Spectroscopic Analysis Part 3 – Spectroscopy Experiments

1. Spectroscopic AnalysisPart 3 – Spectroscopy Experiments Chulalongkorn University, Bangkok, Thailand January 2012 Dr Ron Beckett Water Studies Centre School of ChemistryMonash University, Melbourne, Australia Email: Ron.Beckett@monash.edu Water Studies Centre

2. 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

3. E2 DE = hn DE = hn E1 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

4. E2 DE = hn DE = hn E1 Intensity E2 Intensity n Frequency E1 n Frequency

5. Slit Monochromator Detector Light Source Sample Slit Recorder 1. Absorption Spectroscopy Experiments

6. 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

7. Techniques of Wavelength Selection 2. Prism Monochromators

8. 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 nl = d(sin i + sin r)where n is the diffraction order . n = 1 n = 2 i r

9. 3.Diffraction Grating Monochromators

10. 3.Diffraction Grating Monochromators

11. EMR Detectors for Spectroscopy 1. Photographic Plates

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

13. EMR Detectors for Spectroscopy • 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

14. E2 DE = hn E1 Intensity n Frequency Origin of an Absorption Peak Energy Transition Absorption Spectrum

15. Absorption Spectrum

16. Absorption Spectrum

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

18. E2 Excitation DE = hn E1 Origin of an Emission Peak Energy Transition Emission Spectrum Intensity n Frequency

19. 3. Fluorescence Spectroscopy Experiments Monochromator Sample nem Detector nex Slit Slit Monochromator Recorder Slit Light Source

20. Intensity nem Frequency Origin of a Fluorescence Peak E2 Radiationless energy loss Energy Transition E3 DE = hnex DE = hnem E1 Emission Spectrum

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

23. Excitation to a higher molecular electronic state by a chemical reaction followed by emission of EMR E2 DE = hn 4. Chemiluminescence Excitation by a chemical reaction E1

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