The Astrophysical Journal, 559:L163–L166, 2001 October 1
䉷 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.
THE MILLIMETER-WAVE SPECTRUM OF NiC (X 1S⫹) AND CoC (X 2S⫹)
M. A. Brewster and L. M. Ziurys
Department of Astronomy and Department of Chemistry, University of Arizona, Tucson, AZ 85721;
and Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721
Received 2001 May 29; accepted 2001 July 27; published 2001 September 6
ABSTRACT
The pure rotational spectra of two metal carbide species, NiC (X 1S⫹) and CoC (X 2S⫹), have been measured
in the laboratory using millimeter/submillimeter direct absorption methods. The molecules were created by reacting
metal vapor with CH4 in a DC discharge. Four rotational transitions of CoC were recorded, each consisting of
16 hyperfine components arising from the cobalt nuclear spin of I p 7/2. Multiple transitions of the two most
abundant nickel isotopomers, 58NiC and 60NiC, were additionally measured. The spectra were analyzed using the
appropriate Hamiltonian and spectroscopic constants accurately determined. For CoC, this study included a
complex hyperfine analysis of the magnetic, electric quadrupole, and nuclear spin-rotation terms. Cobalt and
nickel are iron peak elements that may be produced via neutron capture processes in asymptotic giant branch
stars. Dredge-up events mix these elements to the stellar surface, where they are incorporated into the expanding
stellar envelope. CoC and NiC offer a means to trace these elements in carbon-rich circumstellar shells.
Subject headings: ISM: molecules — line: identification — methods: laboratory — molecular data
A possible carrier of iron peak elements are the diatomic
carbide species with the general formula MC. These molecules
are good possibilities because AGB envelopes are often carbonrich. Unfortunately, rotational transition frequencies for such
compounds are generally not available. A few diatomic carbide
species with the iron peak elements have been studied, but only
via electronic spectroscopy. For example, Barnes, Merer, &
Metha (1995) have recorded 2P–X 2S⫹ and 2P–2D bands of
CoC, and Adam & Peers (1997) have investigated the [14.0]
2 ⫹
S –X 2S⫹ transition of this species. In addition, Morse and
collaborators have measured optical spectra of NiC (Brugh,
Ronningen, & Morse 1998), which was determined to have a
1 ⫹
S ground state. No experimental information exists for CrC,
MnC, CuC, or ZnC.
In this Letter, we present the first measurements of the pure
rotational spectrum of CoC (X 2S⫹) and NiC (X 1S⫹). These
species were investigated using submillimeter direct absorption
techniques. Rotational transitions were recorded for the most
abundant nickel isotopomers, 58NiC and 60NiC, and for CoC,
where hyperfine structure was resolved. Here we describe these
results and their analysis.
1. INTRODUCTION
One of the fundamental questions for astrophysics is the
origin of the elements. A particular problem in this area is the
synthesis of the iron peak elements, Cr, Mn, Co, Ni, Cu, and
Zn. According to Woosley & Weaver (1995), isotopes with
mass number less than 57 are created in explosive oxygen and
silicon burning and in nuclear statistical equilibrium (NSE).
Isotopes with A 1 56 are also produced by NSE processes, in
particular, alpha-rich “freeze-out.” However, they are additionally created during hydrostatic helium and carbon burning via
neutron capture mechanisms (or s-processes) in asymptotic giant branch (AGB) stars. Because of the many uncertainties
involved in such pathways, predicting the abundances of the
iron peak elements is difficult (Goswami & Prantzos 2000).
Hence, input of observational data is essential in evaluating
the relative importance of the nucleosynthetic schemes.
Because of the instabilities resulting from helium and hydrogen shell burning, stars on the AGB undergo dredge-up
processes. These events mix the products of interior nucleosynthesis up to the surface of the star (e.g., Blöcker 1999). As
mass loss occurs, the dredged-up material becomes part of the
circumstellar envelope and is consequently available to form
molecules. Evidence for this mixing is readily found in nonsolar
isotope ratios obtained from millimeter-wave observations of
molecular lines (e.g., Kahane et al. 1992). For example, measurements of CO toward the carbon-rich envelope of IRC
⫹10216 suggests 12 C/ 13 C ∼ 44 (Guélin et al. 1995). The enrichment of 13C in this shell is attributed to the CNO cycle.
Studying molecules in circumstellar shells thus offers an
avenue to examine the products of hydrostatic helium and carbon burning and related dredge-up events. Such molecules are
best investigated by measuring their pure rotational transitions,
which are easily excited in these envelopes. Because of the
uncertainties in s-process production, chemical compounds
containing the iron peak elements are of interest. Some of these
elements, in fact, have reasonably large cosmic abundances.
