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 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 1s 1 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 E2E2 E1E1  E = h E2E2 E1E1 3

E2E2 E1E1 E2E2 E1E1 Frequency Intensity Frequency Intensity 4

1. Absorption Spectroscopy Experiments Light Source Slit Sample Monochromator Slit Detector 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 6

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. ir Techniques of Wavelength Selection n = 2 n = 1 8

3. Diffraction Grating Monochromators 9

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1. Photographic Plates EMR Detectors for Spectroscopy 11

2.Phototubes and Photomultipliers Use the photoelectric effect to convert photons into a measureable electric current EMR Detectors for Spectroscopy 12

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 13

Origin of an Absorption Peak E2E2 E1E1  E = h Frequency Intensity Energy Transition Absorption Spectrum 14

Absorption Spectrum 15

Absorption Spectrum 16

2. Emission Spectroscopy Experiments Excitation Energy Heat Electrical EMR Sample Monochromator Slit Detector Recorder 17

Origin of an Emission Peak Frequency Intensity Energy Transition Emission Spectrum E2E2 E1E1  E = h Excitation 18

3. Fluorescence Spectroscopy Experiments Sample Monochromator Slit Detector Recorder Light Source Monochromator Slit ex em 19

Origin of a Fluorescence Peak E3E3 E1E1  E = h ex Frequency Intensity em Energy Transition Emission Spectrum  E = h em E2E2 Radiationless energy loss 20

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Forensic Application of Fluorescence Visualization of fingerprints Cyanoacrylate fumed Cyanoacrylate fumed + Rhodamine 6G Fingerprint visualized by redwop fluorescent fingerprint powder 22

4. Chemiluminescence E2E2 E1E1  E = h Excitation by a chemical reaction Excitation to a higher molecular electronic state by a chemical reaction followed by emission of EMR 23

Chemiluminescence Observed in Nature e.g. firefly, fungi, jellyfish, bacteria, crustacea and fish all may exhibit bioluminescence. 24