THE ASTROPHYSICAL JOURNAL, 472 : L57–L60, 1996 November 20
q 1996. The American Astronomical Society. All rights reserved. Printed in U.S. A.
THE PURE ROTATIONAL SPECTRUM OF FeC (X 3 D i )
M. D. ALLEN, T. C. PESCH,1
AND
L. M. ZIURYS1
Department of Chemistry, Arizona State University, P.O. Box 871604, Tempe, AZ 85287-1604
Received 1996 July 8; accepted 1996 September 9
ABSTRACT
The pure rotational spectrum of the FeC radical (X 3 D i ) has been measured in the laboratory for the first time
using millimeter/submillimeter direct absorption techniques. FeC was created by the reaction of iron vapor,
produced in a high-temperature Broida-type oven, and methane gas under DC discharge conditions. Six
rotational transitions each were recorded for the two lower spin-orbit ladders of this molecule in the frequency
range 240 – 484 GHz, as well as six transitions for the V 5 3 ladder of the iron isotopomer 54 FeC. The data were
analyzed using a 3 D Hamiltonian, and rotational and certain spin-orbit parameters were determined. The
observation of SiC in the circumstellar shell of IRC 110216 suggests that metal carbides such as FeC may be
present as well.
Subject headings: ISM: molecules — line: identification — methods: laboratory — molecular data
available for these elusive molecules. Several optical measurements on various metal carbides, however, have been done.
For example, the 4 S– 4 S system of AlC has been studied via
emission spectroscopy (Brazier 1993), and several optical
transitions of CoC have been recorded using LIF techniques
(Barnes, Merer, & Metha 1995). Very recently, the 3 D 3 3 D
bands of FeC have been observed, again using LIF methods
(Balfour et al. 1995). Aside from optical work, matrix isolation
ESR spectroscopy of AlC has also been accomplished, which
has enabled the investigation of the aluminum hyperfine
interactions in this radical (Knight et al. 1990).
In this Letter we present the first measurements of the pure
rotational spectrum of a metal carbide species, FeC, which has
a 3 D i ground state. We have recorded several transitions
arising from the two lower spin-orbit substates of this radical
using millimeter/submillimeter wave direct absorption spectroscopy, and have also obtained data for the 54 FeC isotopomer. Spectroscopic parameters have been determined for
both species and are presented here as well.
1. INTRODUCTION
Although various metal-bearing molecules have been detected in circumstellar gas since 1987, no species have yet been
found that contain the element iron. Toward the expanding
envelope of the late-type carbon star IRC 110216, only
compounds bearing magnesium (MgCN, MgNC: Kawaguchi
et al. 1993; Ziurys et al. 1995), aluminum (AlCl, AlF: Cernicharo & Guélin 1987), sodium (NaCl, NaCN: Cernicharo &
Guélin 1987; Turner, Steimle, & Meerts 1994), and even
potassium (KCl: Cernicharo & Guélin 1987) to date have been
observed. Because iron has a cosmic abundance a factor of 5
higher than either sodium, aluminum, or potassium, it is
possible that an Fe-bearing compound will eventually be
discovered in circumstellar or even interstellar material. Such
a result will be interesting because iron is the end product of
silicon burning and hence thermal fusion in stars, and its
abundance, including that of its 54 Fe isotope, has important
implications for nucleosynthesis.
There have been several unsuccessful searches in the past
for interstellar iron-containing molecules, including FeO, FeF,
and FeCl (e.g., Merer, Walmsley, & Churchwell 1982; Ziurys
et al. 1996). Such observations were carried out in conjunction
with precise laboratory spectroscopy measurements usually
performed in the millimeter/submillimeter wavelength region,
such as the work of Endo, Saito, & Hirota (1984) for FeO,
Tanimoto, Saito, & Okabayashi (1995) for FeCl, and Allen &
Ziurys (1996) for FeF. It is unclear, however, that any of these
three radicals would be the most likely carrier of iron in an
envelope of a carbon-rich star such as IRC 110216.
