1 spectroscopic analysis part 2 – electromagnetic radiation chulalongkorn university, bangkok,...
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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: [email protected]
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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