rose transmits red light.
c) The rose emits red light.
d) The rose reflects red light.

2- Light as Particles
• ELECTROMAGNETIC ENERGY IS CARRIED BY

PHOTONS:
• A PHOTON is a SINGLE QUANTUM OF LIGHT.
• The energy of one photon of a particular frequency is:
E = hʋ = h c / 
h = 6.63 x 10-34 Joule sec = is PLANCK's CONSTANT.
c is the speed of light.

Thought Question
The higher the photon energy…
a) the longer its wavelength.

b) the shorter its wavelength.
c) energy is independent of wavelength.

Thought Question
The higher the photon energy…
a) the longer its wavelength.

b) the shorter its wavelength.
c) energy is independent of wavelength.

How can light behave as both a wave
and a particle?
1. It doesn’t really
2. It really is simultaneously both a wave and a
particle
3. Light and small objects such as atoms behave
in ways we never see in everyday objects, so
we can’t describe them in everyday terms
4. This is what quantum mechanics describes
5. 3 and 4

Thought Question
Why don’t we glow in the dark?
a) People do not emit any kind of light.

b) People essentially only emit light that is
invisible to our eyes.
c) People are too small to emit enough light for
us to see.
d) People do not contain enough radioactive
material.

WHAT IS THE STRUCTURE OF
MATTER?
Electron
Cloud
Atom
Nucleus
Nucleus size only around 10-15m while electron

clouds are roughly 10-10m = 0.1 nm = 1Å
As nearly all of the mass is in the nucleus, matter is
mostly empty space! Dr. Suzan A. Khayyat 38
Atomic Terminology
• Atomic Number = # of protons in nucleus
• Atomic Mass Number = # of protons + neutrons
• Molecules: consist of two or more atoms (H2O, CO2)

Atomic Terminology
• Isotope: same # of protons but different # of
neutrons. (4He, 3He)

What is found in the nucleus of
atoms?
1. Protons with a + charge
2. Neutrons with no charge
3. Electrons with a – charge
4. All of the above
5. 1 and 2

What is found in the nucleus of
atoms?
1. Protons with a + charge
2. Neutrons with no charge
3. Electrons with a – charge
4. All of the above
5. 1 and 2

How is the isotope 14C different
from 12C?
1. It has more protons
2. It has more neutrons
3. It has more electrons
4. All of the above

How is the isotope 14C different
from 12C?
1. It has more protons
2. It has more neutrons
3. It has more electrons
4. All of the above

Phase Changes
• Ionization: Stripping of
electrons, changing atoms into
plasma
• Dissociation: Breaking of
molecules into atoms
• Evaporation: Breaking of
flexible chemical bonds,
changing liquid into gas
• Melting: Breaking of rigid
chemical bonds, changing solid
into liquid
• Sublimation: from solid to gas
Review of the Nature of Matter
• What is the structure of matter?

– Matter is made of atoms, which consist of a
nucleus of protons and neutrons surrounded by a
cloud of electrons
• What are the phases of matter?
– Adding heat to a substance changes its phase by
breaking chemical bonds.
– As temperature rises, a substance transforms from
a solid to a liquid to a gas, then the molecules can
dissociate into atoms
– Stripping of electrons from atoms (ionization) turns
the substance into a plasma
Absorption and emission of
radiation

• Energy levels in matter are quantized. In
addition to its translational energy, a
species may possess other sorts of
internal energy. Each of which is
quantized: rotational energy, vibrational
energy which arises from the periodic
oscillation of atoms in a molecule, and
electronic energy which depends on the
distance of the electron from the nucleus
and the type of orbital that it occupies.
• When a species absorbs a quantum of
radiation, it becomes excited. How the
spices assimilates that energy __ in
rational, vibrational, or electronic modes
__ depends on the wavelength of the
indicate radiation. The longer the
wavelength of electromagnetic
radiation, the lower the energy.
Absorption & Emission
Rapid process(10-15s)

Absorption and Emission of Photons

http://micro.magnet.fsu.edu/optics/lightandcolor/frequency.html
Absorption and Emission
Absorption Emission
Absorption: A transition from a lower level to a higher level with transfer of

energy from the radiation field to an absorber, atom, molecule, or solid.
Emission: A transition from a higher level to a lower level with transfer of

energy from the emitter to the radiation field. If no radiation is emitted.
http://www.chemistry.vt.edu/chem-ed/spec/spectros.html
• Types of emission:
1- Stimulated emission
eg. laser
2- Spontaneous emission.
eg. Fluoresecence, phosphorescence and
chemiluminescence.

Absorption and emission pathways

McGarvey and Gaillard, Basic Photochemistry at
http://classes.kumc.edu/grants/dpc/instruct/index2.htm
Singlet State
(S 1,S2, ......)
1(n,   Triplet State
(T1, T2, ...)
     Biological
Photochem. Response
A F ISC
b l n   
s u
o o
r r
p
e      photochem. &
s
t c singlet oxygen
i e
o n
c Phosphorescence
n
e
Ground State
So
Jablonski energy diagram

Beer-Lambert Law
• The Beer-Lambert law is of particular
importance in determining the intensity
of absorbed radiation in photochemical
experimentation, and in calculating
concentrations from absorption
measurements.

