Fourier Transform Infrared (FTIR) Spectrometer

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Fourier Transform Infrared (FTIR) Spectrometer

• Subhashree Mishra
• ATMS Grad Student, UNR
• W. P. Arnott
• Physics, UNR

Introduction to Atmospheric Instrumentation (ATMS
360) University of Nevada Reno
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Energy Levels Basic Ideas
Basic Global Warming The C02 dance
About 15 micron radiation
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Wavelength and Wavenumber

• Wavelength 1 / Wavenumber
• For the IR, wavelength is in microns.
• Wavenumber is typically in 1/cm, or cm-1.
• 5 microns corresponds to 2000 cm-1.
• 20 microns corresponds to 500 cm-1.
• 15 microns corresponds to 667 cm-1. Much
  terrestrial IR energy at the wavenumber.

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Carbon Dioxide Concentration
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Example Problem Instantly Double CO2
Concentration.What is the effect on the infrared
spectrum at the surface?
Consequence The Earths surface warms because
of the additional IR comingto the surface from
the Atmosphere.
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Example Problem Instantly Double CO2
Concentration.What is the effect on the infrared
spectrum from space?
Consequence The less IR radiation escapes to
space when the atmosphere has 800 ppmCO2 because
the atmosphere is less transparent to IR emitted
by the Earths surface. TheEarths surface
temperature must increase to again balance the
outgoing IR with the incoming solar radiation.
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LEDs As Detectors
Each photon with enough energy will normally free
exactly one electron, and result in a free hole
as well. If this happens close enough to the
electric field, or if free electron and free hole
happen to wander into its range of influence, the
field will send the electron to the N side and
the hole to the P side. This causes further
disruption of electrical neutrality, and if we
provide an external current path, electrons will
flow through the path to their original side (the
P side) to unite with holes that the electric
field sent there, doing work for us along the
way. The electron flow provides the current, and
the cell's electric field causes a voltage. With
both current and voltage, we have power, which is
the product of the two.
From http//science.howstuffworks.com/solar-cell3.
htm
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LEDs As Detectors Thermal Noise
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FTIRs Often Use MCT Detectors Mercury Cadmium
Telluride
HgCdTe or Mercury cadmium telluride (also Cadmium
Mercury Telluride, MCT or CMT) is an alloy of
CdTe and HgTe and is sometimes claimed to be the
third semiconductor of technological importance
after Silicon and Gallium(III) arsenide. The
amount of cadmium (Cd) in the alloy (the alloy
composition) can be chosen so as to tune the
optical absorption of the material to the desired
infrared wavelength. (from http//en.wikipedia.org
/wiki/Mercury_cadmium_telluride)
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Outline

• Introduction
• Theory
• Design
• Applications
• Measurements
• Discussions

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What is a FTIR Spectrometer?

• A spectrometer is an optical instrument used to
  measure properties of light over a specific
  portion of the electromagnetic spectrum, 5
  microns to 20 microns.
• FTIR (Fourier Transform InfraRed) spectrometer is
  a obtains an infrared spectra by first collecting
  an interferogram of a sample signal using an
  interferometer, then performs a Fourier Transform
  on the interferogram to obtain the spectrum.
• An interferometer is an instrument that uses the
  technique of superimposing (interfering) two or
  more waves, to detect differences between them.
  The FTIR spectrometer uses a Michelson
  interferometer.

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FOURIER TRANSFORMS

• Fourier transform defines a relationship between
  a signal in time domain and its representation in
  frequency domain.
• Being a transform, no information is created or
  lost in the process, so the original signal can
  be recovered from the Fourier transform and vice
  versa.
• The Fourier transform of a signal is a continuous
  complex valued signal capable of representing
  real valued or complex valued continuous time
  signals.

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Fourier Transforms cont.

• The Continuous Fourier Transform, for use on
  continuous signals, is defined as follows
• And the Inverse Continuous Fourier Transform,
  which allows you to go from the spectrum back to
  the signal, is defined as
• F(w) is the spectrum, where w represents the
  frequency, and f(x) is the signal in the time
  where x represents the time. i is sqrt(-1), see
  complex number theory.

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Fourier Transforms cont.

