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Overlapping Orders Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992. d(sin + sin ) = m d(sin + sin ) = m For Example: 1 st Order = 400 nm 2 nd Order = 200 nm 3 rd Order = 133 nm Any equivalent m combinations will share values. Calculate the free spectral range: f = /(m+1)
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Blaze Angle ( ) Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998. Intensity is highest when matches the angle of specular reflection. Gratings: often characterized by angular dispersion (D a ) the angular separation corresponding to a wavelength separation 0 th order
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Blaze Angle ( ) Blazed gratings direct most of the light towards a higher order band. Optimum Intensity: = 2 Blaze Wavelength ( b ): b = dsin2 b = dsin2 Ingle and Crouch, Spectrochemical Analysis Efficiency drops to ~50% at 2/3 b and 3/2 b.
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Ingle and Crouch, Spectrochemical Analysis Czerny – Turner Monochromator Slits: determine resolution and throughput Monochromators: often characterized by linear dispersion (D l ) – dx/d Monochromators: often characterized by linear dispersion (D l ) – dx/d R d : Reciprocal D l
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Can change angle of grating to focus different on the exit slit. Take-off angle ( ) is constant. Change to direct different towards = + = - m = 2 d sin cos Ingle and Crouch, Spectrochemical Analysis
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Are you getting the concept? In our monochromator, we have a grating with d = 833.33 nm operating in first order with fixed at 6.71 °. At what angles ( ) must the grating be positioned in order to select = 300 nm or 500 nm? What incident angle ( ) is achieved for each in order to perform wavelength selection? Sketch the geometry around the diffraction grating to check your answers.
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http://www.jobinyvon.com/usadivisions/osd/product/size_spectrum.pdf Czerny – Turner Monochromaor 1. Dispersion, Resolution, and Bandpass 2. Accuracy, Speed 3. Throughput, Imaging Quality 4. Stray Light Monochromator Characteristics
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Spectral Bandpass and Slit Function Ingle and Crouch, Spectrochemical Analysis Spectral bandpass: s g = R d W
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Spectral Resolution ( ) with Large W http://www.jobinyvon.com/usadivisions/osd/product/size_spectrum.pdf Ingle and Crouch, Spectrochemical Analysis
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Are you getting the concept? Calculate D a, D l, R d and s g for 1 st order diffraction under optimal conditions for the indicated 0.5 m monochromator with 100 m slits.
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Spectral Resolution ( ) with Small W Diffraction-Limited Spectral bandpass: s d = R d f s d = R d f W’ d W’ d s d = s d = D a W’ d D a W’ d Rayleigh Criteron infers: d ~ s d = R d f d ~ s d = R d f W’ d W’ d
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Are you getting the concept? A grating monochromator with a reciprocal linear dispersion of 1.2 nm/mm is to be used to separate the sodium lines at 589.0 nm and 589.6 nm. In theory, what slit width would be required? http://en.wikipedia.org/wiki/Diffraction_grating
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Double and Triple Monochromators http://architect.wwwcomm.com/Uploads/Princeton/Documents/A&S_Modes.pdf Use Double or Triple Systems to: 1.increase spectral resolution 2.increase stray light rejection Two modes of operation: 1.additive 2.subtractive
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Additive Multi-Stage Monochromators http://architect.wwwcomm.com/Uploads/Princeton/Documents/A&S_Modes.pdf All 3 stages contribute to dispersion Grating G 1 disperses light Slit S 1,2 passes only a narrow portion Grating G 2 further disperses light Slit S 2,3 passes only a narrow portion Grating G 3 disperses light before detection Total dispersion = additive dispersion of each stage Slits open relatively wide in spectrographs to permit enough light through to use the entire detector. →significant stray light
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Subtractive Multi-Stage Monochromators http://architect.wwwcomm.com/Uploads/Princeton/Documents/A&S_Modes.pdf 1 st 2 stages act as a filter Grating G 1 disperses light Slit S 1,2 passes only a narrow portion Grating G 2 recombines dispersed light Slit S 2,3 passes filtered light Grating G 3 disperses light before detection Very high stray light rejection Gratings G 1 and G 2 must match in groove density, and thus, their dispersion actions cancel – very sharp bandpass filter.
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Others Ways to Separate Others Ways to Separate Bandpass Filters www.mellesgriot.com Notch Filters High-pass Filters Low-pass Filters
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