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L57
The Astrophysical Journal, 655: L57--L60, 2007 January 20
# 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A.
ROTATIONAL SPECTRA OF THE CARBON CHAIN NEGATIVE IONS C 4 H # AND C 8 H #
H. Gupta, 1,2 S. Bru˜ nken, 1 F. Tamassia, 3 C. A. Gottlieb, 1 M. C. McCarthy, 1 and P. Thaddeus 1
Received 2006 November 1; accepted 2006 December 12; published 2007 January 11
ABSTRACT
The rotational spectra of the butadiyne anion C 4 H # and the octatetrayne anion C 8 H # have been detected in
the laboratory. Precise spectroscopic constants for these closed­shell molecules have been obtained, which enable
their rotational spectra to be calculated to high accuracy throughout the radio band. Deep astronomical searches
can now be undertaken in essentially any molecular source, including TMC­1 and IRC #10216, where the
negative ion C 6 H # has recently been detected. The large dipole moments and high binding energies of both
anions make them good candidates for astronomical detection.
Subject headings: ISM: molecules --- line: identification --- molecular data --- molecular processes ---
radio lines: ISM
We recently detected the radio spectrum of the negative ion
C 6 H # in the laboratory and showed that this suprisingly large
carbon chain anion is the carrier of the unidentified harmonic
sequence of molecular lines B1377 observed by Kawaguchi et
al. (1995) in the circumstellar shell of the carbon star IRC
#10216 (McCarthy et al. 2006). The radio spectra of other
molecular anions should now be detectable in the laboratory
and in space, especially linear closed­shell carbon chains sim­
ilar in structure to C 6 H # . Here we report laboratory detection
of the next shorter and the next longer members of the series,
the butadiyne anion C 4 H # and the octatetrayne anion C 8 H # .
Precise transition frequencies over most of the radio band are
now available for astronomical searches for these anions.
Like C 6 H # , the C 4 H # anion has been observed in both the
millimeter­wave band with a free space (FS) spectrometer (Gottlieb
et al. 2003) and the centimeter­wave band with a Fourier transform
microwave (FTM) spectrometer (McCarthy et al. 2000). The
longer anion C 8 H # has only been detected with the highly sensitive
FTM instrument, since its rotational lines are fairly weak. Sev­
enteen rotational transitions between 9 and 363 GHz of C 4 H # and
nine successive rotational transitions between 9 and 19 GHz of
C 8 H # have been measured to an accuracy approaching 1 part in
10 7 (see Tables 1 and 2).
In the FS spectrometer, C 4 H # was produced by a DC glow
discharge through a flowing mixture of argon (15%) and acet­
ylene (85%) at a total pressure of #10 mtorr with the discharge
cell walls cooled to 100 K. As with C 6 H # , the optimum discharge
current (#150 mA) was substantially lower than that which pro­
duces the most intense lines of the neutral radical (#400 mA).
At this lower current, lines of C 4 H # were only about 8 times
weaker than those of either fine­structure component of C 4 H;
hyperfine structure of C 4 H collapses at high J and is not resolved
at millimeter wavelengths, but spin­doubling does not collapse
and is resolved. Taking the different partition functions and di­
pole moments into account, the abundance of the anion is found
to be #0.14% that of the neutral. The absorption
J p 37 R 36
line of C 4 H # is shown in Figure 1; a signal­to­noise ratio of #75
1 Harvard­Smithsonian Center for Astrophysics, Cambridge, MA; and Division
of Engineering and Applied Sciences, Harvard University, Cambridge, MA;
hgupta@cfa.harvard.edu, sbruenken@cfa.harvard.edu, cgottlieb@cfa.harvard
.edu, mccarthy@cfa.harvard.edu, pthaddeus@cfa.harvard.edu.
2 Also at Institute for Theoretical Chemistry, Department of Chemistry and
Biochemistry, The University of Texas, Austin, TX.
3 Dipartimento di Chimica Fisica e Inorganica, Universita‘ di Bologna, Bo­
logna, Italy; tamassia@ms.fci.unibo.it.
was obtained in only 25 minutes of integration, allowing line
frequencies to be measured to 10--20 kHz.
