Spectroscopic Analysis
Part 3 – Spectroscopy Experiments
Chulalongkorn University, Bangkok, Thailand January 2012
Dr Ron Beckett
Water Studies Centre
School of Chemistry
Monash University, Melbourne, Australia
Email: Ron.Beckett@monash.edu
Water
Studies
Centre
1
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
The lowest energy state
has the single electron in
the 1s orbital
1s1
2
Absorption and Emission of EMR
When EMR is absorbed or emitted by matter is
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
DE = hn
E1
E2
DE = hn
E1
3
E2
DE = hn
Intensity
E1
n
Frequency
Intensity
E2
DE = hn
n
E1
Frequency
4
1. Absorption Spectroscopy Experiments
Slit
Monochromator
Detector
Light
Source
Sample
Slit
Recorder
5
Techniques of Wavelength Selection
1. Filters
• Absorption filters
– Coloured glass or
gelatin
– Normally broad
spectral bandwidth
• Cutoff or bandwidth
filters
– Can be combined to
provide narrower
bandwidth
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Techniques of Wavelength Selection
2. Prism Monochromators
2
Entrance
slit
Collimating
lens
1
Prism
Bunsen prism monochromator
Focussing
lens
Focal
plane
Slit
7
Techniques of Wavelength Selection
3. Diffraction Grating Monochromators
– Glass or plastic plate covered with fine lines
– Reflect light of different wavelengths at different angles.
Condition for constructive interference (transmission)
must be achieved where the path difference between
adjacent beams must be an integral number of
wavelengths
n = d(sin i + sin r)
where n is the diffraction order
.
n=1
i
n=2
r
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3. Diffraction Grating Monochromators
Concave mirrors
Reflection Grating
Entrance slit
1
Exit slit
2
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3. Diffraction Grating Monochromators
10
EMR Detectors for Spectroscopy
1. Photographic Plates
11
EMR Detectors for Spectroscopy
2. Phototubes and Photomultipliers
Use the photoelectric effect to convert photons into
a measureable electric current
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EMR Detectors for Spectroscopy
3. Silicon Photodiodes
Consist of a p-n silicon junction which increases
in conductivity when exposed to UV-visible
radiation. The change in conductivity is used
to measure the light intensity.
Photo Diode Array Detectors
A series of such photodiodes can be constructed
and used to simultaneously detect the radiation
of different wavelengths separated by a
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monochromator
Origin of an Absorption Peak
Energy Transition
E2
DE = hn
E1
Absorption Spectrum
Intensity
n
Frequency
14
Absorption Spectrum
15
Absorption Spectrum
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2. Emission Spectroscopy Experiments
Sample
Monochromator
Detector
Excitation
Energy
Heat
Electrical
EMR
Slit
Recorder
17
Origin of an Emission Peak
Energy Transition
E2
DE = hn
Excitation
E1
Emission Spectrum
Intensity
n
Frequency
18
3. Fluorescence Spectroscopy Experiments
Monochromator
Sample
nem
Detector
nex
Slit
Slit
Monochromator
Slit
Recorder
Light
Source
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Origin of a Fluorescence Peak
E2
Energy Transition
E3
DE = hnex
Radiationless energy loss
DE = hnem
E1
Emission Spectrum
Intensity
nem
Frequency
20
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Forensic Application of Fluorescence
Visualization of fingerprints
Fingerprint visualized
by redwop fluorescent
fingerprint powder
Cyanoacrylate
fumed +
Rhodamine 6G
Cyanoacrylate
fumed
22
4. Chemiluminescence
Excitation to a higher molecular electronic state by
a chemical reaction followed by emission of EMR
E2
Excitation by a
chemical reaction
D E = hn
E1
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Chemiluminescence Observed in Nature
e.g. firefly, fungi, jellyfish, bacteria, crustacea and fish
all may exhibit bioluminescence.
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