One group of metal-bearing species that have yet to be
searched for in interstellar or circumstellar gas are the metal
diatomic carbides. Such molecules are of particular interest
because SiC has already been detected in IRC 110216 (Cernicharo et al. 1989), giving irrefutable proof that refractory
elements combined with carbon exist in circumstellar gas.
Moreover, the envelope of IRC 110216 is carbon-rich, and
C-containing molecules are very likely to be abundant. Interstellar studies of metal carbide species have not been carried
out thus far because accurate rest frequencies have not been
2. EXPERIMENTAL
The measurements were carried out using a second generation millimeter/submillimeter wave direct absorption spectrometer at Arizona State University, which utilizes offset
ellipsoidal mirrors as focusing elements (Allen et al. 1996a).
Briefly, the instrument consists of a tunable source of millimeter-wave radiation, a reaction chamber, and a heliumcooled InSb detector. The sources used are Gunn oscillators
combined with Schottky diode multipliers. The radiation is
launched from a scalar feedhorn/Teflon lens combination and
propagated through the reaction chamber quasi-optically using offset ellipsoidal mirrors. The reaction chamber has a
double-pass optical scheme with a rooftop reflector at one end
which rotates the plane of polarization of the radiation by 908.
After the radiation makes its second pass through the cell and
optics, it is reflected by a wire grid and focused by another lens
into the detector. Phase-sensitive detection is accomplished by
frequency modulation of the source.
Iron carbide was synthesized in a mixture of iron vapor and
methane gas. The iron vapor was produced using a hightemperature Broida-type oven which functions near 14008C.
1 Current address: Departments of Astronomy and Chemistry, and Steward
Observatory, University of Arizona, Tucson, AZ 85721.
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ALLEN, PESCH, & ZIURYS
Vol. 472
TABLE 1
OBSERVED TRANSITION FREQUENCIES
56
FeC: X 3 D i ( v 5 0) a
OF
J9 4 J
V
n obs
(MHz)
n obs 2 n calc
(MHz)
6 4 5...........
2
3
242020.332
240862.951
0.003
20.006
7 4 6...........
2
3
282338.846
280989.165
0.005
20.004
8 4 7...........
2
3
322648.966
321107.258
0.017
0.007
9 4 8...........
2
3
362949.425
361216.048
20.028
0.007
11 4 10 . . . . . . . .
2
3
443516.843
441401.086
20.001
20.012
12 4 11 . . . . . . . .
2
3
483781.334
481475.045
0.006
0.005
a
The estimated experimental uncertainty is
75 kHz.
The iron vapor was entrained in 110 mtorr of helium carrier
gas and reacted with 17 mtorr of methane in a DC discharge
operated near 500 V and 750 mA. FeC could also be produced
using CO gas as the carbon donator instead of CH 4 under
identical conditions, and with C 3 O 2 as well, although the
signals were not as strong. In the latter case, no DC discharge
was found to be necessary. The fact that these three different
precursors produced the same spectra is additional evidence
that the molecule synthesized is FeC.
Center frequencies were determined by fitting Gaussian
curves to the line profiles. Typical line widths were
500 –1400 kHz over the interval 200 –500 GHz.
3. RESULTS
The frequencies of the rotational transitions observed for
FeC and 54 FeC are listed in Tables 1 and 2, respectively. As
Table 1 shows, six rotational transitions each have been
measured for the V 5 3 and V 5 2 ladders of 56 FeC; no
evidence of lambda-type doubling was found in these two
spin-orbit components. Table 2 gives the details of the six
transitions measured for 54 FeC. In this case, only the lowest
spin component (V 5 3) was observed. The data for the iron
54 isotopomer were recorded in the natural isotope ratio of
56
Fe: 54 Fe 5 92;6. One transition of Fe 13 C was also observed
56
TABLE 2
OBSERVED TRANSITION FREQUENCIES
54
FeC: X 3 D i ( v 5 0) a
OF
J9 4 J
V
n obs
(MHz)
n obs 2 n calc
(MHz)
6 4 5...........
