Presentation on theme: "What disperses radiation into component wavelengths? Prisms Uses refractive index of material to bend different wavelengths of light different amounts."— Presentation transcript:
What disperses radiation into component wavelengths? Prisms Uses refractive index of material to bend different wavelengths of light different amounts Gratings
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
An echellete grating with 1200 blazes/mm at an incident angle of 30 o to normal. What wavelength would appear at 45 o reflection? Find the distance between the grooves.
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
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
Prisms are much cheaper than grating monochrometers yet gratings are still the most frequently used type of monochrometers. Why?
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
Slit Width 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 Resolution Intensity Larger slit more light will be coming in Smaller slits allow for better resolution Bottom line: choose large slit width if quantitative analysis is what you need Choose small slit width if qualitative analysis is your need
Detectors Ideally detectors should give signal directly proportional to radiant power S = kPs = electrical response k = calibration sensitivityP = power Reality: s = kP + k d k d = dark current
Types of Radiation Detectors 1.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 but much lower than photoelectric detectors
Photoelectric Detectors 1.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
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
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.
Photomultiplier Tubes Best detectors for sensitive response Detectors are judged on 3 criteria: 1.Spectral sensitivity 2.Gain 3.Rise time
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
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
Photoconductivity Detectors Most sensitive in IR region Resistance decreases when irradiated
Photodiodes/ Photodiode Arrays LPDA big advantage: Can measure entire spectrum simultaneously Allows distinction of overlapping peaks on chromatogram Measurement of very transient phenomena All can be aquired & recorded in milliseconds or less Spectrum is focused on a a series of diodes LPDA has reverse optics, selector is after sample Animation
Instrument Designs Temporal Designs: have a single detector, successive bands are examined sequentially in time 1.Non-dispersive: can only measure a few wavelengths, use filters as wavelength selectors 2.Dispersive: can measure multiple wavelengths through wavelength selectors that disperse light
Suggest Optical Components and Materials for Construction of Instruments for: a.Investigation of the fine structure of Absorption bands in the region of nm b.Portable device for determining iron content in natural water based upon absorption of radiation by red Fe(SCN) 2+ complex c.Determining wavelengths of flame emission lines for metallic elements in region from nm