In the FTM spectrometer, the anions were produced under
discharge conditions similar to those in which C 6 H # is ob­
served: a 600--700 V low­current (10--20 mA) DC discharge,
a gas pulse 300 ms long (for a flow of 20--25 cm 3 minute #1 at
STP), the precursor gas consisting of diacetylene (0.1%) heav­
ily diluted with an inert buffer gas (Ne, He, or H 2 ) at a stag­
nation pressure of 2.5 ktorr behind the pulsed valve of the
nozzle. The cavity mirrors and first­stage amplifier were cooled
to 77 K to reduce thermal noise. Lines of C 4 D # and C 8 D # were
detected in a discharge through deuterated diacetylene. As be­
fore, the polarity of the discharge is important in the production
of the anions, which are best observed with the polarity re­
versed with respect to that which produces the neutrals (Taylor
et al. 1998). The abundance of C 4 H # relative to C 4 H depended
on the buffer gas, ranging from with H 2 to an upper
#3
1.2 # 10
limit of with Ne---considerably lower than the ratio
#5
4 # 10
of C 6 H # relative to C 6 H. Under the best conditions, the stron­
gest lines of C 8 H # were observed with a signal­to­noise ratio
of 10 in 90 minutes of integration (see Fig. 2). The lines of
C 8 H # are about 10% as strong as those of the neutral radical
observed under the same conditions but are only a factor of
2--3 weaker than those of C 6 H # . We estimate that there are
#10 8 C 8 H # molecules per gas pulse (by comparison with lines
obtained with a calibrated sample of 1% OCS in Ne), which
corresponds to about 2% that of C 8 H---a similar ratio to that
found for C 6 H # relative to C 6 H.
The searches for the two anions here were guided by high­
level ab initio molecular structure calculations [at the CCSD(T)/
cc­pVTZ level of theory]. Precise equilibrium structures were
obtained and then corrected for vibrational effects (see Table 3
note). All calculations were performed with the ACES II suite
of programs (Stanton et al. 1992). The theoretical ground­state
rotational constants are summarized in Table 3. For C 4 H # the
calculated rotational constant agrees well with that obtained by
photodetachment spectroscopy of the rotationally resolved
electronic transition ( MHz; Pachkov
1 1 #
A R X S B p 4653 # 6
et al. 2003) and a previous high­level theoretical calculation
( MHz; Botschwina 2000). The initial search for C 4 H #
B p 4658
in the millimeter­wave band covered a frequency range of
#0.15% for three successive transitions ( ,
J p 18 R 17
, and ). A single harmonic series of lines of
19 R 18 20 R 19
comparable strength was found, and the search was then extended
to detect additional lines in narrowband scans close to the fre­

L58 GUPTA ET AL. Vol. 655
TABLE 1
Laboratory Rotational Frequencies of C 4 H #
##
J # J
Frequency
(MHz)
a
O# C
(kHz)
1#0 . . . . . . . . . . . . . . . . . . . . . 9,309.887 #2
2#1 . . . . . . . . . . . . . . . . . . . . . 18,619.758 #3
9#8 . . . . . . . . . . . . . . . . . . . . . 83,787.263 #33
16#15 . . . . . . . . . . . . . . . . . . 148,948.610 #3
17#16 . . . . . . . . . . . . . . . . . . 158,256.620 37
18#17 . . . . . . . . . . . . . . . . . . 167,564.324 11
19#18 . . . . . . . . . . . . . . . . . . 176,871.783 #6
20#19 . . . . . . . . . . . . . . . . . . 186,179.030 33
26#25 . . . . . . . . . . . . . . . . . . 242,015.829 0
27#26 . . . . . . . . . . . . . . . . . . 251,320.800 33
28#27 . . . . . . . . . . . . . . . . . . 260,625.328 4
29#28 . . . . . . . . . . . . . . . . . . 269,929.465 #21
31#30 . . . . . . . . . . . . . . . . . . 288,536.554 #15
32#31 . . . . . . . . . . . . . . . . . . 297,839.465 3
37#36 . . . . . . . . . . . . . . . . . . 344,346.858 #16
38#37 . . . . . . . . . . . . . . . . . . 353,646.854 6
39#38 . . . . . . . . . . . . . . . . . . 362,946.288 3
Note.---Estimated 1 j uncertainties in the line frequencies: 2 kHz in the
two lowest transitions, 15 kHz in the millimeter­wave transitions.
a Calculated from the spectroscopic constants in Table 3.
