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16.2 Scanning Infrared Spectrometers

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16.2 Scanning Infrared Spectrometers 16.2 Scanning Infrared Spectrometers • it's difficult to find materials transparent in the infrared • water vapor and atmospheric CO2 can cause problems • there are three common sources • high diffraction orders cause problems with monochromator design • there are three common detectors • example instruments include both a prism and grating monochromators 16.2 : 1/9 Infrared Transparent Materials In order for a material to be transparent in a spectral region it cannot have spectroscopic transitions in that region. It is easy to find material transparent in the UV/visible because single, covalent bonds generally appear at wavelengths less than 200 nm. To be transparent in the infrared the material cannot have vibrations with infrared energies. material range pluses minuses inexpensive; soft; dissolves in NaCl 40,000 - 625 cm-1 90-95% T H2O; easily fogs CsI 40,000 - 200 cm-1 (see NaCl) (see NaCl) hard; insoluble 50% T Ge crystal 5,500 - 600 cm-1 @ pH ≤ 7 (n ~4) KRS-5 soft; 70-75% T 20,000 - 250 cm-1 insoluble @ pH ≤ 7 (Th/Br/I) (n ~2.4) high density attacked by organic 600 - 30 cm-1 limited range polyethylene solvents See http://infrared.als.lbl.gov/IRwindows.html for detailed data and spectra! 16.2 : 2/9 Miscellaneous Facts Both water and CO2 have strong absorption bands throughout the infrared. For this reason most infrared spectrometers are capable of being purged with dry nitrogen. (N2 has one symmetric vibration which is infrared inactive) Because of a lack of suitable material, lenses are never used. Mirrors are used exclusively. Many metals have very high reflectivity in the infrared. NaCl and CsI prisms have been used in older, scanning instruments. The transparent materials mentioned on the last slide are used to make beam splitters in Fourier Transform spectrometers. 16.2 : 3/9 Lamps The globar is a rod of silicon carbide about 6 mm in diameter and 5 cm in length. It is heated to a temperature between 1300 - 1500 K, and generates a blackbody spectrum from 10,000 to 250 cm-1. A small amount of visible light is emitted since it glows red. The Nernst glower is a rod of ZrO2 or Y2O3 that is 1-3 mm in diameter and 1 - 3 cm in length. It is heated to a temperature between 1200 - 2000 K and outputs a blackbody spectrum from 25,000 to 500 cm-1. A high pressure mercury lamp has a plasma emission as well as the characteristic atomic spectrum. The plasma emits in the far infrared at wave numbers < 250 cm-1, where the globar and Nernst glower have too little output to be useful. For a grating instrument, all three sources have to be coupled with optical filters. This is necessary to ensure that low wavelength, non-infrared radiation does not pass the monochromator in a high order. 16.2 : 4/9 Grating Monochromators A typical infrared spectrometer will obtain a spectrum over the range of 2.5 - 40 μm (4,000 - 250 cm-1). Unfortunately, this wavelength range is so wide that orders with m > 1 are a problem. As an example, consider a monochromator set to pass 40 μm in the first order. This grating angle will also pass 20 μm in the second order, 13.3 μm in the third order, etc., all the way to 2.5 μm in the 16th order. To circumvent this problem, infrared spectrometers use several gratings in combination with optical filters. Example: the Perkin-Elmer Model 67 (vintage 1975) used two gratings and six filters 2.5 - 5 μm, m = 2, 100 lines mm-1, filter changes at 3.2 and 4 μm 5 - 16 μm, m = 1, 100 lines mm-1, filter changes at 5, 8.7 and 14.3 μm 16 - 40 μm, m = 1, 25 lines mm-1, filter change at 25 μm 16.2 : 5/9 Detectors (1) copper light A thermocouple detector is composed of a antimony bimetallic junction where the two metals have bismuth drastically different Fermi levels. As the metals trade charge to equalize the Fermi V levels, the resultant voltage is monitored by a high impedance voltmeter. The most common junction is antimony/bismuth. It can detect ΔT same temperature = 10-6 K, and produces 6-8 μV/μW. ice bath The junction is coated with graphite to absorb infrared radiation. As the graphite warms the Boltzmann distribution changes the relative positions of the two Fermi levels. This changes the measured voltage. There is no way to make a single junction work, since the antimony/copper and bismuth/copper voltages would cancel the antimony/bismuth signal. A two junction system is used where one of the junctions is at a fixed, reference temperature. The two copper/antimony junctions must be held at the same temperature. 16.2 : 6/9 Detectors (2) A more sensitive detector is based on a semiconductor photodiode. The material has to have a very low band gap in order for photons to promote electrons from the valence to conduction bands. In turn, low band gaps require that the detector be cooled to 77 K to keep thermal energy from producing a false signal. An example is PbTe with a band gap near 7 μm. Detection of longer wavelengths is possible by combination with lattice vibrations (called phonons). For rapidly changing signals, like those in FTIR, a pyroelectric detector is used. The detector is composed of a crystalline material, e.g. deuterated triglycine sulfate, held between two electrodes. The pyroelectric crystal has an intrinsic polarization, which when placed between the two electrodes creates a charged capacitor. The amount of polarization is highly temperature dependent. One electrode is transparent in the infrared allowing the radiation to heat the crystal. A varying infrared intensity causes a varying temperature, which causes a varying polarization of the crystal, and finally a varying measured voltage. 16.2 : 7/9 Prism Spectrometer Perkin-Elmer Infracord, circa 1959. This device used an NaCl prism. prisms don't require the prism is used twice, doubling order-sorting filters the resolution Toroidal mirrors have two different radii of curvature to correct for 16.2 : 8/9 off-axis aberrations. Grating Spectrometer Perkin-Elmer Model 67, circa 1975. The device uses two gratings and several filters. Note the similarity of the optical layout! the two gratings are back-to- order-sorting filters back and are changed by rotating 180° Toroidal mirrors have two different radii of curvature to correct for 16.2 : 9/9 off-axis aberrations..
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