• A computer can only work with finite discrete
  signals, not with continuous signals. Thus, we
  need to define the Discrete Fourier Transform
  (DFT).
• In DFT, the infinite borders of the integrals can
  be replaced by finite ones, and the integral
  symbol can be replaced by a sum. So the DFT is
  defined as
• And the inverse DFT is defined as

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FTIR Theory
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• The spectrometer described here is a modified
  Bomem MB-100 FTIR.
• The heart of the FTIR is a Michelson
  interferometer (figure 2).
• The mirror moves at a fixed rate. Its position is
  determined accurately by counting the
  interference fringes of a collocated Helium-Neon
  laser.
• The Michelson interferometer splits a beam of
  radiation into two paths having different
  lengths, and then recombines them.
• A detector measures the intensity variations of
  the exit beam as a function of path difference.
• A monochromatic source would show a simple sine
  wave of intensity at the detector due to
  constructive and destructive interference as the
  path length changes (refer figure 3).

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• In the general case, a superposition of
  wavelengths enter spectrometer, and the detector
  indicates the sum of the sine waves added
  together.
• Figure 3 shows some idealized light sources, and
  the interferograms that they would theoretically
  produce.
• The difference in path length for the radiation
  is known as the retardation d (OM OF d) in
  figure 1 and 2.
• When the retardation is zero, the detector sees
  a maximum because all wavenumbers of radiation
  add constructively.
• When the retardation is l/2, the detector sees a
  minimum for the wavelength l. An interferogram is
  the sum of all of the wavenumber intensities.

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Figure 1.
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Schematic of Michelson Interferometer
Figure 2.
Source MS thesis submitted by Carl George
Schmitt, UNR , 1998.
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Wave Interference
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Sample interferograms and their theoretical
source intensity Source MS thesis submitted by
Carl George Schmitt, UNR , 1998.
Figure 3.
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(No Transcript)
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Calibration of the FTIR spectrometer
Source MS thesis submitted by Carl George
Schmitt, UNR , 1998.
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• The spectrometer produces a complex voltage at
  each wavenumber. A linear model for the
  spectrometer response is assumed, where A is an
  instrument offset, and C is a scaling factor,
• V ACI (1)
• If the spectrometer views a perfect blackbody,
  Eq. (1) gives
• V A CBT (2)
• where BT is the Planck emission curve for a
  blackbody of temperature T.
• The two unknowns (A and C) can be determined from
  blackbody measurements at two different
  temperatures,
• V1 A CBT1
• V2 A CBT2

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• Solving for the unknowns yields
• C (V1-V2)/(BT1-BT2)
• and A V1(BT1-BT2)-BT1 (V1-V2)/(BT1-BT2)
• Returning to Eq (1), The FTIR voltage of another
  target (Vtarget) is related to the target
  radiance (Itarget) by
• Itarget(BT1-BT2)VtargetBT1V2BT2V1/(V1-V2)
• Thus, with measurements of blackbodies at two
  temperatures, the calibrated radiance from a
  target (cloud) can be determined.

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APPLICATIONS

• Identification of inorganic compounds and organic
  compounds
• Identification of components of an unknown
  mixture
• Analysis of solids, liquids, and gasses
• In remote sensing
• In measurement and analysis of Atmospheric
  Spectra
• - Solar irradiance at any point on earth
• - Longwave/terrestrial radiation spectra
• Can also be used on satellites to probe the space

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Source UV thoughts from http//uvb.nrel.colostat
e.edu/UVB/publications/uvb_primer.pdf
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MODIS Solar Irradiance Source
http//en.wikipedia.org/wiki/ImageMODIS_ATM_solar
_irradiance.jpg
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Theoretical Absorption Cross Sections
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Theoretical Absorption Cross Sections for the
indicated gases, averaged to 1 cm-1 resolution
for clarity.
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Measurement Example from Reno
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FTIR Radiance Atmospheric IR Window
13 microns
8 microns
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DEFINITION OF THE BRIGHTNESS TEMPERATURETB
Measured Radiance at wavenumber v
Theoretical Radiance of a Black
Body at temperature TB
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FTIR Brightness Temperatures
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Atmosphere Emission Measurements, Downwelling
Radiance

• Notes
• Wavelength range for CO2, H20, O3, CH4.
• Envelope blackbody curves.
• Monster inversion in Barrow.
• Water vapor makes the tropical window dirty.

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RENO FTIR SPECTRA
Which day is more moist?
Which day is warmer near the surface?
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Ideal Weighting Function Wi Where in the
atmosphere the main contribution to the radiation
at wavenumber ?i comes from.
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Downwelling Intensity Emitted by the Atmosphere
to the Detector (Radiance)
?cos?
BT(z)
emissivity?absdz/cos?
z
dz
q
blackbody radiance, T temperature.
q
ftir
transmission
emission
weighting function
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Weighting Functions for Satellite Remote Sensing
using the strong CO2 absorption near 15.4 um.
(from Wallace and Hobbs, 2nd edition)