High Resolution Spectroscopy and Astronomical Detection of Molecular Anions

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1 High Resolution Spectroscopy and Astronomical Detection of Molecular Anions Sandra Brünken, C. A. Gottlieb, H. Gupta, M. C. McCarthy, and P.Thaddeus Harvard Smithsonian Center for Astrophysics 10th Biennial HITRAN Conference June 22, 2008

2 Overview Introduction Identification of the first anion in space the story of C 6 H Anions in the laboratory Anions in space six anions now detected four anions detected with surprisingly high abundances Discussion and Conclusions

3 Motivation more than 130 molecules detected in space most identified by their rotational spectra probe and influence physical properties: e.g. cooling fractional ionization understanding of chemistry and formation processes R. Ruiterkamp (2000)

4 intensity Identifying molecules in space radiation source absorption cell detector = frequency background source interstellar cloud circumstellar shell radio telescope

5 Identifying molecules in space Astrophysical Observation High Resolution Laboratory Spectroscopy Chemical Modells of Interstellar Objects

6 Dense Interstellar Cloud Cores 10 K 10 4 cm -3 sites of star formation H 2 dominant Molecules seen in IR absorption and radio emission Cosmic rays create weak plasma Fractional ionization < 10-7

7 Molecules in space Number of Atoms H 2 H 2 O NH 3 SiH 4 CH 3 OH CH 3 CHO HCOOCH 3 CH 3 CH 2 OH OH H 2 S H 3 O + CH 4 NH 2 CHO CH 3 NH 2 CH 2 OHCHO (CH 3 ) 2 O SO SO 2 H 2 CO CHOOH CH 3 CN CH 3 CCH CH 3 C 2 CN CH 3 CH 2 CN SO + + HN 2 H 2 CS HC CCN CH 3 NC CH 2 CHCN C 7 H H(C C) 3 CN SiO HNO HNCO CH 2 NH CH 3 SH HC 4 CN H 2 C 6 CH 3 (C C) 2 H SiS SiH 2 HNCS NH 2 CN C 5 H C 6 H HC 6 H C 8 H NO NH 2 CCCN H 2 CCO HC 2 CHO c-ch 2 OCH 2 CH 3 CO 2 H CH 3 (C C) 2 CN NS + H 3 + HCO 2 C 4 H CH 2 =CH 2 CH 2 CHOH C 6 H 2 HCl NNO CCCH c-c 3 H 2 H 2 CCCC 10 NaCl HCO c-ccch CH 2 CN HC 3 NH + CH 3 COCH 3 AlCl OCS CCCS SiC 4 HC 4 H CH 3 (C C) 2 CN AlF CCH HCCH H 2 CCC C 5 S HOCH 2 CH 2 OH PN HCS + HCNH + HCCNC C 4 H 2 CH 3 CH 2 CHO SiN c-sicc HCCN HNCCC HC 4 N NH CCO H 2 CN H 3 CO + CH 2 CNH SH CCS c-sic HF C 3 CH 3 H(C C) 4 CN CN MgNC CH 2 D CO NaCN AlNC A Molecular 12 CS CH 2 Universe 80 c-c 6 H 6 C 2 MgCN SiC HOC CP HCN H 2 O H(C C) 5 CN CO + HNC CH + CO 2 CH NH CH SiCN CH OH 20 Total: 136 N 2 AlCN CN CF + SiNC KCN HCO Year December 2006

8 Molecules in space Number of Atoms H 2 H 2 O NH 3 SiH 4 CH 3 OH CH 3 CHO HCOOCH 3 CH 3 CH 2 OH OH H 2 S H 3 O + CH 4 NH 2 CHO CH 3 NH 2 CH 2 OHCHO (CH 3 ) 2 O SO SO 2 H 2 CO CHOOH CH 3 CN CH 3 CCH CH 3 C 2 CN CH 3 CH 2 CN SO + + HN 2 H 2 CS HC CCN CH 3 NC CH 2 CHCN C 7 H H(C C) 3 CN SiO HNO HNCO CH 2 NH CH 3 SH HC 4 CN H 2 C 6 CH 3 (C C) 2 H SiS SiH 2 HNCS NH 2 CN C 5 H C 6 H HC 6 H C 8 H NO NH 2 CCCN H 2 CCO HC 2 CHO c-ch 2 OCH 2 CH 3 CO 2 H CH 3 (C C) 2 CN NS + H 3 + HCO 2 C 4 H CH 2 =CH 2 CH 2 CHOH C 6 H 2 HCl NNO CCCH c-c 3 H 2 H 2 CCCC 10 NaCl HCO c-ccch CH 2 CN HC 3 NH + CH 3 COCH 3 AlCl OCS CCCS SiC 4 HC 4 H CH 3 (C C) 2 CN AlF CCH HCCH H 2 CCC C 5 S HOCH 2 CH 2 OH PN HCS + HCNH + HCCNC C 4 H 2 CH 3 CH 2 CHO SiN c-sicc HCCN HNCCC HC 4 N NH CCO H 2 CN H 3 CO + CH 2 CNH SH CCS c-sic 3 11 HF C 3 CH 3 H(C C) 4 CN CN MgNC CH 2 D + CO NaCN AlNC 12 CS CH 2 c-c 6 H 6 C 2 MgCN SiC HOC + 13 CP HCN CO + HNC 15 positive molecular ions H(C C) 5 CN CH + CO 2 CH SiCN Total: 136 N 2 AlCN CF + SiNC KCN HCO +