For example, that of nickel is Ni/H ∼ 1.8 # 10⫺6, while for
cobalt, Co/H ∼ 8.1 # 10⫺8 (Savage & Sembach 1996). Moreover, these abundances may be substantially enhanced in AGB
envelopes relative to the cosmic values.
2. EXPERIMENTAL
The spectral measurements were conducted with one of the
quasi-optical millimeter/submillimeter spectrometers of the
Ziurys group (e.g., Ziurys et al. 1994). This instrument consists
of phase-locked Gunn oscillator/Schottky diode multiplier combination as the frequency source, a reaction chamber, and a
liquid-helium–cooled, InSb bolometer as the detector.
Cobalt carbide and nickel carbide were both generated by
the reaction of metal vapor and methane in a DC glow discharge. In each case, the metal vapor (!1 mtorr) was produced
in a high-temperature Broida-type oven, which was lined with
zirconia felt and ceramic pieces to aid in the melting process.
The vapor was then reacted with approximately 10 mtorr of
CH4, which was allowed to flow from beneath the oven and
therefore assist in carrying the metal gas into the discharge
region, which was approximately 5 cm above the oven itself.
No argon carrier gas was used, as is usually the case for our
L163
L164
MILLIMETER-WAVE SPECTRUM OF NiC AND CoC
Vol. 559
TABLE 1
Observed Transition Frequencies of CoC (X 2S⫹)a
N⬙ r N⬘
8 r 9 .........
11 r 10 . . . . . .
11 r 12 . . . . . .
12 r 13 . . . . . .
a
J⬙ r J⬘
F⬙ r F⬘
vobs
vobs ⫺ vcalc
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
12 r 13
5r6
6r7
7r8
8r9
9 r 10
10 r 11
11 r 12
11 r 12
10 r 11
9 r 10
8r9
7r8
6r7
5r6
4r5
14 r 15
7r8
8r9
9 r 10
10 r 11
11 r 12
12 r 13
13 r 14
13 r 14
12 r 13
11 r 12
10 r 11
9 r 10
8r9
7r8
6r7
15 r 16
8r9
9 r 10
10 r 11
11 r 12
12 r 13
13 r 14
14 r 15
14 r 15
13 r 14
12 r 13
11 r 12
10 r 11
9 r 10
8r9
7r8
16 r 17
9 r 10
10 r 11
11 r 12
12 r 13
13 r 14
14 r 15
15 r 16
15 r 16
14 r 15
13 r 14
12 r 13
11 r 12
10 r 11
9 r 10
8r9
373615.896
373686.428
373757.949
373831.919
373911.089
373999.678
374107.194
374263.762
374201.735
374359.151
374468.795
374559.925
374640.908
374715.403
374785.475
374852.110
456698.442
456757.148
456817.497
456879.745
456945.771
457018.209
457103.336
457220.310
457411.551
457528.697
457614.888
457688.655
457755.594
457817.973
457877.188
457934.084
498221.886
498275.791
498331.449
498388.596
498448.641
498513.763
498588.210
498684.287
498995.539
499091.525
499166.665
499232.699
499293.464
499350.594
499405.220
499457.660
539732.251
539781.847
539833.038
539885.591
539940.269
539998.543
540063.117
540139.301
540562.000
540637.880
540702.809
540761.782
540816.924
540869.405
540919.643
540968.423
⫺0.029
⫺0.035
0.046
⫺0.026
0.013
0.024
0.048
⫺0.001
0.033
0.033
⫺0.020
0.032
0.014
⫺0.029
0.041
⫺0.064
0.024
⫺0.010
⫺0.001
⫺0.048
0.035
⫺0.013
⫺0.030
⫺0.006
⫺0.081
0.025
0.021
⫺0.009
0.019
⫺0.021
⫺0.026
0.019
⫺0.053
⫺0.026
0.041
0.018
⫺0.002
0.003
⫺0.016
⫺0.007
0.005
0.034
0.008
⫺0.014
0.011
⫺0.020
0.057
⫺0.075
0.045
0.071
⫺0.047
⫺0.007
⫺0.007
⫺0.042
0.031
0.006
0.032
⫺0.037
⫺0.047
⫺0.037
⫺0.020
0.069
⫺0.013
0.065
8.5
8.5
8.5
8.5
8.5
8.5
8.5
8.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
10.5
10.5
10.5
10.5
10.5
10.5
10.5
10.5
9.5
9.5
9.5
9.5
9.5
9.5
9.5
9.5
11.5
11.5
11.5
11.5
11.5
11.5
11.5
11.5
10.5
10.5
10.5
10.5
10.5
10.5
10.5
10.5
12.5
12.5
12.5
12.5
12.5
12.5
12.5
12.5
11.5
11.5
11.5
11.5
11.5
11.5
11.5
11.5
9.5
9.5
9.5
9.5
9.5
9.5
9.5
9.5
8.5
8.5
8.5
8.5
8.5
8.5
8.5
8.5
11.5
11.5
11.5
11.5
11.5
11.5
11.5
11.5
10.5
10.5
10.5
10.5
10.5
10.5
10.5
10.5
12.5
12.5
12.5
12.5
12.5
12.5
12.5
12.5
11.5
11.5
11.5
11.5
11.5
11.5
11.5
11.5
13.5
13.5
13.5
13.5
13.5
13.5
13.5
13.5
12.5
12.5
12.5
12.5
12.5
12.5
12.5
12.5
In units of megahertz, for v p 0.
