Chapter 5
Ver 1.0
April 2015.
Infrared (IR) Spectroscopy Definition : • Energy absorption by molecule in infrared region • Transition between two vibrational levels • Extract molecular structural information by measuring molecular vibrations spectra • Molecular fingerprints, characterize and identify the molecule • Used in conjunction with Raman Spectroscopy IR Spectrometer Instrumentation : • Dispersive prism or grating • Non-dispersive interference filters, tunable lasers sources, an interferometer (Fourier Transform Infrared Spectrometer • For IR region, front faced mirrors • Glass and quartz absorb opaque to IR Parts of IR spectrometer: 1. Source of Infrared radiation • IR, heat radiation • Heat transfer direct, do not require a direct physical medium • Converted to heat, only on absorption by object • Source, an incandescent solid material which has a radiant energy distribution • Similar to theoretical black body • Tungsten filament lamp, Glober, Nernst Glower, Nichrome heater 2. Infrared monochromator • Means of separating source radiation wavelengths • Prisms and gratings • Prisms of alkali halides • Large crystals, Prisms, windows and cells are cut from them • N aCl 4000 − 650 cm−1 • Potassium bromide 1000 − 400 cm−1 • Cesium iodide 1000 − 260 cm−1 • Below 250 cm−1 , gratings are used, disperse the energy 3. Infrared Detector • Thermal radiant energy into electrical energy Prepared by: Dr. K. Saravana Kumar,
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Asst. Professor, Dept. of Physics, SRM University, Ramapuram Campus.
Chapter 5
Ver 1.0
April 2015.
• Selective response dependent on wavelength incident radiation • Photographic plates, photocells, photoconductive cells, infrared phosphors • Photoconductive important, rapid response and high sensitivity • PbS, PbSe, Lead telluride, selenium • Increase in conductivity, with increase in IR illumination • 0.5 3.5 microns • Non-selective response independent of wavelength of incident radiation • Directly proportional to incident energy • Thermocouples, bolometers, pneumatic cell Thermocouple • Thermocouples: evaporating bismuth antimony, semiconductor alloys, on thin film cellulose nitrate, in evacuated chamber with KBr window Bolometers • Electrical signal, change in resistance of a metallic conductor with temperature • Faster response time due to small thermal capacity • Thermistor bolometer, increased sensitivity • Fused mixture of metal oxides : Ni, Co, Mn, exponential decrease in resistance with temperature Pneumatic detector cell • Two cells, first 3 mm filled with a gas (Xe), connected to second cell through a tube • One end closed with a curved diaphragm, acting as mirror • Heat on gas changes the volume, expansion due to IR absorption • Thereby moving mirror in other cell proportionally Preparation of samples : 1. Gaseous samples • No special preparation • 10 cm long cell for gaseous samples • Vacuum tight cells • Connection problems • Proper operation, due to heating and cooling cycles 2. Liquid or solution samples • Pure liquid samples • No solvents, as they have their own absorption spectra Prepared by: Dr. K. Saravana Kumar,
2
Asst. Professor, Dept. of Physics, SRM University, Ramapuram Campus.
Chapter 5
Ver 1.0
April 2015.
• Liquid and solution, same type of cell • Non-corrosive and not too viscous liquids, normally in sealed cells • Highly viscous liquids, sandwiched as a layer between two salt plates • Cavity cells 3. Solids • Solid Films: solid samples into thin films, on IR transmitting window, or solid melted and cooled into thin film, not recommended for crystalline substances • Mull-Technique: Powdered sample mixed with a little heavy paraffin oil, nujol, mulled into paste, transferred to rock salt plate • Pressed Pellet Technique: Weighed solid material thoroughly mixed with KBr, pressed into pellet under high pressure in evacuated die, resulting in transparent discs Applications of IR Methods : • GC-IR • TG-IR • Tiny samples • Study of light emitted by sample • Photocurrent sepctra
Prepared by: Dr. K. Saravana Kumar,
3
Asst. Professor, Dept. of Physics, SRM University, Ramapuram Campus.
