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![Page 1: Spectroscopic Analysis Part 3 – Spectroscopy Experiments Chulalongkorn University, Bangkok, Thailand January 2012 Dr Ron Beckett Water Studies Centre School.](https://reader036.fdocuments.us/reader036/viewer/2022062315/5697c02a1a28abf838cd7f37/html5/thumbnails/1.jpg)
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]
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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
The lowest energy state has the single electron in the 1s orbital
1s12
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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
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E2
E1
E = h
E2
E1
E = hFrequency
Intensity
Frequency
Intensity
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1. Absorption Spectroscopy Experiments
Light Source
Slit
Sample
Monochromator
Slit
Detector
Recorder5
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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
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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
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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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