Page 331 - 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

848 Microwave Molecular Spectroscopy

FIGURE 28 Illustration of a high-temperature reaction flow system coupled to a microwave spectrometer for producing
short-lived molecules. The gas is made to flow through the Stark cell by means of vacuum system and the flow rate is
adjusted by a needle valve. The 8-mm-inside-diameter quartz tube is heated in a 10-cm-long furnace. The distance
between the cell and furnace is also about 10 cm. [From Kroto, H. W. (1982)]. Chem. Soc. Rev. 11, 435.]

spectra contributes immensely to the task of assigning the cell, where it undergoes a series of collisions with the cold
spectra. Moreover, detection is improved because of an in- background gas as it randomly walks toward the cell wall
crease in the absorption coefficient with decreasing tem- and freezes out. The spectroscopic gas may be heated by
perature. Temperatures near 1 K have been obtained in means of the small oven around the injector tube to tem-
microwave studies. Very low temperature spectroscopy peratures up to ca. 1300 K. Since the spectroscopic gas
provides the opportunity to study weakly bound species, is at low concentration, it does not affect the temperature
discussed in Section IV.E.3, as well as other processes. of the inert buffer gas. The collisions cool the molecule’s
Some of the advantages and applications are summarized various degrees of freedom, and typically some 10,000
in Table XXIX.
Very low temperature conditions can be achieved by
use of free expansion jets. Numerous weakly bound com-
plexes have been produced by this method (see Sec-
tions IV.E.3 and XI.D). Another method is a collisional
cooling technique. This technique is particularly versa-
tile; the system is shown in Fig. 29. The system employs
a variable-temperature injector to produce an enhanced
population in excited vibrational states and liquid nitro-
gen temperature trapping. The cell is maintained at 77 K
and contains a buffer gas of helium or nitrogen, which has
a significant vapor pressure at 77 K. A small amount of
the gas to be studied is injected into the cooled absorption

TABLE XXIX Advantages and Applications of Low-
Temperature Microwave Spectroscopy

Improved resolution
Increased sensitivity
Simplification of complex spectra
FIGURE 29 Schematic illustration of a collisionally cooled cell.
More direct correspondence between observable parameters and
The 4-in.-diameter cell is 3 ft long with the center 1-ft section made
theoretical parameters
of copper and the end sections of stainless steel. The dashed-
Enhance production and lifetime of transient species
line region is enclosed in a 77 K medium. Windows for passing
Energy transfer microwave radiation are -in. Teflon. The injector is a copper tube
1
2
Reaction dynamics at low temperature (0.04-in. inside diameter) surrounded by a small oven at the inlet
Pressure-broadening processes where hν> kT to the cell. The inset shows a random collision scenario. [After
Goyette, T. M., Ebenstein, W. L., and De Lucia, F. C. (1990). J. Mol.
Simulate low-temperature and low-pressure environments (outer space)
Spectrosc. 140, 311.]