7 4 6...........
8 4 7...........
9 4 8...........
11 4 10 . . . . . . . .
12 4 11 . . . . . . . .
3
3
3
3
3
3
242480.335
282875.343
323261.878
363638.714
444358.473
484698.957
0.004
20.002
0.001
20.004
0.003
20.001
a
The estimated experimental uncertainty is
75 kHz.
FIG. 1.—Spectra of the J 5 12 4 11 transition 56 FeC and 54 FeC observed
in this work near 481– 485 GHz. Each spectrum covers 50 MHz in frequency
and was taken in one 30 s scan. The top and middle spectra are the V 5 3 and
V 5 2 spin-orbit sublevels of 56 FeC, respectively, while the bottom figure shows
data for the V 5 3 state of 54 FeC.
as an additional confirmation, the J 5 13 4 12 transition at
488491.0 MHz.
Figure 1 presents spectra of the J 5 12 4 11 rotational
transition of 56 FeC in the V 5 3 ladder near 481.5 GHz (top
panel), 56 FeC in the V 5 2 ladder near 483.8 GHz (middle
panel), and 54 FeC in the V 5 3 ladder near 484.7 GHz (bottom
panel). All three spectra cover a 50 MHz frequency range, and
each was taken in one 30 s scan.
The data were modeled with an effective Hamiltonian of the
form (Brown et al. 1979)
Ĥ eff 5 Ĥ rot 1 Ĥ so 1 Ĥ ss ,
(1)
where the individual terms describe rotational, spin-orbit, and
spin-spin interactions, including their centrifugal distortion
No. 1, 1996
PURE ROTATIONAL SPECTRUM OF FeC
TABLE 3
MOLECULAR PARAMETERS
FeC
FOR
56
IN THE
X 3 D i STATE (in MHz) a
FeC
CONSTANT
Present Work
Previous Values b
54
FeC
(Present Work)
A ...............
AD . . . . . . . . . . . . .
B ...............
B (V 5 3) e . . . .
B (V 5 2) e . . . .
D ...............
D (V 5 3) e . . . .
D (V 5 2) e . . . .
23722680 c
4.66652 (61)
20173.4036 (19)
20075.3976 (66)
20171.9625 (67)
0.0500739 (80)
0.048394 (31)
0.050025 (31)
123297700 d
...
...
20080 (45)
20203 (45)
...
...
...
23722680 c
4.728 c
20309.64703 (62)
20210.3292 (66)
...
0.0522326 (30)
0.050483 (31)
...
a
Errors given are 3 s statistical uncertainties, in units of the last quoted
decimal place.
b
From Balfour et al. (1995).
c
Parameter constrained to this value in the fit (see text).
d
Originally quoted as (2110 H 25 cm 21).
e
Neglecting spin-orbit contributions (see text).
corrections. Neglecting centrifugal distortion, these terms can
be expressed as (e.g., Brown, Cheung, & Merer 1987)
Ĥ eff 5 B@ J~ J 1 1! 2 V 2 1 S~S 1 1! 2 S 2 #
1 ALz Sz 1 ~2/3! l ~3S2z 2 S 2! .
(2)
In this equation, B is the rotational constant, A is the
spin-orbit parameter, and l describes the spin-spin interactions.
Because estimates of the spin-spin constant were not available, the data for FeC were fitted in two different ways. First,
the V 5 2 and V 5 3 ladders of 56 FeC and the V 5 3 sublevel
of 54 FeC were fitted to individual effective B and D constants,
neglecting any spin-orbit or spin-spin terms. The rotational
parameters thus obtained are given in Table 3. Then, spinorbit interactions were considered. Because the spin-orbit
constant is independent of J, it could not be exclusively
determined from our data set, which involves transitions only
within V ladders. However, AD , the centrifugal distortion
correction to A, can be established because it does have a J
dependency. Therefore, the data were fitted by first allowing
B, D, A, and AD to vary. The value obtained for the spin-orbit
constant in this iteration was A 5 23.723 GHz, very similar to the number determined by Balfour et al. (1995)
( A 4 23.297 GHz). It is also close to A 5 24.214 GHz
estimated from the following relation (e.g., Balfour et al.