experiments, because it tended to degrade the carbide signals.
The discharge current used was typically 700 mA at 200 V.
Spectra used for the frequency measurements were recorded
from averages of scan pairs, one taken in increasing frequency
and the other in decreasing frequency. For NiC, only one pair
Fig. 1.—Spectrum of the N p 11 r 12 rotational transition of CoC (X 2S⫹)
near 499 GHz. The transition is split into 16 separate lines that arise from
spin rotation and hyperfine interactions. The individual hyperfine components
are labeled by the quantum number F. They form two groups of lines distinctly
separated in frequency, corresponding to the two spin-rotation components,
J p 23/2 r 25/2 and J p 21/2 r 23/2. The asterisks mark unidentified features. This spectrum is a composite of 11 100 MHz scans, each lasting about
1 minute in duration.
was required; in the case of CoC, two to eight pairs were usually
found necessary. Line widths were typically 700–1200 kHz
over the measurement range of 373–541 GHz.
3. RESULTS AND ANALYSIS
The rotational frequencies obtained for CoC in its ground
vibrational (v p 0) and electronic (2S ⫹ ) state are listed in Table 1. As the table shows, four rotational transitions of CoC
were measured, each of which consists of 16 hyperfine components. These components were individually resolved in every
case.
The coupling scheme used for CoC in this study is case bbJ.
In this basis, the unpaired electron spin couples with the
molecular frame rotation via spin-rotation (N̂ · Sˆ ) interactions,
resulting in a doublet structure labeled by quantum number
ˆ ⫹ Sˆ . (J indicates the total angular momentum,
J, where Jˆ p N
neglecting nuclear spin.) Cobalt has only one isotope, 59Co,
which has a nuclear spin of I p 7/2. This spin additionally
couples with J to create hyperfine splittings, labeled by quantum
number F, where Fˆ p Jˆ ⫹ Iˆ. In contrast, Barnes et al. (1995)
found that the bbs case was most appropriate for their CoC data
set. In this scheme, the electron and nuclear spins couple to form
ˆ p Iˆ ⫹ Sˆ , and then Fˆ p Gˆ ⫹ Nˆ .
quantum number G, where G
Spin-rotation interaction increases with increasing N, however.
While Barnes et al. (1995) recorded a wide range of rotational
transitions, we investigated only the N p 8 r 9 lines and higher.
In our data set, we are evidently observing the transition from
bbs to bbJ coupling. This effect is apparent in the ordering of
hyperfine components. For the N p 8 r 9 transition, the hyperfine lines of the two spin-rotation components are intermingled in frequency. For the N p 10 r 11 transition
and higher, the hyperfine lines are clearly separated into two individual groups for each spin-rotation doublet, as expected when
the N̂ · Sˆ interaction dominates.
The case bbJ pattern is clearly illustrated in Figure 1. This
figure is a spectrum of the N p 11 r 12 rotational transition
No. 2, 2001
BREWSTER & ZIURYS
L165
TABLE 2
Observed Transition Frequencies of NiC (X 1S⫹)a
58
60
NiC
NiC
J⬙ r J⬘
vobs
vobs ⫺ vcalc
vobs
vobs ⫺ vcalc
9 r 10 . . . . . . .
10 r 11 . . . . . .
11 r 12 . . . . . .
12 r 13 . . . . . .
13 r 14 . . . . . .