Fig. 1.--- transition of C 4 H # observed with the free space mil­
J p 37 R 36
limeter­wave spectrometer, in an integration time of 25 minutes. Owing to the
modulation scheme employed, the instrumental line shape is approximately
the second derivative of a Lorentzian.
TABLE 2
Laboratory Rotational Frequencies of C 8 H #
##
J # J
Frequency
(MHz)
a
O# C
(kHz)
8#7 . . . . . . . . . . . . . . . . . . . . 9,333.434 1
9#8 . . . . . . . . . . . . . . . . . . . . 10,500.110 0
10#9 . . . . . . . . . . . . . . . . . . . 11,666.785 #1
11#10 . . . . . . . . . . . . . . . . . . 12,833.460 0
12#11 . . . . . . . . . . . . . . . . . . 14,000.134 0
13#12 . . . . . . . . . . . . . . . . . . 15,166.806 0
14#13 . . . . . . . . . . . . . . . . . . 16,333.476 #1
15#14 . . . . . . . . . . . . . . . . . . 17,500.147 1
16#15 . . . . . . . . . . . . . . . . . . 18,666.814 0
Note.---Estimated 1 j uncertainties in the line frequencies are 2 kHz.
a Calculated from the spectroscopic constants in Table 3.
Fig. 2.--- transition of C 8 H # observed with the FTM spectrom­
J p 11 r 10
eter---a 90 minute integration. The double­peaked instrumental line shape is
caused by the Doppler shift of the supersonic molecular beam relative to the two
traveling waves that compose the confocal mode of the Fabry­Perot cavity; the
rest frequency of the transition is the average of these two components.
quencies predicted on the basis of this limited data set. The search
for C 8 H # in the centimeter­wave region was similar. The spec­
troscopic constants of both species are given in Table 3.
The evidence that the lines are from C 4 H # and C 8 H # is
extremely strong. The derived rotational constants B are within
0.1% of those calculated ab initio, and the small ratios of the
centrifugal distortion constant to the rotational constant
( and , respectively) are character­
#7 #9
D/B p 1.3 # 10 7 # 10
istic of linear carbon chains (Gottlieb et al. 1983; McCarthy
et al. 1999). In addition, the rotational spectra of the deuterated
species were obtained at the expected isotopic shifts (7.1% and
3.3%, respectively) to better than 0.05%. The absence of fine
and hyperfine structure in the high­resolution FTM measure­
ments (line width 10--20 kHz) and the closely harmonic se­
quence of the lines strongly indicate closed­shell linear mol­
ecules with ground states, as predicted (Pan et al. 2003;
1 #
S
Pino et al. 2002).
As a further check, for C 4 H # we determined the charge of
the carrier by measuring the rest frequency of individual ro­
tational lines in single pass absorption in the FS spectrometer.
As expected, Doppler shifts in the line frequencies are observed
when the polarity of the discharge is reversed (owing to the
axial drift of the ion; see Fig. 3), indicating a charged carrier.
The sign of the shift is opposite to that observed for HCO # ,
confirming that the carrier is a negative ion.
From the spectroscopic constants derived here, rotational tran­
sitions of C 4 H # can now be predicted to better than 250 kHz or
0.1 km s #1 in equivalent radial velocity through 800 GHz. For
C 8 H # , although the rotational and centrifugal distortion constants
in Table 3 are derived from measurements below 19 GHz, they
are so accurately determined that the radio spectrum of C 8 H #
can now be calculated well beyond the range of measurement.
For example, the uncertainty in equivalent radial velocity is less
than 0.03 km s #1 at 20 GHz, and 0.4 km s #1 at 50 GHz---less
than the line width in both TMC­1 and IRC #10216 at fre­
quencies where C 8 H has been found. We conclude that the pre­
dicted frequencies for both species are adequate for deep astro­
nomical searches in all the standard molecular sources. At a

No. 1, 2007 ROTATIONAL SPECTRA OF C 4 H # AND C 8 H # L59
TABLE 3
Spectroscopic Constants of C 4 H # and C 8 H #
Constant Laboratory Theoretical
C 4 H #
B . . . . . . . . . . . . . . . . . . . . . 4654.9449(2) 4653.9 a
. . . . . . . . . . . . . . . . .
