Signal Modulation: Michelson Interferometer. â¢ Used primarily for optical measurement. â IR, UV-VISâ¦ â¢ Displacement of moving mirror results i...

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Components for Infrared Spectroscopy • Mid-IR light: 400-4000 cm-1 (25 – 2.5 m wavelength) • Sources: – Blackbody emitters • Globar metal oxides • Nernst Glower: Silicon Carbide

• Detectors: – Not enough energy for photoelectric effect-based detectors. – Pyroelectric Detectors • Crystalline materials whose electric polarization depends on temperature • Changing temperature change in charge distribution current • Deuterated Triglycine Sulfate - DTGS

– Photoconductive Detectors • Semiconductor-based • Thermal energy can promote e- from valence to conduction band • Fast response, susceptible to thermal noise – Liquid Nitrogen Cooled

• Mercury Cadmium Telluride (MCT), Indium Antinomide (InSb)

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Dispersive IR Spectroscopy • Detector “sees” limited number of wavelengths – Small number of photons need sensitive detector • PMT: good for UV-Vis, poor for IR

• S/N in UV-Vis typically source limited – Dispersive UV-Vis instruments work well

• Dispersive techniques are tougher in the IR – Lower photon energies – Poorer detectors Infrared measurements are detector noise limited.

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Multiplex Spectroscopy • Dispersive vs. Multiplex methods – Dispersive: Breaks source light into component wavelengths (“Single at a time”) – Multiplex: Single detector, view all simultaneously • Modulate signal to encode response • Use mathematical algorithm to decode response – Fourier Transform – Often convert from between time domain and frequency domain

• Applied to optical spec., NMR, mass spec...

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Multiplex Spectroscopy • Relationship between time-domain and frequency domain – In multiplex, typically record data in time domain – Time domain data contains all the information of frequency domain data.

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Multiplex Spectroscopy •

As more frequencies are added, time-domain signal gets more complex sum = v + 1.2v + 1.5v 4 3 2 1 0 -1 -2 -3 -4

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Mathematical “transform” can convert between domains – calculation intensive process

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Big Problem: It is very difficult to acquire data with time resolution necessary for optical frequencies (Billions of Hz) – Detectors don’t respond that fast! – Must build in some instrumental modulation at a frequency that we can monitor • Must maintain the same time relationships

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Signal Modulation: Michelson Interferometer • Used primarily for optical measurement – IR, UV-VIS…

• Displacement of moving mirror results in interference at detector – constructive and destructive

• Retardation (): Difference in path lengths of the two beams (2(M-F)) • As retardation changes, different frequencies (wavelengths) will constructively interfere. – Interferogram is plot of resulting power as a function of retardation.

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Michelson Interferometer: Mathematical Relationships •

With mirror moving at constant velocity:

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1/2 is there because of construction of interferometer – But optics don’t behave ideally, need “fudge factor”

1 P() P( ) cos 2ft 2 P() B( ) cos 2ft P() B( ) cos 2

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We can easily relate f to the frequency (wavelength, wavenumber) of light that is “in phase”

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We can write a similar expression for each wavenumber (wavelength). The interferogram is the sum of all wavelengths present. For a continuum: B( ) cos 2 d P()

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We record P() in the time domain. The Fourier transform gets us to the frequency domain P() cos 2 d B( )

– FT is accomplished through numerical integration – Practical considerations: • Kinda tough to go from + to -! • How big is d

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Benefits/Challenges of Multiplex Methods • Resolution:

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– Examples: • What mirror retardation is necessary to resolve bands at 1710 cm-1 and 1700 cm-1 ? • How about 589.0 nm and 590.0 nm?

• Advantages: – Multiplex Advantage • Decrease in observation time compared to dispersive instruments • Allows more scans per unit time - better S/N – Most true where detector noise limited (IR)

– Throughput Advantage: • More photons hit the detector - larger signal!

– High Resolving Power

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IR Sampling • Transmission – Thin film, KBr pellet, mull • Sample-to-sample inhomogeneity makes quantitative analysis challenging.

• Reflection – Attenuated total reflectance – ATR • Crystal must have high refractive index – total internal reflection – Evanescent wave extends slightly above surface of crystal

• Requires good contact between sample and crystal

http://las.perkinelmer.com/content/TechnicalInfo/TCH_FTIRATR.pdf

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