9 Molecules in space Number of Atoms H 2 H 2 O NH 3 SiH 4 CH 3 OH CH 3 CHO HCOOCH 3 CH 3 CH 2 OH OH H 2 S H 3 O + CH 4 NH 2 CHO CH 3 NH 2 CH 2 OHCHO (CH 3 ) 2 O SO SO 2 H 2 CO CHOOH CH 3 CN CH 3 CCH CH 3 C 2 CN CH 3 CH 2 CN SO + + HN 2 H 2 CS HC CCN CH 3 NC CH 2 CHCN C 7 H H(C C) 3 CN SiO HNO HNCO CH 2 NH CH 3 SH HC 4 CN H 2 C 6 CH 3 (C C) 2 H SiS SiH 2 HNCS NH 2 CN C 5 H C 6 H HC 6 H C 8 H NO NH 2 CCCN H 2 CCO HC 2 CHO c-ch 2 OCH 2 CH 3 CO 2 H CH 3 (C C) 2 CN NS + H 3 + HCO 2 C 4 H CH 2 =CH 2 CH 2 CHOH C 6 H 2 HCl NNO CCCH c-c 3 H 2 H 2 CCCC 10 NaCl HCO c-ccch CH 2 CN HC 3 NH + CH 3 COCH 3 AlCl OCS CCCS SiC 4 HC 4 H CH 3 (C C) 2 CN AlF CCH HCCH H 2 CCC C 5 S HOCH 2 CH 2 OH PN HCS + HCNH + HCCNC C 4 H 2 CH 3 CH 2 CHO SiN c-sicc HCCN HNCCC HC 4 N NH CCO H 2 CN H 3 CO + CH 2 CNH SH CCS c-sic 3 11 HF C 3 CH 3 H(C C) 4 CN CN MgNC CH 2 D + CO NaCN AlNC 12 CS CH 2 c-c 6 H 6 C 2 MgCN SiC HOC + 13 CP HCN H(C C) 5 CN CO + HNC CH + CO 2 CH SiCN NO negative molecular ions! Total: 136 N 2 AlCN CF + SiNC KCN HCO +

10 Anions in space not such a bad idea H shown to be the main source of opacity in the solar atmosphere. (Wildt, ApJ, 1939) Molecular anions long postulated to be constituents of the interstellar gas. cold, dense clouds circumstellar shells diffuse interstellar medium (Herbst, Nature, 1981; Lepp & Dalgarno, ApJ, 1988; Petrie, MNRAS, 1996; Petrie & Herbst, ApJ 1997; Millar et al., MNRAS, 2000, Tulej et al., ApJL, 1998) anions studied by low resolution methods in the laboratory. (e.g. Rienstra-Kiracofe et al., Chem. Rev., 2002)

11 Anions in space lack of laboratory data But: only two anions studied by rotational spectroscopy: (Liu & Oka, JCP, 1986; Civis et al., JCP, 1998). OH and SH a few more by IR spectroscopy: NCS, NCO, NH 2, NH, HNO (Owrutsky et al., Phil Trans. R. Soc. Lond., 1987; Miller et al., JMS, 1989; Al Za al et al., Phys. Rev. A, 1987) and rotationally resolved electronic spectroscopy: C 4 H, CH 2 CN, l-c 3 H 2 (Pachkov et al., Mol. Phys., 2003; Lykke et al., JCP, 1987; Yokoyama et al., JCP, 1996) only one published (unsuccessful) search in space: (Morisawa et al., PASJ, 1995) CCH, NCS, NCO

12 B1377: An unidentified sequence of astronomical lines Line survey of IRC (Kawaguchi et al., PASJ, 1995) 7 lines (Nobeyama 45m) circumstellar envelope around carbon rich star extremely rich chemistry: >50 molecules TMC-1 (Taurus-Auriga Molecular Cloud) (McCarthy et al., ApJL, 2006) 2 lines (GBT 100m) cold (10 K), dense (10 4 cm -3 ) rich in carbon-chains no metal-bearing molecules TMC-1 Guélin et al. (1997) Dame et al. ApJ (2001)