1995):
L A 5 2B2 ~V 5 2!/@B~V 5 3! 2 B~V 5 2!# .
(3)
In the second iteration, A was fixed to 23.723 GHz, and B, D,
and AD were allowed to vary. The results of this final fit are
presented in Table 3. For 54 FeC, only one V ladder was
observed such that AD could not be determined. Consequently, to analyze this data set, this constant was fixed to the
value derived by scaling AD of 56 FeC by the appropriate mass
ratio, while A was set equal to that used for 56 FeC. The B and
D parameters obtained are listed in Table 3 as well.
The optical constants derived by Balfour et al. (1995) for the
56
Fe isotopomer are additionally presented in Table 3. The
L59
millimeter-wave rotational constants are in very good agreement with those reported by Balfour et al. (1995). The
millimeter-wave constants, moreover, reproduce the observed
frequencies to residuals of n obs 2 n calc , 30 kHz (better than
the experimental accuracy of 75 kHz).
4. DISCUSSION
Unlike FeO (e.g., Allen, Ziurys, & Brown 1996b) or FeF
(Allen & Ziurys 1996), lambda-doubling was not observed in
FeC for the V 5 2 and the V 5 3 ladders. This result is not
unexpected, because for 3 D states, the lambda-doubling parameters involved are q̃D , p̃D , and õD . FeO and FeF have 5 D
and 6 D ground states, respectively, and lambda-doubling in
these electronic states concerns parameters m̃D and ñD as well
(e.g., Brown et al. 1987), which are usually several orders of
magnitude larger than the other three constants. Although
there are no ab initio calculations for FeC, theoretical studies
of RuC (Shim, Finkbeiner, & Gingerich 1987) suggest that
there might be several low-lying electronic states in iron
carbide that could perturb the ground 3 D state and cause
lambda-doubling interactions. Such splittings would be largest
in the V 5 1 ladder, which was not observed in this work nor
in the optical study of Balfour et al. (1995).
The absence of the V 5 1 component from the present data
set may partly be due to additional perturbations of excited
electronic states. Balfour et al. (1995) noticed that the spinorbit constant of the Fe atom is z 3d 5 2417 cm 21 , while the
value they estimated for the 3 D ground state for FeC from the
V 5 2 and V 5 3 substates was AL 1 2220 cm 21 , a difference of 1200 cm 21 . They concluded that the V 5 2 component is perturbed by a low-lying 1 D state, which lowers the
V 5 2 energy. Consequently, the V 5 1 component may lie
more than 800 cm 21 (1100 K) above ground, if the actual
spin-orbit splitting is 1400 cm 21 . We have been able to
measure lines originating in the V 5 221 spin-orbit component
of FeF, which lies about 780 cm 21 above the ground state,
suggesting that we should have been populating the V 5 1
ladder in FeC. However, the intensities in the lowest substate
of FeC are about a factor of 5 less than those found for FeF.
The intensity of the V 5 1 transitions of FeC should therefore
be considerably weaker than the V 5 221 lines of FeF and may
not be observable given the experimental sensitivity.
Measurements of the FeC rest frequencies will enable
astronomical searches for this radical to be conducted. Observations of SiC in IRC 110216 have shown it to be a relatively
abundant molecule in this object, with a column density of
6 3 10 13 cm 22 (Cernicharo et al. 1989). Other refractory
carbide species such as FeC are likely candidates for new
discoveries in this enriched carbon envelope and perhaps in
other such stars as well.
This research was supported by NSF grants AST-9253682 and AST-95-03274 and NASA grant NAGW 2989. The
authors thank Professor J. M. Brown for the use of his
Hamiltonian, as well as Professor T. C. Steimle for helpful
discussions.
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