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					   CHEM 515
  Spectroscopy


Microwave Spectroscopy I
         Microwave and Millimeter Wave
                 Spectroscopy
• MW covers the range 1-100
  GHz.
• Millimeter wave covers the
  region 100-600 GHz (order
  of 1 mm wavelength).
• The MW techniques mainly         Millimeter
  differ from the IR, visible
  and UV techniques in the
  sense that they use electronic
  devices rather than optical
  devices.

                                                2
 Historical Aspects of MW Spectroscopy




• This was the first MW spectroscopic observation (1934)
  when Cleeton and Williams observed MW absorption
  frequencies for NH3.

                                                           3
       Bands in the Microwave Region

• In MW spectroscopy, a particular range of frequencies is
  called a band. “MW bands” are given labels as sown in the
  table below.




                                                              4
Major Components of a MW Spectrometer

    MW radiation
      source                  Waveguide   Sample cell




                   Detector               Modulator




                                                        5
                 Microwave Sources

• Klystron Source
  Klystron is an evacuated
  electron tube that produces
  low power frequencies
  (signals).
  The electrons travel through
  the klystron in resonant
  cavities, where their speed is
  regulated. As the electrons
  change speed in the klystron,
  they emanate radiation in the
  form of microwaves.
                                     6
                 Microwave Sources

• Klystron Source
  The microwaves accumulate
  inside the cavity and are fed
  into a “waveguide” that
  works as a selector for a
  particular range of
  frequencies.
  The waveguide is a metal
  tube usually made of copper
  and it prevents loss of
  radiation.

                                     7
                 Microwave Sources

• The Backward Wave Oscillator (BWO)
  The electron beam (from an electron gun) passes
  through a wire helix and generates an electric field that
  induces voltage with the helix wire. The resonating
  electric fields (in and out) produce microwaves in the
  direction opposite to the electron beam.




                                                              8
                 Microwave Sources

• The Backward Wave Oscillator (BWO)
  The BWO method is more convenient and can cover a
  complete MW band, unlike the klystron.
  The frequency of the radiation is varied by controlling
  the beam velocity and the helix potential.




                                                            9
                Microwave Sources

• Microwave sources are highly
  monochromatic (it comprises
  an extremely narrow range of
  wavelengths). This one reason
  that characterizes the very
  small bandwidth of absorption
  lines in the microwave region.




                                    10
Millimeter and Terahertz Wave Sources

• Klystron output waves can be
  converted into higher-frequency
  waves by frequency multiplier
  oscillators (FMO) that generate
  harmonics of lower frequency
  MW radiations.
• BWOs have been developed for
  the millimeter wave region up to
  frequency of 1000 GHz (1 THz)
  which corresponds to the
  terahertz spectroscopy .

                                     11
    Sample Cells in MW Spectrometers

• The MW absorption cell has windows
  normally made from “mica” and is
  preferably evacuated.
• It may be several meters in length and is
  plated from the inside with an inert metal,
  such as copper or gold, to prevent loss of
  energy and to avoid troublesome
  absorption with the sample, which is
  normally in the form of gas, being
  studied.
• The waveguide may be used as a sample
  cell in some cases.
                                                12
       Detectors in MW Spectrometers
• MW and millimeter spectrometer detectors are made from
  crystal diode.
• One common problem with MW detectors is their low
  sensitivity as they produce random noise due to electrical
  effects. That random noise is expressed in terms of electrical
  power:
                         P = kTΔν + CI2 Δν/ν
  The “kTΔν” term is called Johnson noise and little can be done
  to reduce it significantly.
  The second term (C is constant, I is current, Δν is the detection
  limit, and ν is the modulation frequency) can be reduced by
  increasing the modulation frequency or by decreasing the
  detector band width limit.                                      13
                       Modulators

• Modulators are electrical devices that experience periodic
  variation (between 10 t o1000 times per second) to the
  radiation beam and adjust the wave amplitudes before it
  reaches to the detector.
• A MW modulator has many advantages:
   – Increasing the sensitivity of the detector.
   – Selecting the characteristic signals to impose amplification
     on, which produce cleaner spectrum results.
   – Causing the detector to send an ac current with confined
     frequency range (10-1000 Hz) to the detector instead of the
     dc current. The ac current is readily amplified and
     translated.
                                                                14
                   Stark Modulator

• The effect of an applied external
  electric field of a given voltage on
  the energy separations between
  energy levels of an atom or
  molecule is known as “Stark
  effect”.
• “Stark modulation” is widely used.
  When the lines are split due to the
  Stark effect it gives rise to ac
  signals at the detector whenever a
  MW absorption line is detected.

                                         15
                        Stark Modulator

              Stark Voltage

 Modulation-frequency
 dependent

    Recorded by a
    phase-sensitive
    detector




• Stark modulation is also useful in measuring the
  permanent dipole moments of molecules.
                                                     16

				
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