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Spectroscopic AnalysisPart 2 – Electromagnetic RadiationChulalongkorn University, Bangkok, Thailand January

2012

Dr Ron Beckett

Water Studies Centre School of Chemistry

Monash University, Melbourne, Australia

Email: Ron.Beckett@monash.eduWaterStudiesCentre

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Spectroscopy• Spectroscopy deals with the interaction of electromagnetic radiation (EMR) with matter.

• For example the absorption of visible light by a solution

• The amount of light absorbed depends on the colour or wavelength of the light and the concentration of the solution

• Spectroscopy can be used to identify compounds by studying the absorbance versus wavelength plots (spectrum)

• Spectroscopy can be used to analyse the concentration of a solution by measuring the amount of light absorbed

• There are other types of interactions of EMR with matter e.g. fluorescence, scattering, photo-electron emission

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Interactions of EMR with Matter

• Absorption – no re-emission energy dissipated as heat

• Absorption/Re-emission – if the same wavelength is emitted then it is called light scattering

• Absorption/Re-emission – if different wavelength then it is called fluorescence or phosphorescence

• Reflection from Surfaces – mirror, special geometry

• Photoemission of Electrons – EMR detectors

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Types of Electromagnetic Radiation (EMR)

1. Visible light – colours (VIBGYOR)

2. Infrared – radiated heat

3. Ultraviolet – sunburn

4. X-rays - medicine

5. -rays – cancer therapy

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Dual Nature of EMRElectromagnetic radiation has properties that

can be described in terms either:

1. A wave train consisting of oscillating electric and magnetic fields travelling through space

e.g. interference patterns, diffraction

2. A stream of particles (photons)e.g. absorption and emission spectroscopy

photoemission of electrons from metals

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(a) (b)

EMR as a Wave

7(a)

EMR as a Wave

8(b)

EMR as a Wave

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Wavelength and Wavenumber

Wavelength (in m) is the distance travelled during a complete oscillation of the wave

Wavenumber ( in m-1) is the reciprocal of wavelength

-

-

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EMR can be Described by a Sine Wave

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Period and Frequency

Period (T in s) is the time for a complete oscillation

Frequency ( in s-1 or Hertz i.e. Hz) is the number of oscillations per second

= 1/T

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Velocity of EMR

The velocity of EMR (c in m s-1) in a given medium is constant irrespective of the wavelength or frequency

In vacuum c = 3.00 x 108 m s-1

And c = /T =

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Effect of Medium on EMR

Air AirWater

• Frequency remains constant

• Velocity decreases with refractive index

• Wavelength decreases with RIsince during one oscillation the wave travels a shorter distance in water than in air

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Einstein’s Interpretation of the Photoelectric Effect

eEMR can eject electrons from certain materials but only if the frequency is above a given threshold

Einstein argued that this must mean EMR consists of a stream of particles (now called photons) and the energy of a photon depends on the frequency of the EMR

For electrons to be emitted the photon energy must exceed the work required for the electron to escape from the material

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EMR as a Particle

EMR consists of a stream of particles called photons

The energy of a photon (E in J photon-1 or J mol-1) is

E = h(J photon-1 )

Or E = hN0(J mol-1 )

Plank’s constant h = 6.63x10-34 J s Avagadro’s number N0 = 6.02x1023

mol-1

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Intensity of EMR• Intensity is the energy transmitted through unit area at right angles to the beam per second

• It is the energy per photon (Ephoton ) times the number of photons passing unit area per second (N)

I = Ephoton x N Units – J m-2 s-1

= h N

Unit Area

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The type of EMR depends on wavelength, frequency, energy,etc

Infrared - 3x1012 – 3x1014 Hz 100

m – 800 nm

Visible light - 3x1014 – 6x1014

Hz 800 nm – 400

nm

Ultraviolet - 6x1014 – 3x1016 Hz800 nm – 10 nm

X-rays - 3x1016 – 3x1018 Hz 10 nm – 100 pm

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Absorption or emission of specific types of EMR cause changes in different kinds of

atomic or molecular energy

1. Microwave - molecular rotation

2. Infrared – molecular vibration

3. Visible light – outer shell electrons

4. Ultraviolet – outer valence electrons

5. X-rays – inner shell electrons

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The type of EMR depends on wavelength, frequency, energy,etc

Absorption or emission of specific types of EMR cause changes in different kinds of atomic or molecular energy

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The EMR Spectrum

1020 1018 1016 1014 1012 108

Cosm

ic

rays

-rays X-rays UV Vis

ible

Infrared Microwave

Electronic excitation

Bond breaking and ionization Vibration Rotation

Visible Spectrum

400 500 600 700

Wavelength (nm)

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Energy is Quantized

The energy of atoms and molecules is quantized. They can only exist in allowed energy states or levels

Electronic energy levels in a H atom

1s 2s 2p 3s 3p 3d

The lowest energy state has the single electron in the 1s orbital

1s1

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Absorption and Emission of EMR

When EMR is absorbed or emitted by matter it does so in whole photons only (NOT fractions)

Absorption involves promotion from a lower energy state to a higher one

Emission results in a jump from a higher energy level to a lower energy level

E2

E1

E = h

E2

E1

E = h