Page 284 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN009N-447 July 19, 2001 23:3

Microwave Molecular Spectroscopy 801

whole molecule and very sensitive to the molecular struc- P-band (12.4–18 GHz), K-band (18–26.5 GHz), and
ture, isotopic composition, and numerous other molecu- R-band (26.5–40 GHz). The simplest absorption cell con-
lar properties. Information that can be obtained from the sists of an X-band waveguide 2–5 m long with mica win-
observation and analysis of rotational transitions includes dows at each end to provide a vacuum seal. Small holes
precise molecular structures, dipole moments, centrifu- can be made in the waveguide, usually on its wide face, to
gal distortion constants, vibrational potential functions, allow evacuation of the cell and for sample introduction.
internal rotation barriers, nuclear masses and spins, nu- This cell is used for the various higher frequency bands
clear quadrupole coupling constants, molecular magnetic mentionedpreviouslybyemploymentoftaperedtransition
moments, conformations of rotational isomers and ring sections to match the cell to the smaller waveguide of the
compounds, magnetic susceptibility and electric polariz- source and detector. Sometimes, the interior of the absorp-
ability constants, molecular quadrupole moments, struc- tion cell is gold-plated to limit chemical decomposition.
tures and other properties of hydrogen-bonded complexes, The microwave radiation passing through the absorp-
rare atom–molecule complexes, molecular ions, nonpo- tion cell is detected by a crystal detector mounted at the
lar spherical-top molecules, and qualitative and quan- end of the waveguide. The small output voltage is ampli-
titative analysis. The field of microwave spectroscopy fied and displayed on an oscilloscope or chart recorder.
continues to expand with new experimental and theo- A microwave spectrum is obtained by sweeping the mi-
retical developments. The contributions of microwave crowave source over a range of frequencies and observing
molecular spectra to the fields of microwave–optical and the small variations in power at the detector. An absorption
microwave–infrared double-resonance spectroscopy, mi- line appears as a sharp dip on the recording system.
crowave molecular astronomy, and structures of weak To increase the sensitivity of a microwave spectrome-
complexes attest to its vitality. ter, Stark modulation may be added. For this purpose, a
thin metal septum is mounted in the absorption cell par-
allel to the broad side of the waveguide, using insulating
II. EXPERIMENTAL TECHNIQUES material such as Teflon (see Fig. 2). An alternating (5–
100 kHz) voltage, usually a zero-based square wave, is
The microwave region has its own methods of generat- applied to the septum. Typically, voltages from a few to
ing, detecting, and measuring microwave radiation. Dur- 2000 V are applied. As discussed later, in the presence of
ing World War II considerable work was done to develop an external electric field, the rotational energy levels and,
microwave radar systems. This led to new sources—the therefore, the absorption lines are split into a number of
klystron and other hardware associated with the propa- components. If, for example, the source frequency is tuned
gation and measurement of microwaves. After the war, to the resonant frequency of an absorption line, the Stark
this provided the impetus for a rapid development of mi- voltage periodically shifts the absorption frequency away
crowave spectroscopy as an area of study. from the frequency of the source, resulting in a modulation
(field on and off) of the absorption line at the frequency
of the Stark modulator. In general, a signal reaches the
A. Conventional Microwave Spectrometer
amplifier when the source being swept reaches the field-
A typical microwave spectrometer is illustrated in Fig. 2. off absorption frequency or the frequency of the field-on
The essential elements of a microwave spectrometer are a absorption lines (Stark components or lobes). The mod-
microwave source, absorption cell, detection system, and ulated signal is detected and amplified by a preamplifier
a system for measuring the source frequency. Microwave tuned to the modulation frequency. By employing phase-
sources—the klystron and, more recently, the backward- sensitive detection, one can obtain additional sensitivity.
wave oscillator (BWO)—generate a very narrow band of Here, a final amplifier is referenced to the modulation fre-
frequencies so that the source is essentially monochro- quency, and only the noise that has the same phase and
matic. Furthermore, the source frequency can be conve- frequency as the signal is amplified. With phase-sensitive
niently varied and is often phase stabilized to give good detection, the Stark components are displayed in opposite
frequency stability. phase to the zero-field line. Figure 3 shows the appearance
Microwave radiation is conveniently transmitted of a rotational transition obtained with a Stark-modulated
through hollow, rectangular metallic conductors called spectrometer with application of different modulation
waveguides. These are usually made of brass or copper, voltages.
and the rectangular dimensions depend on the frequency Frequency measurements are made by comparing the
rangetobetransmitted,thesizedecreasingwithincreasing frequency of the microwave source with the appropriate
frequency. Typical frequency bands used in the commonly harmonic of a stable oscillator that is calibrated with a
studied low-frequency region are X-band (8–12.4 GHz), frequency standard. The difference frequency between the