3
10 D 0.5875(1) 0.55 b
C 8 H #
B . . . . . . . . . . . . . . . . . . . . . 583.34014(8) 583.2 a
. . . . . . . . . . . . . . . . .
6
10 D 4.3(2) 3.5 c
Note.---Units are MHz.
a from a CCSD(T)/cc­pVTZ calculation. The vibration­rotation correction was
B e
calculated at the CCSD(T)/cc­pVDZ level of theory for C 4 H # , and the SCF/DZP
level for C 8 H # .
b From a CCSD(T)/cc­pVTZ calculation.
c From a SCF/DZP calculation.
Fig. 3.---Ion drift measurements of C 4 H # . Plotted are a series of measurements
of the line with either positive or negative high voltage (HV) applied
J p 37 R 36
to the electrode near the radiation source; the electrode on the detector side was
at ground potential in either case. The frequency shift (relative to a center fre­
quency of 344,346.874 MHz) corresponds to a drift velocity of 25 m s #1 .
typical cold cloud temperature of 5 K, strong transitions of C 4 H #
are expected from the centimeter­wave band below 50 GHz
through the 3 mm wavelength region. At the slightly higher
temperatures of 30--40 K found for carbon chain molecules in
IRC #10216, the intensity peaks near 150 GHz. For C 8 H # the
maximum in intensity lies in the centimeter­wave band in both
cases. These frequency regions are all readily accessible with
present large radio telescopes.
The ground states of both C 4 H # and C 8 H # have been pre­
dicted from ab initio calculations and photodetachment exper­
iments to be (Pan et al. 2003; Pino et al. 2002). The
1 #
S
expected rotational spectra therefore are those of simple linear
molecules, devoid of the fine and hyperfine structure that is
present in the neutral radicals, which possess or ground
2 # 2
S P
states. This collapse of substructure alone results in a gain of
4--8 in intensity for individual lines of the anion relative to
those of the neutral. Furthermore, our high­level ab initio cal­
culations indicate that the dipole moments of the anions are
significantly larger than those of the neutrals (6 D vs. 0.87 D
for the four­carbon chain [Woon 1995] and 11.9 D vs. 6.3 D
for the eight­carbon chain [McCarthy et al. 1996]), so that lines
of the anion relative to those of the neutral are enhanced by
1--2 orders of magnitude or more---an effect that greatly ben­
efits detection in the radio band.
We have failed to find lines of C 4 H # in the available astro­
nomical line surveys of TMC­1 and IRC #10216. In TMC­1,
a rotational temperature of #5 K and a column density of
cm #2 were found for neutral C 4 H (Gue’­
15
N p (0.3--1.3) # 10
lin et al. 1982; Bell et al. 1983). The strongest predicted C 4 H #
line ( ) in the bandwidth of the 8.8--50 GHz survey
J p 5 R 4
with the Nobeyama 45 m telescope (Kaifu et al. 2004) toward
this source is #20 mK. Assuming an excitation temperature of
5 K, this yields an upper limit for the C 4 H # column density of
cm #2 , which is a factor of lower
10 4
N p 2.2 # 10 (1--6) # 10
than that of the neutral. In IRC #10216, the C 4 H radical has
been detected with a column density of N p (2.4--3.0) #
cm #2 and an excitation temperature of 15--35 K, both in
15
10
the centimeter­wave band (28--50 GHz; Kawaguchi et al. 1995)
and the 2 mm wavelength region (129--172.5 GHz; Cernicharo
et al. 2000). No lines of C 4 H # were found in either survey; on
the assumption of an excitation temperature of 35 K, an upper
limit is obtained for the C 4 H # column density of N p 2 #
cm #2 , or 0.1% that of C 4 H.
