1. What disperses radiation into component wavelengths?
Prisms
Uses refractive index of material to bend different wavelengths of light different amounts
Gratings

2. How does a grating disperse light?
Tiny parallel grooves are etched on reflective surface
Grooves are spaced on order of magnitude of wavelength of light from one another
Reflection produces constructive and destructive interference
About groves/mm
Show equation
Show them a CD

3. Grating Movie

4. An echellete grating with 1200 blazes/mm at an incident angle of 30o to normal. What wavelength would appear at 45o reflection?
Find the distance between the grooves.

5. Echellete Grating: Has relatively broad faces where reflection occurs and narrow unused faces.
Concave grating: Gratings formed on concave surfaces do not require auxillary collimating and focusing mirrors or lenses.
Concave surface both disperses and focuses the radiation on the exit slit. Advantage in terms of cost and in decreasing the amount of optical surfaces therefore increasing energy throughput

6. Holographic gratings: Blazes etched with light instead of mechanically, cheaper and better reproductions
Echelle monochrometer: Higher dispersion than echellete uses higher angle of incidence. Short side of blaze is used instead of long. Grating is relatively coarse (<300 groves/mm). Gives lots more orders of light, must use prism to get rid of higher orders. Resolution order of magnitude greater than others

7. Prisms are much cheaper than grating monochrometers yet gratings are still the most frequently used type of monochrometers. Why?

8. Why are grating MCs used more frequently than prism MCs?
Criteria monochrometers are evaluated on:
a.) Spectral purity:
b.) dispersion of the monochrometer
c.) resolution
d.) light gathering power

9. Slit Width
Bottom line: choose large slit width if quantitative analysis is what you need
Choose small slit width if qualitative analysis is your need
Larger slit more light will be coming in
Smaller slits allow for better resolution
Resolution
Intensity
More light coming in will illuminate more of the monochrometer, more constructive interference of light & thus more intense but if slit width is too wide it is difficult to resolve adjacent spectral lines

11. Detectors
Ideally detectors should give signal directly proportional to radiant power
S = kP s = electrical response
k = calibration sensitivity P = power
Reality:
s = kP + kd kd = dark current

12. Types of Radiation Detectors
Photoelectric detectors
a. signal results from absorption of single photons
b. UV, VIS, IR
c. do not have constant response with wavelength
2. Thermal detectors
a. signal results from average power of incident radiation
b. IR
c. does have constant response with l but much lower than photoelectric detectors

13. Photoelectric Detectors
Photovoltaic cells
a. used primarily in visible region
b. produces a current that is proportional to radiant power striking it
c. no external electrical energy required, cheap
d. amplification of signal is difficult
e. can easily measure response at high levels but difficult at low levels

14. Vacuum Phototubes
Tube operated at a high voltage so that current is proportional to radiant power
Photon beam strikes surface material & it emits electrons which go to the anode
Different surfaces can be used for sensitivity in different spectral regions

15. Spectral Response for different photoemissive surfaces
Curve 1 is the response of a bialkali type of cathode with a sapphire window; curve 2 is for a different bialkali cathode with a lime glass window; curve 3 is for a multialkali cathode with a lime glass window; and curve 4 is for a GaAs cathode with a 9741 glass window. The curves labeled 1% and 10% denote what the response would be at the indicated value of quantum efficiency.

16. Photomultiplier Tubes
Best detectors for sensitive response
Detectors are judged on 3 criteria:
Spectral sensitivity
Gain
Rise time

17. Dark Current PMT will have current even when no signal is present.
Can be minimized by cooling detector or be offset because magnitude is generally constant

18. Rise time:
Origin is spread of arrival times of avalanched electrons caused by range of velocities and path lengths that electrons may have
Minimized by electrostatic focusing based on geometries
Using high gain dynodes reduces # stages
High electric field strength increases velocities of ejected electrons

19. Photoconductivity Detectors
Most sensitive in IR region
Resistance decreases when irradiated

20. Photodiodes/ Photodiode Arrays
LPDA big advantage: Can measure entire spectrum simultaneously
Allows distinction of overlapping peaks on chromatogram
Measurement of very transient phenomena
All l can be aquired & recorded in milliseconds or less
Spectrum is focused on a a series of diodes
LPDA has reverse optics, l selector is after sample
Animation

21. Instrument Designs
Temporal Designs: have a single detector, successive bands are examined sequentially in time
Non-dispersive: can only measure a few wavelengths, use filters as wavelength selectors
Dispersive: can measure multiple wavelengths through wavelength selectors that disperse light

22. Spatial Instrument Designs
Multi-channel
Non-dispersive: 3 wavelengths, 3 slits, 3 detectors
Dispersive: LPDA

23. Suggest Optical Components and Materials for Construction of Instruments for:
Investigation of the fine structure of Absorption bands in the region of nm
Portable device for determining iron content in natural water based upon absorption of radiation by red Fe(SCN)2+ complex
Determining wavelengths of flame emission lines for metallic elements in region from nm