1. The Rotational Spectra of the Carbon Chain Anions C2nH¯ (n = 1, 2, 4)
Sandra Brünken, C. A. Gottlieb, H. Gupta, M. C. McCarthy,
and P.Thaddeus
Harvard Smithsonian Center for Astrophysics
62nd International Symposium on Molecular Spectroscopy
WI09 – June 20, 2007

2. C2nH¯ Characteristics
Recent discovery of C6H¯ suggests specific anions to target for laboratory and astronomical searches:
closed-shell 1S ground states and large dipole moments (m)
large electron affinities (EA)
radicals are detected in astronomical sources
anions do not react with H2 (Barckholtz et al., ApJL, 547, 2001)
EA (eV) m (D) m (D) B (MHz)
CCH 2S CCH¯ 1S ,639
C4H 2S C4H¯ 1S ,656
C6H 2P C6H¯ 1S ,377
C8H 2P C8H¯ 1S
Also mention non-reactivity with H2???
References?
Sandra Brünken WI /20/2007

3. Spectral Compression
collapse of fine and hyperfine structure upon electron attachment
higher polarity of the anions vs. the neutrals
Enhancement x
Factor 300 for C4H
Factor 40 for C2H
Factor 20 for C8H
Factor 15 for C6H
→ benefits laboratory and astronomical detection
Sandra Brünken WI /20/2007

4. Experimental Setup - FTM
Supersonic nozzle coupled to a high-Q Fabry-Perot cavity
1 msec
movable
mirror
Q ~ 40,000
cooled to 77 K
discharge nozzle
FID ~100 msec
C8H- and C4H- (although with difficulties)
Discharge V (10-20 mA), significantly lower as that which produces the best signal of the neutrals
Search ranges typically 0.2%
Polarity reversed with respect to optimal conditions for neutrals
Gas pulse 300 mus
Flow cm^3/min at stp
Stagnation pressure 2.5 ktorr, 3 bar
~ 10^8 C8H- molecules per gas pulse (calibrated against OCS)
FFT
Frequency range: 5 – 42 GHz resolution: 20 kHz accuracy: 1-2 kHz
C4H¯ and C8H¯ produced in low current discharge of C4H2 (0.1 %) in Ne (He, H2)
Sandra Brünken WI /20/2007

5. Experimental Setup – mm-wave absorption spectrometer
InSb detector
power supply
solenoid
Gunn × n
2 m
LN2
LN2
Low current 150 mA
Cell walls cooled to K
85% acetylene, 15% Ar
< mtorr pressure, 1-2 Pa, mubar
low current dc glow discharge of C2H2 (85 %) + Ar (15 %)
Frequency coverage: 68 – 500 GHz
Frequency accuracy: 10 – 50 kHz
cell walls cooled by LN2 to 100 K
Sandra Brünken WI /20/2007

6. Octatetrayne, C8H¯ B ~ 0.6 GHz 9 transitions in the 9-19 GHz range
Only FTM measurements
Also D species measured, with deuterated diacetylene
9 transitions in the 9-19 GHz range
Gupta et al., ApJL 655, 2007
Sandra Brünken WI /20/2007

7. Butadiyne, C4H¯ B ~ 5 GHz 19 transitions in the 9 – 360 GHz range
Frequency (MHz)
Relative Intensity
Relative Intensity
Frequency (MHz)
J =
25 min
J = 2 - 1
B ~ 5 GHz
T = 150 K
Both in FTM and mm-wave
Also C4D- measured
Frequency (GHz)
19 transitions in the 9 – 360 GHz range
Gupta et al., ApJL 655, 2007
Sandra Brünken WI /20/2007

8. Acetylide, CCH¯ B ~ 42 GHz 5 transitions in the 80 – 420 GHz range
Frequency (MHz) U
Relative Intensity
integration time: ~ 10 min !
J = 4 - 3
B ~ 42 GHz
T = 150 K
Also 13C measured
Frequency (GHz)
5 transitions in the 80 – 420 GHz range
Brünken et al., A&AL 464, 2007
Sandra Brünken WI /20/2007