Electronic excitation
• First, the energies are of the same order
of magnitude as bond energies, so that
electronic excitation can have a
considerable on the bond in species.
Secondly, the energies correspond
roughly with typical activation energies
for reactions, and the excitation energy
can help a species to either partially or
completely overcome an activation
barrier.
Singlet and Triplet Excited States
http://www.shu.ac.uk/schools/sci/chem/tutorials/molspec/lumin1.htm
Selection Rules
In electronic spectroscopy there are three selection rules which
determine whether or not transitions are formally allowed:
1. Spin selection rule: DS = 0
allowed transitions: singlet  singlet or triplet  triplet
forbidden transitions: singlet  triplet or triplet  singlet
Changes in spin multiplicity are forbidden

http://www.shu.ac.uk/schools/sci/chem/tutorials/molspec/lumin1.htm
Selection rules
2. Laporte selection rule: there must be a change in the parity

(symmetry) of the complex
Laporte-allowed transitions: g  u
Laporte-forbidden transitions: g  g or uu
g stands for gerade – compound with a center of symmetry

u stands for ungerade – compound without a center of symmetry
3. Selection rule of Dℓ = ± 1 (ℓ is the azimuthal or orbital quantum

number, where ℓ = 0 (s orbital), 1 (p orbital), 2 (d orbital), etc.)
allowed transitions: s  p, p  d, d  f, etc.

forbidden transitions: s  s, d  d, p  f, etc.
s and s* orbitals

http://www.cem.msu.edu/~reusch/VirtualText/intro3.htm#strc8a
 and * orbitals

Electronic Transitions:   *
http://www.cem.msu.edu/~reusch/VirtualText
/Spectrpy/UV-Vis/uvspec.htm#uv2
The   * transition involves orbitals that

have significant overlap, and the probability is
near 1.0 as they are “symmetry allowed”.

  * transitions - Triple bonds
Organic compounds with -C≡C- or -C≡N groups, or transition

metals complexed by C≡N- or C≡O ligands, usually have “low-
lying” * orbitals
Electronic Transitions: n  *
http://www.cem.msu.edu/~reusch/VirtualText
/Spectrpy/UV-Vis/uvspec.htm#uv2
The n-orbitals do not overlap at all well with the
* orbital, so the probability of this excitation is
small. The e of the n* transition is about 103
McGarvey and Gaillard, Basic
times smaller than e for the * transition as
Photochemistry at it is “symmetry forbidden”.
http://classes.kumc.edu/grants/dpc/instruc Dr. Suzan A. Khayyat 68
t/index2.htm
Absorption and emission pathways

Fates of excited state
Photophysical processes Lead to emission of
radiation Energy converted to heat
• Photochemical processes Dissociation,
ionization, reaction, isomerization
Laws of photochemistry
• The first law of photochemistry:

Theodor Grotthuss and John W. Draper),
states that light must be absorbed by a
chemical substance in order for a
photochemical reaction to take place.

• The second law of photochemistry:
the Stark-Einstein law, states that for each
photon of light absorbed by a chemical
system, only one molecule is activated for a
photochemical reaction

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Chapter 1

Dr. Suzan A. Khayyat 1

• Photochemistry is concerned with the absorption,

Dr. Suzan A. Khayyat 2

Dr. Suzan A. Khayyat 3

Dr. Suzan A. Khayyat 4

Light is an ELECTROMAGNETIC WAVE

Dr. Suzan A. Khayyat 8

Dr. Suzan A. Khayyat 9

Remember that for all waves, v = f.

• There are two types of waves:

Electromagnetic field= (electric +magnetic) field

• Electric (E) fields oscillate perpendicular to

Dr. Suzan A. Khayyat 13

Dr. Suzan A. Khayyat 14

• Frequency (f or  [nu]): oscillations per sec (Hz)

Dr. Suzan A. Khayyat 16

wavelength x frequency = speed of light = constant

Dr. Suzan A. Khayyat 20

Dr. Suzan A. Khayyat 21

• White light is made up of many different colors

Mirror reflects Movie screen scatters light

Interactions between light and matter determine the appearance of

Dr. Suzan A. Khayyat 27

Dr. Suzan A. Khayyat 28

1. Be absorbed by the atoms in the matter

Dr. Suzan A. Khayyat 29

Dr. Suzan A. Khayyat 30

a) The rose absorbs red light.

Dr. Suzan A. Khayyat 31

a) The rose absorbs red light.

Dr. Suzan A. Khayyat 32

• ELECTROMAGNETIC ENERGY IS CARRIED BY

Dr. Suzan A. Khayyat 33

a) the longer its wavelength.

Dr. Suzan A. Khayyat 34

a) the longer its wavelength.

Dr. Suzan A. Khayyat 35

Dr. Suzan A. Khayyat 36

a) People do not emit any kind of light.

Dr. Suzan A. Khayyat 37

Nucleus size only around 10-15m while electron

• Molecules: consist of two or more atoms (H2O, CO2)

Dr. Suzan A. Khayyat 40

Dr. Suzan A. Khayyat 41

Dr. Suzan A. Khayyat 42

Dr. Suzan A. Khayyat 43

Dr. Suzan A. Khayyat 44

• What is the structure of matter?

Dr. Suzan A. Khayyat 48

Dr. Suzan A. Khayyat 52

Dr. Suzan A. Khayyat 53

Absorption: A transition from a lower level to a higher level with transfer of

Emission: A transition from a higher level to a lower level with transfer of

Dr. Suzan A. Khayyat 55

Dr. Suzan A. Khayyat 56

Jablonski energy diagram

Dr. Suzan A. Khayyat 57