Page 332 - 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 849

collisions occur before the molecule collides with the cell surements are made at various pressures. The linewidth
wall. It is found, that in less than 100 collisions, the trans- ν is extracted from the spectral line shape. These data
lational and rotational temperature is cooled to that of the provide γ from Eq. (131) and n from the temperature de-
buffer gas. On the other hand, the relaxation of the vi- pendence of Eq. (132).
brational degrees of freedom is slower. The vibrational As an example, typical of pressure broadening stud-
temperature is found to depend on the ejection tempera- ies, we consider the 2 2,0 → 3 1,3 transition of water at
ture, the number of collisions with the background gas, 183 GHz with H 2 as a collision partner in the tempera-
and the vibrational relaxation cross section. It is there- ture range 80–600 K and pressure range 0.05–1.0 Torr.
fore possible to attain separate translational/rotational Above 150 K, the temperature dependence is given by the
and vibrational temperatures by control of the injec- above relation with γ H2 (300 K) = 3.20 ± 0.08 MHz/Torr
tion temperature and the pressure and temperature of the and n = 0.95 ±0.07. However, below 150 K, there is a sig-
buffer gas. nificant deviation from the power law. This result indicates
that H 2 as a collision partner is considerably more com-
plex, and H 2 does not act as a classical hard sphere. This
Collisional Broadening
is also evident from the value of n since for a hard-sphere
To extract molecular concentration information and to model n = 0.50. On the other hand, for He as a collision
model the earth’s atmosphere accurate knowledge of pres- partner, n over the whole temperature range is found to be
sure and temperature effects on spectral line shapes is 0.49 ± 0.02, indicating that He behaves essentially as a
needed. Such pressure broadening studies are necessary classical hard sphere in collisions with H 2 O.
to develop models of the chemistry and physics of the Similarly, the proper interpretation of radio astronom-
atmosphere and to gain insight into problems associated ical spectral lines from dense interstellar clouds requires
with air pollution, the greenhouse effect, ozone hole, etc. collisionalinformationinvolvingionsatlowtemperatures.
Spectroscopic remote sensing of planetary and interstellar By combining the considerations reflected in Figs. 27 and
atmospheres also requires such line broadening informa- 29, it is possible to obtain the pressure broadening of
tion. Line shape information provides direct information molecular ions at very low temperatures. This has been
+
on the environment of the molecule, viz., temperature, demonstrated for the ion HCO and the collision partner
pressure, collision partner, etc. Principal collision partners H 2 . Likewise, extension of direct time-resolved measure-
for minor atmospheric species are N 2 and O 2 . Pressure ments, as discussed in Section IV.D, has been recently
broadening studies have been carried out at temperatures carried out incorporating the collisional cooling technique
characteristic of the earth’s atmosphere. However, for discussed here.
planetary species, the dominant collision partners are He
and H 2 . Furthermore, the atmospheres are characterized
D. Fourier-Transform Microwave Spectroscopy
by low pressures and temperatures, which are difficult to
simulate with conventional approaches. At the low tem- The technique of Fourier-transform microwave spec-
peratures, with conventional techniques, the vapor pres- troscopy (FTMS) has been applied to the study of a num-
sures would be vanishingly small due to condensation. ber of weakly bonded complexes, the observation of weak
On the other hand, the collisional cooling technique dis- isotopic species, and the resolution of hyperfine struc-
cussed here provides an ideal laboratory method to sim- ture. It is characterized by higher resolution and sensi-
ulate planetary conditions and similar low-temperature, tivity than conventional Stark-modulated spectros-
low-pressure conditions. copy. The superior resolution is demonstrated in Figs. 24
In linewidth studies, the width is measured versus pres- and 25.
sure and temperature. The linewidth varies with pressure In this method, a short, intense microwave pulse is ap-
at a given temperature as plied to the sample. This pulsed microwave excitation of
the sample produces a transient emission signal which is
ν = γP + ν 0 , (131) detected by a transient signal averager. The time response
of the system is hence observed. Both waveguide-based
where P is the pressure and γ is the pressure broaden-
sample cells and cavity-based cells have been employed.
ing coefficient. Here ν denotes the total linewidth. The
To produce significant transient emission, a high power
temperature dependence of γ is taken as
source (order of watts) is required for the waveguide-
n
γ (T ) = γ 0 (T 0 /T ) , (132) based system. Lower power sources are applicable to the
cavity systems because of the very narrow bandwidth of
such systems. The transient emission is usually averaged
where γ 0 is the coefficient at the reference temperature T 0
and n is a constant. At each temperature, linewidth mea- over many cycles to improve the signal-to-noise ratio. This