9. Spectroscopic constants
Constant CCH¯ C4H¯ C8H¯
B (MHz) (4) (2) (8)
D (kHz) (9) (1) (2)
H (Hz) [0.13]1
1 theoretical value provided by P. Botschwina and P. Sebald (2007), private communication
excellent agreement (< 0.1 %) with ab initio values (CCSD(T)/cc-pVTZ), see talk by H. Gupta WI08
providing accurate transition frequencies for astronomical searches:
CCH¯, C4H¯ < 0.2 km/s up to 1 THz
C8H¯ < 0.4 km/s up to 50 GHz
For C4H- there has been rotationally resolved photodetachment spectroscopy (B=4653 MHz), Pachkov, Pino,… Maier 2003, Mol. Phys. 101, 583
P. Botschwina presented C4H- calculations on the 55th Int. Symp. TC06
For C2H- also variational calculations (with the CCSD(T)/250 cGTO potential energy surface) Mladenovic, Botschwina, Sebald, Carter 1998, Theor. Chem. Acc. 100, 234. At the same level H was calculated
C2H- IR CC-stretch by Gruebele, Polak, Saykally, Journal of Chemical Physics 87 (1987) 144, their B-value is MHz
Influence of H 1 km/s at 2 THz, comparable to line uncertainty
C8H- only in lower frequency range, therefore larger uncertainty, however, well below linewidth in TMC-1 and IRC10216 at frequencies near the Boltzmann peak
Sandra Brünken WI /20/2007

10. Detection of C8H¯ in TMC-1
100m Green Bank Telescope
NT = 2.1(4) ×1010 cm-2
for Tex = 5 K
C8H¯ / C8H ~ 5(1) %
Measurements in April 2007
Brünken et al., ApJL accepted, 2007
Sandra Brünken WI /20/2007

11. Anions in Space
C4H¯ and C8H¯ now been detected in IRC (c Cernicharo et al. ApJL 467, 2007; a Remijan et al. ApJL acc., 2007)
Detection of C6H¯ in a third source: IRAS in L (Sakai et al. ApJL subm., 2007)
CnH¯ / CnH TMC IRC Lab
n obs calc.e obs calc.e
a 28 (2) b
< c
< 0.01d
all ratios in %, b Kasai et al. ApJL 661, 2007, d Cernicharo et al. priv. com., e Millar et al. ApJL 662, 2007
First mention relatively high abundance for long chains, other anions likely to be found, easier to be detected than previously thought
Point to two discrepancies: c4H- lower than predicted
Second: lab abundancies higher than astro-ratios
Go to next slide
Sandra Brünken WI /20/2007

12. Formation Process
high abundances point to a simple and efficient formation process:
radiative e¯ attachment: e¯ + M  M¯*  M¯ + hn
two step process: - formation of a temporary negative ion M¯*
- relaxation to anionic ground state
Overestimated in current models?
Role of dipole bound states? Requires dipole moment ≥ 2 D
De Bleecker et al: investigation of polymerization in an acetylene discharge
in the laboratory dissociative attachment possible1 [Ekin(e¯) ~ 3eV]
e¯ + HCCH  CCH¯ + H (threshold 1.76 eV)
CCH¯ + HCCH  C4H¯ + H2
1 De Bleecker et al., Phys. Rev. E 73, 2006
Sandra Brünken WI /20/2007

13. Summary
Precise transition frequencies now available for C2nH¯ (n=1..4)
C4H¯, C6H¯, and C8H¯ have already been detected in space with surprisingly high abundances
Other anions likely to be detected in the laboratory and in space
A galactic survey of molecular anions, i.e. C6H¯ is timely
other, better sources
abundance as function of temperature, density and composition
Laboratory studies of reaction and formation rates of anions necessary
Sandra Brünken WI /20/2007

14. THANKS! Sam Palmer Filippo Tamassia
NRAO staff for assistance with the GBT observations
Eric Herbst, Bill Klemperer, John Stanton, Takeshi Oka, Peter Botschwina, Bob McMahon, John Maier, Stephan Schlemmer
Funding agencies: NSF, NASA, Robert A. Welch Foundation, HCO
Sandra Brünken WI /20/2007