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Chapter 18. Spectroscopy. Interaction of Light and Matter. Light = photons waves Oscillating electric and magnetic fields Frequency ( n ) - number of full oscillations per second (sec -1 ) or Hertz

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chapter 18

Chapter 18

Spectroscopy

interaction of light and matter
Interaction of Light and Matter
  • Light = photons waves
  • Oscillating electric and magnetic fields
    • Frequency (n) - number of full oscillations per second (sec-1) or Hertz
  • As light travels thru space – distance from crest to crest is the wavelength l (cm)
  • The relations between the two is
    • nl = c
properties
Properties
  • Speed of light
    • c = 2.998 x 108 m/sec in vacuum
    • Light will slow down in other media
      • vx = c/n
      • n is the refractive index of the media
  • Energy of light
    • E = hn
      • h is Planck’s constant 6.626 x 10-34 J.sec
measuring the interaction
Measuring the Interaction
  • Irradiance, P
    • The energy per second per unit area of a light beam.
    • When a sample is in the light beam some energy is absorbed and less energy gets past the sample
transmittance
Transmittance
  • The fraction of the of light that gets to the detector.
  • T = P/Po
  • Po is the power that would arrive at the detector without a sample in the light path
  • Percent T (%T) is just T * 100
absorbance
Absorbance
  • A = log(Po/P) = - logT
  • Beer’s Law (Beer Lambert Law)
    • A = ebC
  • A is the abosorbance - unitless
  • b is the path length - usually in cm
  • C is the concentration - usually in M
  • e is a measure of the compounds ability to absorb light – depends on chemical structure and will have units that are _______________ ??
light source
Light Source
  • Can be a simple lamp
  • Mercury Discharge tube.
  • Sun
wavelength selection
Wavelength Selection
  • Filter
  • Prism
  • Grating

The latter two break white light into a spectrum of color.

  • Laser could serve as both source and wavelength selector.
detection
Detection
  • Phototube.
    • Converts light energy into an electric current
  • Photodiode array
  • Eye
method development
Method Development
  • “Colored” compounds can be directly analyzed.
  • “Colored” complexes can be made.
  • Many metal analyses can be developed using color forming ligand. Colorimetry. Very mature field with good selectivity and good sensitivity.
other types of spectroscopy
Other Types of Spectroscopy
  • Infrared - Bond vibrations
  • UV – higher energy than visible
  • Fluorescence – reemission of a photon at a different wavelength.
homework
Homework
  • Chapter 18
  • 1 ,2, 4, 7, 10 and 16