THE J = 1 – 0 ROTATIONAL TRANSITIONS OF 12 CH +, 13 CH +, AND CD + T. Amano Department of Chemistry and Department of Physics and Astronomy The University of Waterloo

In the past………..  First molecular ion identified in interstellar space Dunham, Publ. Astron. Soc. Pac. 49, 26 (1937) Douglas and Herzberg, Ap. J. 94, 381 (1941)  Pearson and Drouin reported the first laboratory identification of the J = 1 – 0 lines of 12 CH +, and calculated the best predicted rest frequencies for 13 CH +, 12 CD +, and 13 CD +. Ap. J. 647, L83 (2006)  Falgarone et al claimed an interstellar detection of 13 CH + based on Pearson and Drouin’s data. Ap. J. 634, L149 (2005)  Cernicharo et al identified FIR lines ( J = 2 – 1, 3 – 2, 4 – 3, 5 – 4, 6 – 5 ) of CH +. Ap. J. 483, L65 (1997) No infrared measurements involving low-v states.

In this work……………  The frequencies are found different from the reported values.  Large Zeeman effect and spin-rotation interaction Sub-mm system……..  Russian BWO(Backward-wave oscillator) GHz ~1 mW stabilized, using double phase-lock loop  Double modulation technique frequency-discharge frequency-magnetic field extended negative glow discharge

 Extended negative glow discharge source. (Magnetic field: 160 G).  Double modulation.  CH 4 ~ 0.5 mTorr ( 13 CH 4, CD 4 ) He ~ mTorr For extended negative glow discharges, usually Ar is used as buffer gas. However, Ar buffer does not produce CH +. The He pressure and the optimum magnetic field are correlated. Experimental Details Rapidly react with H 2 and O 2

An example of observed signals

Zeeman splitting of the J = 1 – 0 line of 12 CH +

Hyperfine structure of the J = 1 – 0 line of 13 CH +

Transition frequencies / MHz This work Pearson and Drouin a From optical data 12 CH (20) (75) b 13 CH (10) (estim’d) c F = 3/2 – ½ (20) F = ½ - ½ (40) 12 CD (20) (189) (estim’d) d Spin-rotation constant/MHz C I 1.087(50) e Rotational g-factor g J 7.65(29) a J. C. Pearson and B. J. Drouin, Astrophys. J. 647, L86 (2006) b R. Hakalla et al, Eur. Phys. J. D. 38, 481 (2006) c Z. Bembenek, J. Mol. Spectrosc. 181, 136 (1997) d Z. Bembenek et al, J. Phys. B, 20, 6197 (1987) e S. P. A. Sauer and I. Paidarova, Chem. Phys. 201, 405 (1995)

Zeeman effect and the spin-rotation interaction Small admixture of the excited 1  electronic state induced by the L-uncoupling causes both Zeeman effect and the spin- rotation interaction. However, each does not appear in same fashion, resulting in distinct effects for 12 CH +, 13 CH +, and CD +.  12 CH + More prominent Zeeman effect.  13 CH + Both effects are observable.  12 CD + Both Zeeman and the spin-rotation interaction are smaller. Theoretical calculations of magnetic properties of CH +  Fowler and Steiner, Mol. Phys. 74, 1147(1991) “paramagnetic molecule of closed shell”  Sauer and Paidarova, Chem. Phys. 201, 405 (1995)

12 CH + 13 CH + (a) 12 CH + C I = kHz (b) 13 CH + C I = MHz

Zeeman effect and spin-rotation interaction in 1 Σ states Combined with L-uncoupling term, lead to the second order Zeeman and hyperfine terms

Rotational g-factor Λ-doubling constant and rotational g-factor q = cm -1 ( v = 0 in A 1  ) g e ∼ g J = 5.17 ( 7.65 obs ) 11.3 (ab initio calculation, HF-SCF/aug-pV5Z) ( Hui Li, Waterloo)

Spin-rotation interaction This interaction is essentially between the πelectron and the nuclear spin(s); H in 12 CH +, 13 C and H in 13 CH +, and D in 12 CD +. The π electron is localized on the C atom. Therefore, the interaction with the 13 C nuclear spin is more prominent than that with either H or D. Ab initio calculation by Sauer and Paidarová, Chem. Phys. Lett. 201, 405 (1995) C C = MHz, C H = -73 kHz, C D = -21 kHz → C I = MHz Obs. C I = MHz for 13 CH +

Distinctively different behavior of the isotopic species is understood theoretically. It provides firm support to the identifications. Conclusion

University of Waterloo Acknowledgments NSERC (Natural Science and Engineering Research Council of Canada) H. Li (Waterloo) R. J. Le Roy (Waterloo)

Double modulation sub-mm system at Waterloo

FIR lines, are they consistent with our data? Cernicharo et al, Ap. J. 483, L65 (1997) 2 – GHz ( µm ) 3 – GHz ( µm ) 4 – GHz ( µm ) 5 – GHz ( µm ) 6 – GHz ( µm ) 1 – (20) MHz o – c / MHz present Hakalla et al B 0 /MHz (51) (57) D 0 /MHz (26) (63) R. Hakalla et al, Eur. Phys. J. D. 38, 481 (2006)