Optical Atomic Spectroscopy

1
Optical Atomic Spectroscopy

• Fundamentals

2
Processes in Atomic Spectroscopy

• Excite atom
• Observe emission or absorption of electromagnetic
  radiation

3
Excitation Processes for Atomic Spectroscopy
Sample
Energy

• M ? M
• Bombardment with electrons or x-rays
• Thermal excitation with electric current, flame,
  or radio-frequency (plasma)
• Direct excitation with UV, visible, IR, rf
  radiation

4
Bombardment with Electrons or X-rays

• Ejection of core electrons
• A e- ? A 2e-
• Emission occurs as x-rays when valence electrons
  relax to lower states vacated by ejected
  electrons.
• Techniques
• X-ray fluorescence spectroscopy
• Widely used for qualitative and semi-quantitative
  analysis of solids
• Electron Spectroscopy for surface analysis
• XPS X-ray photoelectron spectroscopy
• AEC Auger electron spectroscopy

5
Excitation with Electrical Current, Flame, or
Plasma

• Excitation of valence electrons
• Absorption/emission in UV, visible
• Techniques (quantitative elemental analysis)
• Flame Atomic emission spectroscopy (metals only)
• ICP-AE. Inductively coupled plasma atomic
  emission spectroscopy
• For metals and nonmetals.
• Powerful multi-element analysis technique, widely
  used.

6
Direct Excitation with Electromagnetic Radiation

• Excitation with UV, visible, IR, microwave,
  radio-frequency
• Most techniques are based on absorption of
  radiation
• Also may produce fluorescent radiation in UV,
  visible
• Techniques for atomic systems
• Atomic absorption spectrophotometry (quantitative
  analysis for metals)
• Atomic fluorescence (potential alternative for
  AA, not widely used)
• Techniques for molecular systems
• Molecular absorption spectrophotometry (UV,
  visible). Widely used for quantitative analysis.
• Infrared absorption spectrophotometry. For
  structure elucidation, also quantitative
  analysis.
• Molecular fluorescence (fluorometry). Highly
  sensitive, quantitative analysis.
• Raman (infrared)
• NMR (radio frequency)

7
Atomic Spectra
8
Emission Spectra. General Features

• Lines
• Emission from individual atoms (electronic states
  only, no vibrational states)
• Bands
• Emission from small molecules (electronic and
  vibrational states involved)
• Continuum
• Due to thermal emission from incandescent
  particles in flame.

See Flame Emission Spectrum of Brine Solution
(next page)
9
Flame Emission Spectrum of a Brine Solution.

• Major Features
• Lines from Na, K, Ca
• Bands from MgOH
• Background continuum
• Black body radiation (incandescence)

10
A Closer Look at Atomic Spectra

• Energy level diagrams
• Atomic emission, absorption, and fluorescence
  spectra
• Atomic line widths
• Temperature effects

11
Energy Levels for Na (Single Outer Electron)

• Multiple levels available to the single electron
  originating from level 3s
• Selection rules limit transitions (eg, ?l ? 1)
• s ? p p ? d d ? f
• Most probable transitions shown in heavier lines.
• Diagram for Mg is similar, but ?E values are
  greater

3p orbitals are split into 2 levels due to
spin-orbit coupling (3d also split, but
difference is small and not shown)
Diagram from Skoog, Holler and Nieman Instrumental
Analysis, 5e
Ground state is 3s1
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Emission Spectra
Off scale
Non-resonance emission (4d ? 3p)

• Resonance lines
• Involve transitions to ground state
• Are generally more intense than other transitions.

Resonance emission (3p ? 3s)
Flame emission from Na From Skoog, Holler and
Nieman, Instrumental Analysis, 5e
13
Energy Levels for Mg. Two Outer Electrons.

