Presentation on theme: "What disperses radiation into component wavelengths?"— Presentation transcript:
1 What disperses radiation into component wavelengths? PrismsUses refractive index of material to bend different wavelengths of light different amountsGratings
2 How does a grating disperse light? Tiny parallel grooves are etched on reflective surfaceGrooves are spaced on order of magnitude of wavelength of light from one anotherReflection produces constructive and destructive interferenceAbout groves/mmShow equationShow them a CD
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 reproductionsEchelle 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 monochrometerc.) resolutiond.) light gathering power
9 Slit WidthBottom line: choose large slit width if quantitative analysis is what you needChoose small slit width if qualitative analysis is your needLarger slit more light will be coming inSmaller slits allow for better resolutionResolutionIntensityMore 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 DetectorsIdeally detectors should give signal directly proportional to radiant powerS = kP s = electrical responsek = calibration sensitivity P = powerReality:s = kP + kd kd = dark current
12 Types of Radiation Detectors Photoelectric detectorsa. signal results from absorption of single photonsb. UV, VIS, IRc. do not have constant response with wavelength2. Thermal detectorsa. signal results from average power of incident radiationb. IRc. does have constant response with l but much lower than photoelectric detectors
13 Photoelectric Detectors Photovoltaic cellsa. used primarily in visible regionb. produces a current that is proportional to radiant power striking itc. no external electrical energy required, cheapd. amplification of signal is difficulte. can easily measure response at high levels but difficult at low levels
14 Vacuum PhototubesTube operated at a high voltage so that current is proportional to radiant powerPhoton beam strikes surface material & it emits electrons which go to the anodeDifferent 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 responseDetectors are judged on 3 criteria:Spectral sensitivityGainRise 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 haveMinimized by electrostatic focusing based on geometriesUsing high gain dynodes reduces # stagesHigh electric field strength increases velocities of ejected electrons
19 Photoconductivity Detectors Most sensitive in IR regionResistance decreases when irradiated
20 Photodiodes/ Photodiode Arrays LPDA big advantage: Can measure entire spectrum simultaneouslyAllows distinction of overlapping peaks on chromatogramMeasurement of very transient phenomenaAll l can be aquired & recorded in milliseconds or lessSpectrum is focused on a a series of diodesLPDA has reverse optics, l selector is after sampleAnimation
21 Instrument DesignsTemporal Designs: have a single detector, successive bands are examined sequentially in timeNon-dispersive: can only measure a few wavelengths, use filters as wavelength selectorsDispersive: can measure multiple wavelengths through wavelength selectors that disperse light
23 Suggest Optical Components and Materials for Construction of Instruments for: Investigation of the fine structure of Absorption bands in the region of nmPortable device for determining iron content in natural water based upon absorption of radiation by red Fe(SCN)2+ complexDetermining wavelengths of flame emission lines for metallic elements in region from nm
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