AM0907_Molecular Spectroscopy_Widia and Yahia

8/7/2019 AM0907_Molecular Spectroscopy_Widia and Yahia

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Presented By :Presented By :WidiastutiWidiastuti SetyaningsihSetyaningsih and Mohamedand Mohamed YahiaYahia ( EMQAL Students).( EMQAL Students).Under supervision Committee by : Professors /Under supervision Committee by : Professors / J.Ayuso J.Ayuso , J.A. Alvarez., J.A. Alvarez.


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BESTEC GmbH, based on the former centre for scientific instruments of the academy of science, Berlin Adlershof http://www.bestec.de/

Cairn Research Ltd. Is an independent scientific instrumentsmanufacturer based in Faversham in UK. http://www.cairnweb.com/

Eddy Company's leadership for supplying solutions for production of complex precision optical, Apple Valley, US http://www.eddyco.com/

McPherson, Inc. manufactures Instruments and system for Spectroscopy,Chelmsford, USA http://www.mcphersoninc.com/

OhioLumex Co. is a manufacturer of analytical and scientificinstrumentation, Twinsburg, US http://www.bestec.de/

Verity Instruments is a manufacturer of optical emission instruments inCarrollton http://www.verityinst.com/

HORIBA is a worldwide companies provides an extensive array of

instruments, Kyoto, Japan http://www.perkinelmer.com/


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T he important Specification of Monochromators :1) Dispersion, resolution, band-pass , T he spectral response:

A) Dispersion:The dispersion of a monochromator is defined by d /dx, where dx is the spatial separation at the exit plane between two spectral lines whose wavelengths are and +d . So dispersion shows thecapability to disperse light.

The dispersion of a given monochromator depends on its length and on the particulars of the usedgrating.

As the number of optical components is increased, the transmitted intensity decreases, due toreflection losses. However, the resolution reached by double monochromators is greatly improvedcompared to that of single monochromators.


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B) T he Spectral Resolution.This is the ability of the monochromator to separate two adjacent spectral lines. If the minimumspectral distance between two lines in the vicinity of that can be isolated by the monochromator is

, then the spectral resolution, R0, is given by

In grating monochromators, the resolution depends on the number of grooves (resolution increaseswith the number of grooves) as the optical length traveled by light beam inside the monochromator increase ( longer monochromators have higher spectral resolutions).

The resolution increases as the slit width decreases as in the large slits output spectrum becomes broader, leading to a larger and then reducing the monochromator resolution.

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large distances enhance the spatial separation between the spectral components, so reduce the number of

the outgoing spectral components for a fixed slit width. For this reason, large monochromators are usedin high spectral resolution experiments.For narrow structure analysis (resolution better than 0.1 nm in the visible range), large monochromatorsare the best choice because they offer increased spectral dispersion and thus a higher spectral resolution

The principle of the grating show that the path length difference between the reflected lights by the twoadjacent grooves is ( S=d(sin E sin F).


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C) Band-pass.

M onochromators are not perfect, as they produce an apparent spectral broadeningfor an ideal input monochromatic beam.

The full width at half maximum(FWH M ) of the output beam is calledthe monochromator band-pass.

Figure shows Spectrum broadening of amonochromatic light when it passesthrough a real monochromator.


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D) T he spectral response: blaze :M any factors affecting the spectral response of a monochromator, the grating reflectance efficiencyis the most important.The wavelength at which the grating operates with its highest efficiency is called blaze wavelength.The efficiency of any grating (the most commonly used gratings) is strongly dependent on thedispersed wavelength.The following figure shows the spectral response of two gratings (in each case, the blaze wavelengthis indicated by an arrow).


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2 ) Accuracy and speed:

Accuracy and Speed depend on the grating motor drive. Instruments with 0.5-m or higher focal length are usually equipped with sine bar mechanisms that give excellent accuracy(better than 0.1 nm) and repeatability (better than 0.01 nm).

Speed can take several minutes to scan a large spectral range with high spectral resolution inmonochromator mode. M onochromators work by scanning through the spectral features of theoptical signals, step by step.

Smaller devices commonly use direct drives because their resolution specifications are lower.In this case, they set the grating position within a few seconds.

Precision (Repeatability) is the accuracy to which a wavelength setting can be repeatedly set.It can be specified in either wavelength units or in percent of wavelength.

The figure shows that the rotating the diffraction grating position scans the wavelengthsacross the monochromators exit slit.The accuracy and speed of the rotation depend on the grating motor drive, with sine bar mechanisms providing excellent accuracy and repeatability at the cost of speed.


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The grating equation specifies the angle that is required to bring each wavelengththrough the exit slit .

where k is the diffraction order, n the grating groove density and the vacuumwavelength in nanometers.


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3 ) T hroughput, Imaging quality:Small monochromators have better throughput than large ones because of their larger numerical (f

numbers) and simpler design (often with fewer optical components).

(f number): The input aperture of the monochromator. It measures the ability of a device to collectradiation in the entrance slit.

M anufacturers of small instruments usually find that to preserve spectral resolution, they can produce only a small image. producing a larger image, especially across a large spectral range, isvery difficult because of the spatial corrections required.

Imaging spectrographs has partially corrected the imaging quality as these instruments use toroidalgratings or toroidal mirrors to correct for astigmatism in the image plane and to improve imagequality while keeping the numerical ( f number ) at the same level as non-imaging devices.

Spectrograph: An instrument that presents a range of wavelengths at the exit focal plane for detection by an array detector.

Well-corrected, fixed, compact spectrographs can provide excellent image quality.


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4 ) Stray light, design, focal length.

Stray light relates to the quality of the devices optical components (mirrors and grating). The user isgenerally not aware of stray light which can produce poor results.

Stray light: is light in an optical system , which was not intended in the design. The light may befrom the intended source, but follow paths other than intended, or it may be from a source other thanthe intended source.

Because of slit/slit configuration, monochromators have less stray light or re-entrant light than dospectrographs, which have no exit slit.

When the stray light is important in an application, large-focal-length instruments or doublemonochromators are the best choice.

Small devices present more risk of stray light than larger ones. In terms of optical design, most largemonochromators /spectrographs use the asymmetric Czerny-Turner configuration.


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Some important Conclusions for monochromators:

If you need to analyze a short spectral range at low resolution, you can probably use an inexpensive,compact monochromator or spectrograph. Even if these devices have stray light, chemometriccalibration methods can correct it without influencing the results.

If you need high resolution, accuracy or versatility, large monochromators and spectrographs areoften the safest buy. That is why these are generally the best instruments for research or high-technology industries.

The most important significant that effect on the cost of monochromators can be classified as thefollowing:

1) Dispersion, resolution, band-pass , The spectral response (Top criteria).2) Accuracy and speed (Second criteria).3) Throughput, imaging quality (Third criteria).4) Stray light, design, focal length(Final criteria).

The most used monochromator is Czerny turner design that has better resolution than 0.1 nmresolution.


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AM0907_Molecular Spectroscopy_Widia and Yahia

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