12
10
The upper limit for the anion­to­neutral ratio of C 4 H derived
both in the laboratory and toward the two astronomical sources
IRC #10216 and TMC­1 is surprisingly low, more than an
order of magnitude lower than that found for the longer chain
C 6 H. This low ratio may indicate anion formation from the
neutral molecule by radiative electron attachment, a process
that tends to become more efficient with increasing molecular
size, i.e., a larger density of states, and a higher electron affinity,
which promote electron capture and a faster relaxation to the
anionic ground state. As predicted from a statistical approach
by Terzieva & Herbst (2000) for pure carbon chain molecules,
a critical size of about six carbon atoms is needed for efficient
electron attachment, and the reaction rate is found to be an
order of magnitude lower for smaller chains, which may explain
the observed low abundance of C 4 H # in space. There may be
other factors affecting the formation rate of hydrocarbon ani­
ons, in particular the role of intermediate dipole­bound states
in electron capture. As discussed by Pachkov et al. (2003) a
dipole­bound state is probably not formed from the ground
state of C 4 H, as it is for the longer chains, but rather from the
low­lying excited electronic state, which may be poorly
2 P
populated in cold interstellar and circumstellar clouds. Detec­
tion of C 4 H # as a function of temperature, density, and ex­
tinction is probably required to understand the formation of
this anion in space.
Similarly, we tried without success to find C 8 H # in existing
astronomical surveys of IRC #10216, specifically in the 45 m
Nobeyama survey by Kawaguchi et al. (1995), where the har­
monic lines of C 6 H # were found. There are 16 rotational tran­
sitions of C 8 H # within the 22 GHz covered by the Nobeyama
survey between 28 and 50 GHz, from to
J p 28 r 27 43 r
, the first about 23 K above the ground state and the
42 J p 0
last about 53 K---fairly close to the maximum in the Boltzmann
distribution. A reasonable limit on the peak radiation temper­
ature for the transitions at 47 and 48 GHz is about 15 mK,
which yields an upper limit of cm #2 in IRC
12
N # 2 # 10
#10216 for the column density of C 8 H # ---about 2.5 times
lower than the column density of neutral C 8 H (Cernicharo &
Gue’lin 1996). Chemical models predict that the column density
of C 8 H # may be as high as 25% that of C 8 H in IRC #10216
(Millar et al. 2000), so C 8 H # might be detectable with a two­
to threefold deeper integration than that required for C 6 H # .
Following our recent detection of C 6 H # in TMC­1, it may now
be possible to detect C 8 H # in this source. Examination of the
recent 45 m Nobeyama survey between 8.8 and 50 GHz (Kaifu
et al. 2004), reveals no lines of C 8 H # , but C 8 H and C 6 H # are
not observed in this survey and were found only after deep

L60 GUPTA ET AL. Vol. 655
searches at precise frequencies. We estimate that lines of C 8 H #
near 19 GHz should be about 10 mK on the assumption that
the abundance of C 8 H # is 2.5% that of neutral C 8 H (the same
ratio as C 6 H # /C 6 H; McCarthy et al. 2006)---sufficient to be
detected with the 100 m Green Bank Telescope in a deep search
at the laboratory rest frequencies.
In conclusion, the present work provides the spectroscopic data
required to search for the two anions C 4 H # and C 8 H # in astro­
nomical sources. Because of our recent detection of the closely
related C 6 H # anion, they are likely candidates for astronomical
detection. Both anions have been detected with high signal­to­
noise ratios in the laboratory, suggesting that other anions may be
found with our present instrumentation. Of particular interest are
those with high stability, i.e., high binding energies, large dipole
moments, and simple rotational spectra. Good candidates include
the cyano compounds , which are the isoelectronic ni­
#
C N
2n#1
trogen analogs to the hydrocarbon chains , and the
#
C H
2n
and anions.
# #
H C N H C H
2 2n 2 2n#1
We thank J. F. Stanton for guidance with the quantum cal­
culations, E. S. Palmer for assistance with microwave elec­
tronics, and K. Higgins for the use of his workstations. F. T.
acknowledges the University of Bologna for funding under the
``Programma Marco Polo'' scheme. This work is supported by
National Science Foundation Grant CHE­0353693; additional
support is provided by the Robert A. Welch Foundation through
a grant to J. F. Stanton at the University of Texas.
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