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Spectroscopic AnalysisPart 3 – Spectroscopy Experiments
Chulalongkorn University, Bangkok, Thailand January 2012
Dr Ron Beckett
Water Studies Centre School of Chemistry
Monash University, Melbourne, Australia
Email: [email protected]
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
1s12
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
E1
E = h
E2
E1
E = h
3
E2
E1
E = h
E2
E1
E = hFrequency
Intensity
Frequency
Intensity
4
1. Absorption Spectroscopy Experiments
Light Source
Slit
Sample
Monochromator
Slit
Detector
Recorder5
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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Entrance slit
Collimating lens
Prism
Focussing lens
Slit
Focal plane
1
2
Bunsen prism monochromator
2. Prism Monochromators
Techniques of Wavelength Selection
7
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
.
i r
Techniques of Wavelength Selection
n = 2n = 1
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3. Diffraction Grating Monochromators
1 2
Exit slit
Reflection Grating
Entrance slit
Concave mirrors
Czerny-Turner grating monochromator
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3. Diffraction Grating Monochromators
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1. Photographic Plates
EMR Detectors for Spectroscopy
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2. Phototubes and Photomultipliers
Use the photoelectric effect to convert photons into a measureable electric current
EMR Detectors for Spectroscopy
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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 monochromator
EMR Detectors for Spectroscopy
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Origin of an Absorption Peak
E2
E1
E = h
Frequency
Intensity
Energy Transition
Absorption Spectrum
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Absorption Spectrum
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Absorption Spectrum
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2. Emission Spectroscopy Experiments
Excitation Energy
HeatElectricalEMR
SampleMonochromator
Slit
Detector
Recorder
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Origin of an Emission Peak
Frequency
Intensity
Energy Transition
Emission Spectrum
E2
E1
E = hExcitation
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3. Fluorescence Spectroscopy Experiments
SampleMonochromator
Slit
Detector
Recorder
Light Source
Monochromator
Slit
Slit
ex
em
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Origin of a Fluorescence Peak
E3
E1
E = hex
Frequency
Intensity
em
Energy Transition
Emission Spectrum
E = hem
E2
Radiationless energy loss
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Forensic Application of Fluorescence
Visualization of fingerprints
Cyanoacrylatefumed
Cyanoacrylatefumed +Rhodamine 6G
Fingerprint visualized by redwop fluorescent fingerprint powder 22
4. Chemiluminescence
E2
E1
E = hExcitation by a chemical reaction
Excitation to a higher molecular electronic state by a chemical reaction followed by emission of EMR
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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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