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Introduction to Spectroscopy: Methods, Spectra, and Energy States

This chapter provides an introduction to spectroscopy, including different spectroscopic methods, the spectrum of electromagnetic radiation, and the energy states of chemical species. It covers topics such as line spectra, band spectra, absorption of radiation, relaxation processes, uncertainty processes, and absorbance.

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Introduction to Spectroscopy: Methods, Spectra, and Energy States

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  1. Chapter 6 Introduction to Spectroscopy

  2. I. Spectroscopic Methods

  3. 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.

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

  5. II. 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

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

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

  8. Line spectra

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

  10. Band Spectra

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

  12. 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

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

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

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

  16. F. Absorbance • Transmission • Absorbance

  17. Beer’s Law

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