Page 334 - 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 851

A high-temperature (ca. 1000 K) capillary tube can be is in the microwave region and the pump transition is in

used to pyrolyze a molecule in a buffer gas followed by the millimeter-wave region. The pump radiation is intro-
gas expansion duced into the cavity by means of an oversized microwave
A dc glow discharge of an appropriate gas mixture can horn mounted inside the vacuum chamber. The interaction

be used to produce the desired molecule. Alternatively, region for the millimeter waves with the microwave radi-
the electric discharge can be used to excite vibrational ation is a small cylindrical region above the horn.
states in one component of a gas mixture and these The inset in Fig. 31 depicts a typical DR experiment.
excited molecules form a vibrationally excited The double arrow line represent a microwave signal tran-
complex, for example, Ar···HCl (10 0 state). sition and the single arrow a millimeter-wave pump tran-
◦
Laser ablation of solids can be used to provide metal sition. This is only one of four possible double resonance

reactants. connections for this three-level system. In an actual ex-
periment, the 2 0,2 –3 0,3 microwave transition is monitored
In laser ablation, a metal is vaporized by a high-powered while the millimeter-wave pump transition is slowly swept
laser (Nd:YAG, 532 nm) and the metal vapor is allowed through a given spectral region. A change in the signal
to react with other species present in a buffer gas. The transition (DR effect) confirms the energy level structure
product is stabilized by expansion through a nozzle into and provides an accurate measure of the pump transition
the cavity. This method has been used to produce and study 2 0,2 –3 1,3 . This DR experiment illustrates how spectral as-
such species as InX (X = F, Cl, Br), AlCl, YCl, YBr, and signments may be confirmed. Also, note that the high-
others. frequency pump transition, which lies above the frequency
Besides the perpendicular configuration shown in region of the cavity MWFT spectrometer, can be indirectly
Fig. 31, it is possible to use a parallel configuration where measured with this DR experiment.
the nozzle is mounted in one of the mirrors and the molec-
ular beam from the expansion nozzle travels parallel to
the axis of the cavity. This configuration, in fact, provides E. Terahertz Region
improved resolution.
As apparent from Eq. (1), line intensity increases with
increased frequency and hence the terahertz region offers
1. Pulsed-Nozzle MWFT
significant advantages for the detection of weak signals.
Double-Resonance Spectroscopy
The major methods to obtain high frequencies are (i)
With the configuration of Fig. 31, it is possible to con- frequency multiplication of lower frequency sources and
veniently perform microwave–millimeter wave double- (ii) high-frequency tunable backward-wave oscillators
resonance (DR) experiments. Here, the signal transition (BWOs). BWOs have been developed that provide radi-
ation to 1.3 THz. These sources coupled with broadband
scanning techniques provide capabilities similar to the
lower microwave and millimeter-wave frequency regions.
Significant improvements in multiplier design employing
a planar Schottky diode also allow efficient conversion
of fundamental frequencies to over a terahertz. Here, a
high-power, lower frequency BWO operating in the range
300–600 GHz is used as the primary source. The region is
hence characterized by high sensitivity, high resolution,
high measurement accuracy, and broadband tuning
capabilities.

SEE ALSO THE FOLLOWING ARTICLES
FIGURE 31 A schematic representation of a pulsed cavity FTMW
spectrometer with both molecular beam and cavity axis parallel.
For DR experiments, a microwave horn is used to inject millimeter- ANALYTICAL CHEMISTRY • COLLISION-INDUCED SPEC-
wave radiation into a small interaction zone of the molecular ex- TROSCOPY • HYDROGEN BOND • INFRARED SPEC-
pansion. The inset shows the energy level scheme for a DR ex-
TROSCOPY • INTERSTELLAR MATTER • ION KINETICS AND
periment. Here the signal frequency was fixed at the 2 0,2 → 3 0,3 ENERGETICS • MICROWAVE COMMUNICATIONS • MIL-
transition and this transition was monitored as the pump radiation
was scanned. For details of the DR experiment see Jager, W., and LIMETER ASTRONOMY • QUANTUM CHEMISTRY • TIME
Gerry, M. C. L. (1995). J. Chem. Phys. 102, 3587. AND FREQUENCY