Page 283 - 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

800 Microwave Molecular Spectroscopy

magnetic), the coupling of internal and overall rota- of the electromagnetic spectrum, which lies between the
tion, or the effects of nuclear coupling. conventional radiowave region and the infrared region.
Internal rotation Rotation of two parts of a molecule The distinguishing features that characterize microwave
about a single bond. Such an internal rotation is often spectroscopy are high resolution and highly accurate
hindered by a barrier. frequency measurements. These characteristics, coupled
Klystron, backward wave oscillator, YIG oscillator with a sensitivity to molecular structure, isotopic com-
Sources of tunable monochromatic electromagnetic ra- position, and various other molecular properties, make
diation in the microwave region. it a powerful technique for obtaining detailed molecular
Microwave region Part of the electromagnetic region ex- information.
tending from about 1000 MHz (λ = 30 cm) to 1 mil-
lion MHz (λ = 0.3 mm).
Microwave spectrometer A microwave source, absorp- I. BACKGROUND
tion cell, and detector. Used to investigate the rotational
spectrum. In microwave molecular spectroscopy, absorption spec-
Microwave spectrum Usually an absorption spectrum tra arise from molecular rotation and correspond to tran-
(i.e., the abstraction of energy from the radiation field). sitions between the rotational energy levels associated
The spectrum is associated with transitions between with a given vibrational state of a particular electronic
rotational energy levels in a given vibrational and elec- state (see Fig. 1). The rotational transitions, which fall
tronic state and consists of a series of lines, each char- in the microwave region, are induced through the inter-
acterized by a frequency and an intensity. action of the molecular electric dipole with the electric
Rigid rotor Idealization in which a molecule is treated vector of the radiation. The microwave region extends
as a rigid, nonvibrating rotor. roughly from 1000 MHz (λ = 30 cm) to 1 million MHz
Rotational constants Constants that mainly determine (λ = 0.3 mm). In this spectral region, frequencies are ex-
the rotational energy of a molecule. They are inversely pressed in megahertz (MHz) or gigahertz (GHz) units,
proportional to the moments of inertia, which in turn where the Hertz unit denotes cycles per second; 1 MHz =
depend on the mass and geometry of the molecule. 10 Hz and 1 GHz = 10 Hz. In terms of wavelengths, the
6
9
Rotational energy Quantum mechanics requires mole- region may be conveniently divided into the centimeter-
cules to rotate only at certain rates and hence to have wave (λ = 1–30 cm), millimeter-wave (λ = 1–10 mm),
only discrete values of rotational energy and total an- and submillimeter-wave regions (λ< 1 mm). Note that 30
gular momentum. and 300 GHz correspond to 1 cm and 1 mm, respectively.
Rotational quantum numbers Numbers that specify The rotational absorption spectrum depends on the prin-
various angular momenta and distinguish the rotational cipal moments of inertia and, hence, is characteristic of the
energy levels.
Satellite spectrum Rotational absorption lines that arise
from an excited vibrational state rather than the ground
vibrational state.
Selection rules Rules that specify whether or not a
given transition between two particular energy levels is
allowed.
Symmetric top Molecule for which two of the three prin-
cipalmomentsofinertiaareequal.Symmetrictopsmay
be further divided into prolate and oblate tops.
Waveguide Rectangular-shaped metal pipe used to trans-
mit microwave radiation. Different frequency regions
require different-size waveguides.

MICROWAVE molecular spectroscopy involves the ob-
servation and analysis of absorption transitions between
molecularrotationalenergylevelsofgas-phasemolecules. FIGURE 1 Vibrational energy levels associated with the ground
electronic state. With each vibrational level there is a set of ro-
These transitions between rotational levels are associated
tational energy levels. Pure rotational transitions, transitions be-
with a given vibrational state of the ground electronic tween rotational levels of a given vibrational level, are on the right
state, and the transitions fall in the microwave region and pure vibrational transitions are on the left.