Spectroscopic Analysis Part 3 – Spectroscopy Experiments Chulalongkorn University, Bangkok, Thailand January 2012 Dr Ron Beckett Water Studies Centre School.

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

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

6

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

15

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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