382159.485
420337.338
458504.324
496659.441
534801.715
0.010
⫺0.004
⫺0.006
⫺0.007
0.008
…
417943.458
455893.366
493831.554
531757.092
…
0.012
⫺0.005
⫺0.020
0.013
a
In units of megahertz, for v p 0.
of CoC recorded in this work near 499 GHz. The 16 hyperfine
components, labeled by quantum number F, form two distinct
groups in frequency space corresponding to the spin-rotation
doublets, J p 21/2 r 23/2 and J p 23/2 r 25/2, as indicated
on the spectrum. There is, in fact, a gap in frequency between
the two sets of lines. Each hyperfine component is visible in
the spectrum. Features marked by asterisks are unidentified
lines.
The frequencies measured for NiC are presented in Table 2.
Five transitions were recorded for the 58Ni isotopomer and four
for the 60Ni isotopomer. These lines were observed in the natural
elemental abundances of 67.7% (58Ni) and 26.2% (60Ni).
(Nickel has three other stable isotopes that compose only a few
percent of the total abundance.) Because NiC has a 1S ⫹ ground
electronic state, its spectrum is very simple, as shown in Figure 2. This figure presents the J p 12 r 13 transition of the
58
Ni isotopomer near 496 GHz (lower panel) and the J p
11 r 12 line of the 60Ni isotopomer at 455 GHz (upper panel).
In contrast to CoC, these transitions are composed of single
lines. The 60NiC signal is weaker, as expected.
The CoC and NiC data were analyzed with the appropriate
Hamiltonian. For NiC (X 1S⫹), only molecular frame rotation
and its centrifugal distortion needed to be considered. In the
case of CoC (X 1S⫹), on the other hand, the effective Hamiltonian included fine-structure (spin-rotation) and hyperfine interactions:
ˆ ef f p H
ˆ rot ⫹ H
ˆ sr ⫹ H
ˆ hf .
H
(1)
The spin-rotation term is characterized by the constant g and
its centrifugal distortion correction, gD. The hyperfine Hamiltonian consists of three separate terms: magnetic, electric quadrupole, and nuclear spin-rotation interactions, i.e.,
ˆ hf p H
ˆ mhf ⫹ H
ˆ eqQ ⫹ H
ˆ nsr .
H
(2)
In this case, the Fermi contact term (bF), its centrifugal distortion correction (b FD), and the spin-spin dipolar coupling (c)
compose the magnetic hyperfine contribution (e.g., Hirota
1985):
Fig. 2.—Spectrum of the J p 12 r 13 transition of 58NiC near 496 GHz
and the J p 11 r 12 line of the 60NiC isotopomer near 455 GHz, both observed
in the natural abundance of nickel. NiC has a 1S⫹ ground state such that both
transitions appear as single lines. Each spectrum was recorded as a 100 MHz
scan, lasting approximately 1 minute.
The spectroscopic parameters obtained from these analyses
are given in Table 3. For the two isotopomers of NiC, the rotational constants B and D were determined; for CoC, fine structure and hyperfine parameters were established as well. Also
given in the table are the constants for CoC derived from the
optical studies of Barnes et al. (1995) and Adam & Peers (1997).
Within the uncertainties, all three data sets are in reasonable
agreement. From the millimeter-wave analysis, however, three
additional constants were determined (gD, b FD, and cI).
4. DISCUSSION
ˆ ˆ 2 ⫹ c (3Iz Sz ⫺ Iˆ · S).
ˆ
ˆ mhf p b F Iˆ · Sˆ ⫹ b F (Iˆ · S)N
H
D
3
(3)
The quadrupole and the nuclear-spin rotation interactions take
the form
2
2
ˆ eqQ ⫹ H
ˆ nsr p eqQ(3Iz ⫺ I ) ⫹ cI Iˆ · N,
ˆ
H
4I(2I ⫺ 1)
where eqQ and cI are the respective constants.
(4)
These measurements confirm that the electronic ground
states of NiC and CoC are 1S⫹ and 2S⫹, respectively. Establishing the ground states of such species allows the electron
configuration to be predicted, which in turn gives insight into
the bonding of the molecule. Such bonding is of interest because it concerns the unusual combination of a transition metal
and a single carbon atom. As predicted by Shim & Gingerich
(1999), a 1S⫹ ground state for NiC implies that its primary
electron configuration is (8j)2(9j) 2(3p)4(1d) 4. Here the 9j and
1d orbitals are nonbonding, arising from atomic orbitals of the
L166
MILLIMETER-WAVE SPECTRUM OF NiC AND CoC
Vol. 559
TABLE 3
Spectroscopic Constants for NiC (X 1S⫹) and CoC (X 2S⫹)a
Parameter
B ..............
