1. 1 Spectroscopic Analysis Part 2 – Electromagnetic Radiation Chulalongkorn University, Bangkok, Thailand January 2012 Dr Ron Beckett Water Studies Centre School of Chemistry Monash University, Melbourne, Australia Email: Ron.Beckett@monash.edu Water Studies Centre

2. 2 Spectroscopy Spectroscopy deals with the interaction of electromagnetic radiation (EMR) with matter. For example the absorption of visible light by a solution The amount of light absorbed depends on the colour or wavelength of the light and the concentration of the solution Spectroscopy can be used to identify compounds by studying the absorbance versus wavelength plots (spectrum) Spectroscopy can be used to analyse the concentration of a solution by measuring the amount of light absorbed There are other types of interactions of EMR with matter e.g. fluorescence, scattering, photo-electron emission

3. 3 Interactions of EMR with Matter Absorption – no re-emission energy dissipated as heat Absorption/Re-emission – if the same wavelength is emitted then it is called light scattering Absorption/Re-emission – if different wavelength then it is called fluorescence or phosphorescence Reflection from Surfaces – mirror, special geometry Photoemission of Electrons – EMR detectors

4. 4 Types of Electromagnetic Radiation (EMR) 1. Visible light – colours (VIBGYOR) 2. Infrared – radiated heat 3. Ultraviolet – sunburn 4. X-rays - medicine  -rays – cancer therapy

5. 5 Dual Nature of EMR Electromagnetic radiation has properties that can be described in terms either: 1.A wave train consisting of oscillating electric and magnetic fields travelling through space e.g. interference patterns, diffraction 2.A stream of particles (photons) e.g. absorption and emission spectroscopy photoemission of electrons from metals

6. 6 (a) (b) EMR as a Wave

7. 7 (a) EMR as a Wave

8. 8 (b) EMR as a Wave

9. 9 Wavelength and Wavenumber Wavelength ( in m) is the distance travelled during a complete oscillation of the wave Wavenumber ( in m -1 ) is the reciprocal of wavelength  - -

10. 10 EMR can be Described by a Sine Wave

11. 11 Period and Frequency Period (T in s) is the time for a complete oscillation Frequency ( in s -1 or Hertz i.e. Hz) is the number of oscillations per second = 1/T

12. 12 Velocity of EMR The velocity of EMR (c in m s -1 ) in a given medium is constant irrespective of the wavelength or frequency In vacuum c = 3.00 x 10 8 m s -1 And c = /T =

13. 13 Effect of Medium on EMR Air Water Frequency remains constant Velocity decreases with refractive index Wavelength decreases with RI since during one oscillation the wave travels a shorter distance in water than in air

14. 14 Einstein’s Interpretation of the Photoelectric Effect e EMR can eject electrons from certain materials but only if the frequency is above a given threshold Einstein argued that this must mean EMR consists of a stream of particles (now called photons) and the energy of a photon depends on the frequency of the EMR For electrons to be emitted the photon energy must exceed the work required for the electron to escape from the material

15. 15 EMR as a Particle EMR consists of a stream of particles called photons The energy of a photon (E in J photon -1 or J mol -1 ) is E = h (J photon -1 ) Or E = h  N 0  (J mol -1 ) Plank’s constant h = 6.63x10 -34 J s Avagadro’s number N 0 = 6.02x10 23 mol -1

16. 16 Intensity of EMR Intensity is the energy transmitted through unit area at right angles to the beam per second It is the energy per photon ( E photon ) times the number of photons passing unit area per second (N) I = E photon x NUnits – J m -2 s -1 = h  N Unit Area

17. 17 The type of EMR depends on wavelength, frequency, energy,etc Infrared - 3x10 12 – 3x10 14 Hz 100  m – 800 nm Visible light - 3x10 14 – 6x10 14 Hz 800 nm – 400 nm Ultraviolet - 6x10 14 – 3x10 16 Hz  800 nm – 10 nm X-rays - 3x10 16 – 3x10 18 Hz 10 nm – 100 pm

18. 18 Absorption or emission of specific types of EMR cause changes in different kinds of atomic or molecular energy 1. Microwave - molecular rotation 2. Infrared – molecular vibration 3. Visible light – outer shell electrons 4. Ultraviolet – outer valence electrons 5. X-rays – inner shell electrons

19. 19 The type of EMR depends on wavelength, frequency, energy,etc Absorption or emission of specific types of EMR cause changes in different kinds of atomic or molecular energy

20. 20 The EMR Spectrum Wavelength (nm)

21. 21 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 3d The lowest energy state has the single electron in the 1s orbital 1s 1

22. 22 Absorption and Emission of EMR When EMR is absorbed or emitted by matter it 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