CHEM 515Spectroscopy

Microwave Spectroscopy I

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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 Sample cell

ModulatorDetector

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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 detector band width limit.

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

Stark Voltage

Recorded by a phase-sensitive detector

Modulation-frequency dependent