Instrumentation for UV and visible absorption

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I

• Instrumentation for UV and visible absorption

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Lamps

• Generally need a continuous source
• Tunable laser would be ideal (not available)
• Choice depends on wavelength region
• Visible Tungsten
• UV H2 or Deuterium (160 -350nm)
• Visible Tungsten ( 350 2500 nm)

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Deuterium (arc) lamp

• Low power discharge (100w) through low pressure
  (10 torr) of deuterium.
• D2 Ee ?D2 ? D D h?
• As the two atomic species can have a variety of
  kinetic energies, so the light emitted will be a
  continuum.

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Deuterium lamps
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Tungsten Filament Lamp

• Visible and Near Infrared
• Filament temperature 2870 K
• Stable because of good voltage control

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Quartz/halogen lamps

• Iodine is added
• Higher operating temperature (3500 K) allows
  higher energy output but requires quartz envelope
  (melts at higher temp than glass)
• W I2 ?WI2 (volatile)
• When they hit the hot filament they decompose and
  release W
• Increases lamp life

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Ruby laser
Some atoms emit photons
which stimulate further emission
Light from flash tube excites ruby atoms
Leaves through half-silvered mirror
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Optical materials

• Need light to be able to pass through sample
  holder, etc.
• Visible glass strong, cheap
• Usually cuts off 360 nm
• UV quartz
• Below 200 nm, O2 absorbs so purge with dry
  nitrogen (gets you to 160 nm)
• lower vacuum UV

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Useful transmission rangea for optical
materials Material Range fused silica 170 nm -
3.6 µm glass 360 nm - 2.5 µm sodium
chloride 200 nm - 15 µm potassium bromide 230 nm
- 25 µm potassium chloride 200 nm - 18 µm
thallium bromide-thallium iodide 500 nm - 35 µm
cesium iodide 230 nm - 50 µm calcium
fluoride 125 nm - 9 µm barium fluoride 130 nm -
12 µm lithium fluoride 104 nm - 7 µm sodium
fluoride 195 nm - 10.5 µm cadmium fluoride 200 nm
- 10 µm lead fluoride 290 nm - 11.6 µm lanthanum
fluoride 400 nm - 9 µm magnesium fluoride 110 nm
- 7.5 µm aLimits are taken as wavelengths where
percent transmittance falls to 60 percent for a
1-cm thickness.
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Absorption filters

• Just in visible region
• Coloured glass or dye between plates
• Cheap
• Cut-off or band-pass

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Interference Filters

• Two transparent plates coated wth partially
  reflecting metal films
• Separated by dielectric material- CaF2 or,MgF2 (
  thickness t)
• Exiting beams can have travelled extra distances
  multiples of 2t
• If 2t n ?/?, constructive interference will
  occur orders of that ? of light will pass
  through the filter
• Smaller bandpass than absorption filters

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Transmission Gratings

• Light interference
• Diffraction or reflection

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Reflection Gratings

• Holographic gratings
• 2 collimated beams of light are used to produce
  interference fringes in a photosensitive material
  on flat glass.
• The light-exposed material is washed away and the
  grooves are coated with a reflective layer, eg Al

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Grating normal
Monochromatic Beam at incident Angle i
CD extra distance travelled
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n? CD AB d(sini sinr)
CD dsini AB -dsinr
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Grating Characteristics

• Resolution

The more grooves, the better the resolution
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Dispersion
Dispersion is better if the spacing between
grooves is smaller
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Monochromator

• Grating and slits
• Usually other mirrors as well

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Slit width

• The slit width is defined by the bandwidth of
  radiation it allows through.
• Resolution of closely spaced bands is achieved at
  the expense of decreased S/N.
• Slits should be as wide as possible, but small
  compared to width of absorbance band

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Unwanted orders of light

• Need a filter to remove these
• Always have filter as well as a grating

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Errors Stray radiation
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• Low A P similar to Po
• High A P is small - low S/N ratio
• For most modern instruments, once above a certain
  concentration, the error is mostly in the cell
  positioning