• Singlet and triplet states if multiple valence
  electrons
• More energy levels, more complex spectra
• Single-triplet transition less probable than
  singlet-singlet

Diagram from Skoog, West and Holler, Instrumental
Analysis, 5e
14
Atomic Absorption Spectra

• Physical Process
• Sample is atomized in flame
• MX ? Mo(g) Xo(g)
• Atoms in flame absorb source radiation
• M h? ? M
• Electronic transitions
• Flame AA is based on absorption by ground state
  atoms at wavelengths corresponding to resonance
  lines. There are few atoms in the excited state
  (Boltzman distribution)

Resonance lines 3s ? 3p 3s ? 4p 3s ? 5p
15
Atomic Fluorescence Spectra

• Atomize sample in flame, excite with UV
  radiation, observe fluorescence at right angle
• Alternative to AA, no commercial instruments
  available

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Atomic Line Widths

• General Issues
• In AA and AE, very narrow line width provides
  freedom from interferences
• In AA, line widths are important in design of
  instruments.

Effective Line Width Width of line at one half of
maximum intensity
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Line Widths. Uncertainty Effect

• To measure frequency, let us use an interference
  technique where the unknown frequency interacts
  with a known frequency. Interference produces a
  beat with period ?t, where
• We must measure over a period of one or more
  beats, so

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Estimation of Natural Line Width due to
Heisenberg Uncertainty
M ? M

• Typical excited state lifetime is about 10 ns.
• Estimate the natural line width for the Hg
  emission line at 253.7 nm, which has an average
  lifetime of 2 x 10-8 s.

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Doppler Effect
The wavelength of emitted radiation is affected
by the velocity of the object.
?0 is wavelength emitted by source at rest
20
Line Widths. Doppler Broadening

• The wavelength of emitted radiation is affected
  by the velocity of the object.

21
Line Widths. Collisional Broadening

• Also called Pressure, or Lorentz, broadening
• Origin
• Collisions with other atoms or molecules perturb
  energy levels
• Magnitude
• Flames 2 to 3 times natural line width
• In high-pressure Hg or Xe discharge lamps, it
  results in so much overlap that lines overlap to
  yield a continuum of radiation in UV and visible

22
Xenon Emission SpectraEffect of Pressure
Broadening
Low-pressure lamp, 400-700 nm http//home.achilles
.net/jtalbot/data/elements/
23
Emission Spectra of Hg and Xe Discharge Lamps
Hg
Doppler Broadening Collisional Broadening
400 500 600 700
Xe
400 500 600 700 800 Wavelength, nm
http//www.olympus-biosystems.com/technical/lights
ources.html
24
Emission Spectrum of Xe Lamp
http//www.pti-nj.com/a-702.html
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Emission Spectra of Xe, Deuterium, Tungsten
These are essentially continuum sources
http//optoelectronics.perkinelmer.com/library/pap
ers/tp9.asp
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Temperature and Atomic Spectra

• The Boltzman equation describes the effect of
  temperature on population distribution of energy
  levels

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Temperature and Atomic Spectra

• The fraction of excited atoms in an atomic Na
  vapor at flame temperatures is very small.

The fraction of Na atoms in the 3p state at 2500
K is 0.017.
The fraction of Na atoms in the 3p state at 2510
K is 0.018.
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Temperature Effects in Flame Atomic Spectroscopy

• Emission methods
• signal ? population of excited state atoms
• precise temperature control is required
• Absorption methods
• signal ? population of ground state atoms
• temperature control is less critical
• Other factors
• Effects on ionization equilibria, other reactions
  in flame, may be significant.

29
Other Effects of Measurement Conditions on Signal
in Flame/Plasma Spectroscopy

• In flame methods, signal based on concentration
  of nonionized atoms
• Emission Mo ? Mo h?
• Absorption Mo h? ? Mo
• In plasma emission, signal may be due to emission
  form either neutral atoms or ions.
• Chemical conversion of analyte Mo to other
  species will decrease signal. For flame methods,
• Ionization at high temperature Mo ? M e-
• Prevent with ionization suppressor (e.g., NaCl)
• Nao M ? Na Mo
• Formation of stable metal oxides M O ? MO
  (eg, MgO, AlO)
• For flames, use reducing conditions (excess fuel
  ? C, CO)
• MO CO ? M CO2