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Abstract:
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Near-infrared spectroscopy can be a workhorse technique for materials
analysis in industries such as agriculture, pharmaceuticals, chemicals and polymers. A
near-infrared spectrum represents combination bands and overtone bands that are
harmonics of absorption frequencies in the mid-infrared. Near-infrared absorption
includes a combination-band region immediately adjacent to the mid-infrared and three
overtone regions. All four near-infrared regions contain "echoes" of the fundamental mid-infrared
absorptions. For example, vibrations in the mid-infrared due to the C-H stretches will
produce four distinct bands in each of the overtone and combination regions. As the
bands become more removed from the fundamental frequencies they become more
widely separated from their neighbors, more broadened and are dramatically reduced in
intensity. Because near-infrared bands are much less intense, more of the sample can be
used to produce a spectra and with near-infrared, sample preparation activities are
greatly reduced or eliminated so more of the sample can be utilized. In addition, long
path lengths and the ability to sample through glass in the near-infrared allows samples
to be measured in common media such as culture tubes, cuvettes and reaction bottles.
This is unlike mid-infrared where very small amounts of a sample produce a strong
spectrum; thus sample preparation techniques must be employed to limit the amount of
the sample that interacts with the beam. In the present work we describe the successful the fabrication and calibration of a linear high resolution linear spectrometer using tunable diode laser and a 36 m path length cell and meuurement of a highly resolved structure of OH group in methanol in the transition region A v =3. We then analyse the NIR spectrum of certain
aromatic molecules and study the substituent effects using local mode theory |