High-Resolution Jet-Cooled Spectroscopy of SF
6
:
The
2
6
Combination Band of
32
SF
6
and
the
3
Band of the Rare Isotopomers
V. Boudon,* M. Hepp,† M. Herman,† I. Pak,‡ and G. Pierre*
*Laboratoire de Physique de l’Universite ´ de Bourgogne, CNRS, B.P. 400, F-21011 Dijon, France; †Laboratoire de Chimie Physique Mole ´culaire,
Universite ´ Libre de Bruxelles, CP 160/09, Av. Roosevelt 50, B-1050 Bruxelles, Belgium; and ‡I. Physikalisches Institut,
Universita ¨t zu Ko ¨ln, Zu ¨lpicher Str. 77, D-50937 Ko ¨ln, Germany
Received June 8, 1998; in revised form July 29, 1998
The Fourier transform infrared spectrum of SF
6
was recorded in a supersonic expansion jet of an SF
6
/argon mixture. The SF
6
:Ar
seeding ratio was 2:3. The instrumental bandwidth was 0.005 cm
-1
. A globar source and an MCT detector were used. A rotational
temperature of approximately 30 K was achieved. The
2
+
6
combination band of
32
SF
6
was analyzed using a modified version
of the spherical top data system (STDS) programs developed in Dijon. A very good fit was obtained for this band with an rms of
0.0036 cm
-1
. The effective Hamiltonian was developed up to fourth order for the
2
+
6
part, to second order for the
2
and ground
state parts, and to first order for the
6
part. Five hundred twenty-one transitions were assigned, 40 of them reaching the F
2u
forbidden
sublevel. The positions of the two F
1u
and F
2u
sublevels are found to be 991.276 and 989.487 cm
-1
, respectively. The
3
band of
the
33
SF
6
and
34
SF
6
isotopomers were also analyzed. Parameters and simulations are presented. A first detection of the
3
Q branch
of
36
SF
6
(0.02% natural abundance) is reported. © 1998 Academic Press
1. INTRODUCTION
For a long time sulfur hexafluoride has been considered as a
very interesting molecule for several reasons. First of all, its
high degree of symmetry combined with a relatively high
molecular weight makes it a good candidate for the study of
fundamental problems related to molecular physics and sym-
metry (1, 2). Moreover, due to its chemical inertness it is easier
to study than other hexafluoride molecules (MoF
6
, WF
6
, UF
6
,
PuF
6
, . . . ). More recently, it appeared that SF
6
is used by
many industries and has been recognized as an atmospheric
pollutant. This molecule could significantly contribute to the
greenhouse effect, due to strong absorption in the midinfrared
region.
Apart form the two strong fundamental bands
3
(3–5)
and
4
(6) and from the overtones of
3
(7–9), only a few
high-resolution studies were published on this molecule,
compared with other spherical tops like CH
4
or SiH
4
for
example. The most complete work ever published on com-
bination, overtone, and hot bands of SF
6
was that of Mc-
Dowell et al. (10, 11), who observed 29 bands, but only at
a relatively moderate resolution (0.05 cm
-1
) that did not
allow a very detailed analysis. These difficulties are due to
the weakness of the combination and overtone bands and to
the congestion of the fine structure, for this heavy species. In
addition, hexafluorides possess low-energy vibrational lev-
els highly populated at room temperature, leading to hot
bands and further increasing the line density. Supersonic jet
expansions provide adequate means to cope with some of
these problems. The experimental setup developed at ULB
(12, 13) combines this technique with the advantages of
FTIR spectroscopy. A long absorption path allows weak
spectroscopic features to be observed. The group in Dijon
developed expertise in the challenging topics of the analysis
of the fine structure in spherical tops molecules (14). The
present merging of both available theoretical and experi-
mental know-hows was thus found most adequate in this
problem.
In this paper we report the first detailed spectroscopic
observation and analysis of a combination band of SF
6
, i.e.,
2
+
6
. We also observed and analyzed in details the
3
fundamental of two isotopomers,
33
SF
6
and
34
SF
6
, and de-
tected the
3
Q branch of
36
SF
6
(only 0.02% natural abun-
dance). The experimental details are provided in Section 2.
The theoretical background is explained in Section 3 and the
results are given in Sections 4 for
32
SF
6
and in Section 5 for
the rare isotopomers.
2. EXPERIMENT
The FTIR slit jet setup used in Brussels has already been
described in the literature (12, 13). The spectrometer is a
Bruker IFS120HR Fourier transform interferometer with a
maximum resolution of 0.00185 cm
-1
. It was equipped with a
globar source and a KBr beam splitter. An optical filter was
used to cut all wavelengths shorter than about 7.5 m. For the
jet measurements a parallel light beam was coupled out of the
interferometer through a KBr window allowing the interferom-
JOURNAL OF MOLECULAR SPECTROSCOPY 192, 359 –367 (1998)
ARTICLE NO. MS987699
359
0022-2852/98 $25.00
Copyright © 1998 by Academic Press
All rights of reproduction in any form reserved.