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

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 2.97 0.8 CCH¯ 1S 3.1 41,639 C4H 2S 3.56 0.9 C4H¯ 1S 6.1 4,656 C6H 2P 3.81 5.6 C6H¯ 1S 8.2 1,377 C8H 2P 3.97 6.3 C8H¯ 1S 11.9 583 Also mention non-reactivity with H2??? References? Sandra Brünken WI09 06/20/2007

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 WI09 06/20/2007

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 600-700 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 20-25 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 WI09 06/20/2007

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 100-120 K 85% acetylene, 15% Ar <10 - 15 mtorr pressure, 1-2 Pa, 10-15 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 WI09 06/20/2007

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 WI09 06/20/2007

Butadiyne, C4H¯ B ~ 5 GHz 19 transitions in the 9 – 360 GHz range Frequency (MHz) Relative Intensity Relative Intensity Frequency (MHz) 18619.6 18619.9 J = 37 - 36 25 min J = 2 - 1 B ~ 5 GHz T = 150 K Both in FTM and mm-wave Also C4D- measured 0 100 200 300 400 500 600 Frequency (GHz) 19 transitions in the 9 – 360 GHz range Gupta et al., ApJL 655, 2007 Sandra Brünken WI09 06/20/2007

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 100 200 300 400 500 600 700 Frequency (GHz) 5 transitions in the 80 – 420 GHz range Brünken et al., A&AL 464, 2007 Sandra Brünken WI09 06/20/2007

Spectroscopic constants Constant CCH¯ C4H¯ C8H¯ B (MHz) 41639.237(4) 4654.9449(2) 583.34014(8) D (kHz) 96.97(9) 0.5875(1) 0.00043(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 40836 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 WI09 06/20/2007

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 WI09 06/20/2007

Anions in Space C4H¯ and C8H¯ now been detected in IRC+10216 (c Cernicharo et al. ApJL 467, 2007; a Remijan et al. ApJL acc., 2007) Detection of C6H¯ in a third source: IRAS 04368+2557 in L1527 (Sakai et al. ApJL subm., 2007) CnH¯ / CnH TMC-1 IRC+10216 Lab n obs. calc.e obs. calc.e 8 5 5.4 28a 28 (2) 6 1.6 8.9 8.6b 30 3 4 < 0.014 0.2 0.024c 0.8 0.14 2 - < 0.01d 0.1 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 WI09 06/20/2007

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 WI09 06/20/2007

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 WI09 06/20/2007

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 WI09 06/20/2007