Basic Review of Optical Spectroscopy

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Basic Review of Optical Spectroscopy

• Types of Optical Spectroscopy
• The Electromagnetic Spectrum
• Molecular
• Atomic
• Absorbance versus Fluorescence
• Beers Law, Absorbance, Transmittance, etc.

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Topics You Should Be Familiar With

• From Physics Courses
• EM radiation and its properties
• Diffraction
• Refraction
• Coherent and incoherent radiation
• Polarization of radiation
• Scattering of radiation

• From Chemistry Courses
• Photoelectric effect
• Electromagnetic spectrum
• Beers Law, etc.
• Quantized states in atoms
• lead to line spectra
• Quantized states in molecules
• lead to broad or continuum spectra

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Spectroscopic Methods
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EMR Properties are Dependent Upon the Media the
Radiation is In!
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EMR Crossing Media Boundaries is Refracted
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Photoelectric Effect

• EMR striking a surface has the ability to eject
  an electron from that surface. This is the
  Photoelectric effect.
• The energy required to eject electrons from the
  surface of a material is known as its work
  function.
• Different materials have different work functions.

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• When EMR strikes a photocathode (or any
  photo-emissive surface), the energy required to
  eject and accelerate the photo electron equals
  the energy required to break the electron free
  from the surface ( ?, work function) plus its
  kinetic energy (KE).
• This energy is equal to the energy of the
  incoming EMR.
• E h x ?

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Dont Forget.

• E h x c/? or E h x ?
• E ? ? x b x c (Beers Law)
• Molar absorptivity is wavelength dependent
• Beers Law can fail at high concentrations!
• The color you see in something that absorbs light
  is the complement of what it has absorbed!

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When Electromagnetic Radiation is Absorbed..
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Atomic versus Molecular Transitions

• Atomic transitions are usually very discreet
  changes of electrons from one quantum state to
  another (energy levels, shells, spins, etc.).
  What you absorb is what you get back (resonance).
• Molecular transitions can involve intermediate
  states and therefore resonance is less common!

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ATOMIC Line SPECTRUM
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Molecular SPECTRUM
From http//www.uis.edu/trammell/trammell.html
? ranges for molecular transitions are often 20
nm or more!
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Some Basic Concepts...

• Why are even line spectra not truly lines?
• They are really broad distributions that are just
  over a range of about .01 nm or less.
• Some of this (especially with respect to lines)
  is due to the uncertainty principle!
• Remember, than an atom or molecule does not go
  from one distinct energy state to another
• it goes from some high probability state to
  another high probability state
• we can never know the exact energy
• limited by h/?t
• Heisenbergs Uncertainty Principle in action!
• In addition
• Doppler effect
• Variability due to random error in the instrument
  components (e.g. wobbly grating, vibration, etc.)

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When a sample we are analyzing absorbs light
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Quantitative Relationships for Optical
Spectroscopy

• Beers Law (you should know)
• Definitions
• P0 incident light intensity,
• P transmitted light intensity
• Transmittance
• Absorbance

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Instruments for Measuring Absorption of Light.
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Absorption Spectroscopy (UV-VIS, IR, AAS, etc.)
b
Transmitted Light (P)
Incident Light (Po)
Sample (which in this case has absorbed
violet-blue light. See Harris Table 18.1)
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Fluorescence and Phosphorescence
Excitation Beam
Emitted Beam
Detector
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Resonance Fluorescence

• Resonance Fluorescence
• Usually atomic
• Emitted light has same E as excitation light
• Simpler, atomic systems with fewer energy states
  (vs molecules) undergo resonance fluorescence
• Not as widely used in analytical chemistry as
  non-resonance fluorescence
• Hg analysis is one example

Excitation Beam
Emission (identical E)
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Non-resonance Fluorescence

• Typical of molecular fluorescence
• Large number of excited states
• rotational
• vibrational
• etc..
• Molecules relax by stepping from one state to
  another
• Resulting emitted light shifts to lower
  energies
• longer wavelengths lower energy

Excitation Beam
Emission (lower E, longer ? )
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