D ..............
g ...............
gD . . . . . . . . . . . . .
bF . . . . . . . . . . . . . .
c ...............
eqQ . . . . . . . . . . . .
cI . . . . . . . . . . . . . .
bFD . . . . . . . . . . . . .
rms of fit . . . . . .
a
b
c
58
NiC
19116.216(13)
0.041212(41)
…
…
…
…
…
…
…
0.007
60
NiC
CoC
19007.279(18)
0.040701(54)
…
…
…
…
…
…
…
0.014
20798.1661(43)
0.045741(15)
⫺1231.04(25)
⫺0.01102(56)
3927.16(75)
187.1(3.0)
303(10)
0.447(15)
⫺0.00739(99)
0.035
CoC (Optical)
20797.94(28)b
0.00457(15)
⫺1232.45(99)
…
3925.9(3.3)
193.1(6.3)
306(36)
…
…
…
20798.1(5.5)c
0.0537(81)
⫺1225.9(2.2)
…
3896.3(7.0)
78(20)
760(540)
…
…
…
In units of megahertz, for v p 0. Errors quoted are 3 j.
From Barnes et al. 1995.
From Adam & Peers 1997.
nickel atom. The 8j and 3p levels are bonding orbitals and
have corresponding antibonding 10j and 4p orbitals, which
are unoccupied. Therefore, NiC has essentially a triple bond
between the carbon and nickel atoms.
Cobalt carbide has one less electron than NiC. If the electron
configuration of NiC applied to CoC, removal of one electron
would result in a valence configuration of (1d)3. This configuration does not generate a 2S⫹ term. In order to create this
electronic state, a j1 valence configuration must exist; the likely
configuration is (8j)2(3p)4(1d)4(9j)1. Consequently, the 9j orbital must shift to higher energy in CoC relative to NiC. Another possibility is that the electron configuration for nickel
carbide is (8j)2(3p)4(1d)4(9j)2, which also results in a 1S⫹ term.
Evidence for the (9j)1 valence configuration in CoC is apparent in the hyperfine constants. As noted by Adam & Peers
(1997) and Barnes et al. (1995), the Fermi contact parameter
bF is quite substantial—almost 4 GHz. It is sufficiently large
that a centrifugal distortion correction had to be added to this
term for our millimeter-wave analysis. This parameter is usually
not necessary. In fact, the bF-value for CoC of 3.927 GHz is
very close to that of the 4s electron in the cobalt atom, which
is 4.411 GHz (Guthöhrlein & Keller 1990). Therefore, the unpaired electron in CoC must be primarily located on the cobalt
atom in a 9j orbital very similar to a 4s orbital, which would
be expected if it is nonbonding. However, a nonnegligible dipolar hyperfine constant of c p 187 MHz additionally indicates
that the orbital of the unpaired electron has some d character,
perhaps arising from sdj hybridization.
A bond length of r0 p 1.5612 Å has been determined for
CoC, in complete agreement with the work of Barnes et al.
(1995). For NiC, the bond distance was calculated to be
r0 p 1.6308 Å, a value that is quite close to the theoretical
equilibrium bond length of re p 1.621 Å (Shim & Gingerich
1999) and to that found in the optical experiments of Morse
(1.631 Å). The increase in bond distance for NiC relative to
CoC likely is a manifestation of Pauli repulsion arising from
the addition of an extra electron to the nonbonding 9j orbital.
The most important result of these spectroscopic measurements is that highly accurate constants and rest frequencies
have been established for CoC and NiC for astronomical
searches. The carbon-rich envelopes of AGB stars where SiC
and SiC2 have been observed are prime objects. For NiC, the
best transitions for such measurements are likely to be the
J p 4 r 3 and J p 3 r 2 lines near 152.9 and 114.6 GHz,
respectively. Because the Galactic abundance of nickel is relatively high, NiC may indeed be a feasible tracer of this element
in carbon-rich envelopes. Although the cobalt abundance is
about an order of magnitude smaller than that of nickel, sprocesses in AGB stars may enhance its concentration in such
objects (Goswami & Prantzos 2000). Cobalt carbide can be
identified on the basis of one rotational transition because of
the presence of a very distinct fine structure/hyperfine pattern.
The N p 4 r 3 transition near 166.4 GHz may be an optimal
choice for astrophysical observations. Detection of either molecule would have broad implications for AGB nucleosynthesis,
dust grain composition, and gas-phase chemistry in circumstellar material.
This research is supported by NSF grants AST 98-20576
and CHE 98-17707.
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