1. Instrumentation for UV and visible absorption

5. Lamps Generally need a continuous source
Tunable laser would be ideal (not available)
Choice depends on wavelength region
Visible – Tungsten
UV – H2 or Deuterium (~ nm)
Visible – Tungsten (~ 350 – 2500 nm)

6. 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.

7. Deuterium lamps

8. Tungsten Filament Lamp
Visible and Near Infrared
Filament temperature 2870 K
Stable because of good voltage control

9. 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

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

14. 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

15. potassium bromide 230 nm - 25 μm potassium chloride 200 nm - 18 μm
Useful transmission rangea for optical materials
Material Range
fused silica 170 nm μm
glass 360 nm 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 μm
cadmium fluoride 200 nm - 10 μm
lead fluoride 290 nm μm
lanthanum fluoride 400 nm - 9 μm
magnesium fluoride 110 nm μm
aLimits are taken as wavelengths where percent transmittance falls to 60 percent for a
1-cm thickness.

16. Absorption filters Just in visible region
Coloured glass or dye between plates
Cheap
Cut-off or band-pass

18. 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

19. Transmission Gratings
Light interference
Diffraction or reflection

21. 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

22. Grating normal
Monochromatic
Beam at incident
Angle i
CD = extra distance
travelled

25. n = CD – AB = d(sini + sinr)
CD = dsini
AB = -dsinr

27. Grating Characteristics
Resolution:
The more grooves, the better the resolution

28. Dispersion: Dispersion is better if the spacing
between grooves is smaller

29. Monochromator
Grating and slits
Usually other mirrors as well

30. 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

32. Unwanted orders of light
Need a filter to remove these
Always have filter as well as a grating

33. Errors – Stray radiation

35. 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