1. INTRO TO SPECTROSCOPIC METHODS (Chapter 6 continued ) Quantum-Mechanical Properties Of Light Photoelectric Effect Photoelectric Effect Energy States of Atoms and Molecules Energy States of Atoms and Molecules Emission of Radiation Emission of Radiation –Line Spectra –Band Spectra –Continuum Spectra

2. Absorption of Radiation –Atomic Absorption –Molecular absorption Relaxation Processes –Nonradiative Relaxation –Fluorescence and Phosphorescence Relaxation

3. When exposed to incident light, a metal surface under vacuum will emit photoelectrons. First observed in 1887 by Hertz. Explained by Einstein in 1905

4. SHC, 6e Fig 6-13

5. Observations: 1) At constant ν, photocurrent ~ intensity of radiation. 2) Magnitude of the stopping voltage, V o depends on the frequency. 3) The stopping voltage depends on the metal of the photocathode. 4) The stopping voltage is independent of the intensity of radiation.

6. SHC, 6e Fig 6-14 Max KE of photoelectrons vs incident frequency

7. Absorption of light increases the energy of an atom or molecule. Emission of light decreases its energy. E0E0 E1E1 For atoms or ions only electronic states exist. Molecules have electronic, vibrational, and rotational states. Energy States of Atoms and Molecules

8. Fig 6-16 Emission methods

9. Emission of Radiation  Electromagnetic radiation produced when excited species (atoms, ions, or molecules) relax to lower energy levels  Excitation by light, heat, electricity, high speed particles, etc.  Result is an emission spectrum of signal vs. λ or ν E.g., Line Spectra of atoms

11. Emission of Radiation Line Spectra Band Spectra √

12. Fig 6-19 Flame emission spectrum of a brine solution

13. Emission spectrum of the phosphor used in fluorescent lamps blueyellow

14. Fig 6-21 Energy level diagrams for (a) sodium atom and (b) a simple polyatomic moleucle

15. Emission of Radiation Line Spectra Band Spectra Continuum Spectra √ √

16. Fig. 6-22Blackbody radiation curves

17. Output intensities of a D 2 lamp (UV) and a W lamp (vis) at 3200K.

18. Fig 6-16 Absorption methods

19. Absorption of Radiation  Atomic Absorption Electronic process alone Occurs in UV-vis region Only a few well-defined frequencies observed

20.  Molecular Absorption E = E electronic + E vibrational + E rotational Complex spectra over a wide range of λ  Typical energy separations: Electronic: 10,000 – 40,000 cm -1 Vibrational: 100 – 5000 cm -1 Rotational: 1 – 10 cm -1  In general, the larger the molecule the smaller the separations

21. Fig. 6-23 Typical UV-vis absorption spectra

22. Fig. 6-24 Partial energy level diagram for a molecule

23. Relaxation Processes I  Nonradiative Relaxation (radiantionless transitions) Vibrational relaxation (R) to lowest vib level of S 1 Internal conversion (IC): S 1  high vib level in S 0 Intersystem crossing (ISC): S 1  T 1

24. Relaxation Processes II  Radiative Relaxation Fluorescence: S 1  S 0  Fast transition ∼ ns –μs  Resonance fluorescence  Nonresonance fluorescence

25. Phosphorescence:T 1  S 0  Slower process ∼ μs – s  Involves spin-flip of electron

26. Physical processes that may occur after a molecule absorbs UV or vis light. continuum S0S0 allowedforbidden S0S0

27. Fig. 6-24 Attenuation of a beam of radiation by an absorbing solution Beer’s Law A = εbc