Chapter 6

1. Chapter 6 Introduction to Spectroscopy

2. I. Spectroscopic Methods

3. II. Properties of EM Radiation • Talk about EM waves by describing it as a sine wave

5. II. Properties of EM radiation • 2-D electric field

6. Spectrum of Electromagnetic Radiation Electromagnetic radiation is light. Different energy of light can interact with different energetics in matter. Radiofrequency MHz Nuclear spin NMR Microwave GHz Electron spin ESR (EPR) Molecular rotation Microwave Spec. Infrared THz Molecular vibration IR Spec. Vibrations in solids UV/Visible PHz Electronic transitions UV/Vis Spec. (valence shell) Far UV/X-ray PHz Electronic transitions UV/X-ray Spec. (core shells) Gamma ray EHz Nuclear transitions Mössbauer Spec.

8. A. Superposition of waves • Constructive and destructive interference • Important aspect of superposition is the use of Fourier transform which can deconvolute complex waves into just sin and cos waves

9. B. Diffraction of Radiation • Process in which parallel beams are bent as they pass by a narrow slit

10. C. Transmission of Radiation • Radiation is propagated through material slower than through a vacuum • It is a polarization of the material on the atomic and molecular in the medium that is only momentarily retained (10-14 – 10-15s)

11. D. Refraction of Radiation • When radiation passes at an angle through an interface between 2 transparent media the beam refracts

12. E. Reflection of Radiation • If one of the media is not air then reflection occurs

13. Example • Calculate the percent loss of intensity due to reflection of a perpendicular beam of yellow light as it passes through a glass cell containing water.

14. F. Photoelectric Effect • Photocathode is hit with monochromatic radiation which causes electrons to be emitted with range of kinetic energy

15. F. Photoelectric Effect • EM radiation releases electrons from metal surfaces and give them enough kinetic energy to travel to a negatively charged electrode • Also the number of photoelectrons is proportional to the intensity of the incident beam

16. G. Energy States of Chemical Species • Max Planck proposed quantum theory in 1900 • Has 2 main postulates

17. G. Energy States of Chemical Species • For atoms in their elemental states are called electronic states • For molecules they also have quantized vibrational and rotational states

18. IV. Emission of Radiation • EM radiation is produced when an excited particle such as an atom or ion molecule relaxes giving up its excess energy

19. A. Line Spectra • Excitation of atoms in the gas phase using UV-Vis

20. Line spectra

21. B. Band Spectra • Arise from molecules • Radiation excites electron from ground into a lowest vibrational level of excited state and transition back releases photon

22. Band Spectra

23. C. Absorption of radiation • Atomic absorption • Molecular Absorption For polyatomic molecules is made up of all of the electronic, vibrational and rotational states

24. C. Absorption of radiation • Visible radiation can excite transition of an electron from ground Eo to any of the n vibrational levels in the E1 level

26. C. Absorption of radiation • Molecular spectra have large wavelength ranges which will have several closely spaced absorption bands which include vibrational and rotational bands

27. D. Relaxation Processes 1. Nonradiative Relaxation 2. Fluorescence and Phophorescence

29. E. Uncertainty Process • Nature places limits on the precision in which we can make pairs of measurements

30. F. Absorbance • Transmission • Absorbance

31. Beer’s Law