13 B1377: An unidentified sequence of astronomical lines rotational lines are harmonic: separated in frequency by ratio of integers resolving molecular shell carrier almost certainly a closed-shell ( 1 S) linear molecule likely a rather fundamental molecule rotational constant about 1% lower than C 6 H (B=1391 MHz), an abundant radical in this source C 6 H suggested as carrier based on ab initio calculations (Aoki, Chem. Phys. L., 2000) J= mk But: positive ions (HCO + ) not abundant in these sources

14 B1377 in the laboratory Exact match to astronomical lines has now been achieved using two hydrocarbon gases 15+ lines detected over two decades of frequency Constant (MHz) Laboratory Astronomical B (7) (294) 10 6 D 32.35(1) 33.35(993)

15 B1377 in the laboratory and in space

16 Experimental setup - FTM supersonic nozzle coupled to a high-q Fabry-Perot cavity discharge nozzle Q ~ 40,000 movable mirror 1 msec FID ~100 msec cooled to 77 K FFT frequency range: 5 42 GHz resolution: 20 khz accuracy: 1-2 khz low current (~20 ma) discharge of C 4 H 2 or C 2 H 2 (0.1 %) in Ne (He, H 2 )

17 Experimental setup mm-wave absorption spectrometer InSb detector power supply solenoid Gunn n 2 m LN 2 LN 2 low current dc glow discharge of C 2 H 2 (85 %) + Ar (15 %) frequency coverage: GHz frequency accuracy: khz cell walls cooled by LN 2 to 150 K

18 Identification of B1377 as C 6 H composed of both C and H harmonic relation of lines requires linear geometry B constant requires chain of six carbon atoms; one more of less yields B 40% too high or low Isotopic shift observed on deuteration requires one H at end of carbon chain (4.53% vs. 4.55% for C 6 H) Neutral C 6 H ( 2 P) and C 6 H + ( 3 S) lack required symmetry carrier must be C 6 H one of the most securely identified astronomical molecules

19 Why hexatriyne, C 6 H? larger than most of the neutral molecules and all of the cations Four factors favor formation: radical is abundant in both IRC and TMC-1 exceptionally stable: high electron binding (3.8 ev)

20 Why hexatriyne, C 6 H? larger than most of the neutral molecules and all of the cations Four factors favor formation: radical is abundant in both IRC and TMC-1 exceptionally stable: high electron binding (3.8 ev) size confers stability: C 6 H apparently large enough where e attachment becomes efficient (Taylor et al., JCP, 1998)

21 e radiative attachment versus size Statistical Treatment: C n H + e k f [C n H ]* k d C n H + e C n H + hn k rad autodetachment vs. size total radiative attachment C 6 H density of states Herbst et al. ApJ, in press

22 Why hexatriyne, C 6 H? larger than most of the neutral molecules and all of the cations Four factors favor formation: radical is abundant in both IRC and TMC-1 exceptionally stable: high electron binding (3.8 ev) size confers stability: C 6 H apparently large enough where e attachment becomes efficient (Taylor et al., JCP, 1998) spectral simplification: lines of anion x10 more conspicuous than those of neutral!

23 Spectral compression C 6 H ( 2 P) e C 6 H ( 1 S) 10 mk IRC P 1/2 2 P 3/2 Dipole Moments: = 2.1 hfs L-doubling

24 More anions discovery of C 6 H suggested specific anions to target closed-shell 1 S ground states and large dipole moments (m) large electron affinities (EA) radicals are detected in astronomical sources anions do not react with H 2 (Barckholtz et al., ApJL, 547, 2001) EA (ev) m (D) m (D) B (MHz) CCH 2 S CCH 1 S ,639 C 4 H 2 S C 4 H 1 S 6.1 4,656 C 6 H 2 P C 6 H 1 S 8.2 1,377 C 8 H 2 P C 8 H 1 S CN 2 S CN 1 S ,133 C 3 N 2 S C 3 N 1 S 3.1 4,852

25 Ab initio calculations laboratory searches guided by high level CCSD(T) ab initio calculations (cc-pvtz or higher) (Gupta & Stanton, private communication) calculated rotational constants B 0 accurate to better than 0.1 % calculations considerably speed up laboratory searches additional information: centrifugal distortion constants quadrupole coupling constants dipole moments

26 Laboratory measurements six molecular anions detected in less than one year H C C CCH CN C N H C C C C C 4 H C 3 N C C C N H C C C C C C C 6 H H C C C C C C C C C 8 H detections secured by: elemental composition harmonicity close agreement with calculations isotopic shift determination of charge state

