Download CHEM 515 Spectroscopy

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
differ from the IR, visible
and UV techniques in the
sense that they use electronic
devices rather than optical
devices.
Millimeter
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Historical Aspects of MW Spectroscopy
• This was the first MW spectroscopic observation (1934)
when Cleeton and Williams observed MW absorption
frequencies for NH3.
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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.
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Major Components of a MW Spectrometer
MW radiation
source
Waveguide
Detector
Sample cell
Modulator
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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.
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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.
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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.
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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.
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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.
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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 .
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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.
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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
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detector band width limit.
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.
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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.
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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.
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