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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
                    Energy Levels: Basic Ideas




Basic Global Warming: The C02 dance …   About 15 micron radiation
    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.
Carbon Dioxide Concentration
Example Problem: Instantly Double CO2 Concentration.
What is the effect on the infrared spectrum at the surface?




Consequence: The Earth’s surface warms because of the additional IR coming
to the surface from the Atmosphere.
    Example Problem: Instantly Double CO2 Concentration.
    What is the effect on the infrared spectrum from space?



  Satellite with FTIR
  Looking Down




     B(Te)        Te




Consequence: The less IR radiation escapes to space when the atmosphere has 800 ppm
CO2 because the atmosphere is less transparent to IR emitted by the Earth’s surface. The
Earth’s surface temperature must increase to again balance the outgoing IR with the
incoming solar radiation.
                   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
LEDs As Detectors: Thermal Noise
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)
                   Outline

   Introduction
   Theory
   Design
   Applications
   Measurements
   Discussions
          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.
             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.
              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.
             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:
FTIR Theory
   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).
   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.
Figure 1.
  Schematic of Michelson Interferometer




Figure 2.



            Source: MS thesis submitted by Carl George Schmitt, UNR , 1998.
Wave Interference
Figure 3.


     Sample interferograms and their theoretical source intensity

       Source: MS thesis submitted by Carl George Schmitt, UNR , 1998.
Calibration of the FTIR spectrometer




  Source: MS thesis submitted by Carl George Schmitt, UNR , 1998.
   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= A+CI                      (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
   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)Vtarget–BT1V2+BT2V1]/(V1-V2)


   Thus, with measurements of blackbodies at two temperatures,
    the calibrated radiance from a target (cloud) can be
    determined.
                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
Source : UV thoughts from http://uvb.nrel.colostate.edu/UVB/publications/uvb_primer.pdf
              MODIS Solar Irradiance
Source : http://en.wikipedia.org/wiki/Image:MODIS_ATM_solar_irradiance.jpg
Theoretical Absorption Cross Sections
Theoretical Absorption Cross Sections for the indicated gases, averaged to 1 cm-1 resolution for clarity.
Measurement Example from Reno
FTIR Radiance: Atmospheric IR Window
 13 microns         8 microns
DEFINITION OF THE BRIGHTNESS TEMPERATURE
                     TB




  Measured Radiance at wavenumber v
                   =
  Theoretical Radiance of a Black Body at temperature TB
FTIR Brightness Temperatures
 Atmosphere
  Emission
Measurements,
 Downwelling
  Radiance
Notes:
1.   Wavelength
     range for CO2,
     H20, O3, CH4.
2.   Envelope
     blackbody
     curves.
3.   Monster
     inversion in
     Barrow.
4.   Water vapor
     makes the
     tropical window
     dirty.
Which day is more moist?


Which day is warmer
near the surface?


  RENO FTIR
   SPECTRA
Ideal Weighting Function Wi: Where in the atmosphere the main
  contribution to the radiation at wavenumber i comes from.
Downwelling Intensity Emitted by the Atmosphere to the Detector
                          (Radiance)
                                       =cosq
     z dz emissivity=absdz/cosq         q      B[T(z)]
                                               blackbody
                                               radiance,
                              q                T = temperature.
                             ftir
          emission     transmission




                                               weighting
                                               function
   Weighting Functions for
Satellite Remote Sensing using
  the strong CO2 absorption
near 15.4 um. (from Wallace
   and Hobbs, 2nd edition)
I  B(T )exp  
 i           s
                          All Atmos
                          abs          (surface)
 
  B[T (z)]exp( abs (z)) abs (z) dz (atmos)
     0
or
I  B(T )exp  
 i           s
                          All Atmos
                          abs          (surface)
 
  B[T (z)]Wi (z) dz (atmos)
     0
         Satellite with FTIR
         Looking Down




            B(Te)        Te

				
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