27 Cyanide, CN B ~ 56 GHz hyper fine structure acts as spectral fingerprint 5 transitions in GHz range (Gottlieb et al., JCP 2007)

28 Cyanide, CN B ~ 56 GHz integration time ~10 min! anion / neutral ratio of 1 % in glow discharge dissociative attachment to (CN) 2 (Kühn et al., Chem. Phys. L., 1987) CN / (CN) 2 = 4 x 10-6

29 Ion Drift Measurements CN measurements in single pass +HV -HV radiation with alternating polarity direct proof of the charge state of the carrier

30 C 3 N Petrie & Herbst (ApJ, 1997) predicted C 3 N to have abundance ~1 % of neutral C 3 N in TMC-1 discharge production: dissociative attachment to HCCCN (Graupner et al., New J. Phys., 2006) 12 transitions detected up to 380 GHz C 3 N / HC 3 N = 5 x 10-6 B ~ 4.9 GHz C 3 N / C 3 N = 2 %

31 Formation in the laboratory in the laboratory dissociative attachment possible [E kin (e ) ~ 3eV] e + HCCH CCH + H threshold 1.76 ev (De Bleecker et al., Phys. Rev. E, 2006) CCH + HCCH C 4 H + H 2 Questions to be addressed: plasma characteristics, influence of carrier gas, suitable precursors? wall surface chemistry, metallization of discharge cell? DC discharge vs. H + or H + 2 abstraction HC 2n H + OH C 2n H + H 2 O reduction of background lines? velocity modulation

32 Detection of C 8 H in TMC-1 100m Green Bank Telescope (April 2007) N T = 2.1(4) cm -2 for T rot = 5 K C 8 H / C 8 H = 5(1) % Brünken et al., ApJL, 2007

33 Detection of C 3 N in IRC IRAM 30m telescope N T = 1.6 x cm -2 T rot = 24 K C 3 N /C 3 N = 0.52 % Thaddeus et al., ApJ 2008

34 Anions in space C 6 H and C 8 H detected in TMC-1 C 4 H, C 6 H, C 8 H and C 3 N now been detected in IRC (Cernicharo et al. ApJL, 2007; Remijan et al., ApJL, 2007, Kawaguchi et al., PASJ, 2007, Thaddeus et al., ApJ 2008) C 8 H anion to neutral ratio 28-37%! detection of C 4 H, C 6 H in a third source: IRAS in L1527 (Agundez et al. A&A, 2008; Sakai et al. ApJL, 2007, 2008) tentative detection of J = 2-1 of CN in IRC (Guelin, private communication)

35 Abundances of anions in space TMC-1 IRC C n H / C n H obs. calc. e obs. calc. e /37 a b 30 4 < f c 4 C 3 N / C 3 N < 0.8 f 0.52 f 0.05 e,f all ratios in %, a Remijan et al., ApJL, 2007; Kawaguchi et al., PASJ, acc.; b Kasai et al. ApJL, 2007, c Cernicharo et al. ApJL, 2007; d Agundez et al. ApJ 2008., e Millar et al. ApJL, 2007, Herbst et al. ApJ 2008, f Thaddeus et al. ApJ 2008 formation and destruction processes not fully understood lack of laboratory data

36 What can we learn? radiative electron attachment most likely formation process in space: A + e (A )* A + hn k ratt dominant destruction processes: k aa A + H AH + e k neu A + B + A + B [A ] k ratt [e ] k ratt [e ] = [A] k aa [H] + k neu [B + ] k aa [H] fractional ionization fractional ionization dependent upon: Flower et al., A&A, 2007 cosmic ray ionization rate hydrogen density grain characteristics PAH and metal abundance

37 Conclusions accurate transition frequencies available for all six anions C 4 H, C 6 H, C 8 H and C 3 N have already been detected in space with surprisingly high abundances many other anions likely to be detected in the laboratory and in space (CH 2 CN, l-c 3 H 2, C 3 H 3, CH 3 O ) a galactic survey of molecular anions, i.e. C 6 H is timely other, better sources abundance as function of temperature, density and composition laboratory studies of reaction and formation rates of anions necessary

38 THANKS! Sam Palmer Filippo Tamassia Christian Endres, Frank Lewen NRAO staff for assistance with the GBT observations Eric Herbst John Stanton Peter Botschwina Stephan Schlemmer Alex Dalgarno Pepe Cernicharo Bill Klemperer Takeshi Oka John Maier Bill Doering Michel Guelin Funding agencies: NSF, NASA, Robert A. Welch Foundation, HCO