1. Chemistry of Candidate Molecular Ion Carriers of the Diffuse Interstellar Bands
Theodore P. Snow
Nicholas Betts
Meredith Drosback
Veronica Bierbaum
Outline of Talk
* Overview of the Colorado program
* The FA-SIFT apparatus
* The DIBs problem
* Studies of PAH cations
* Studies of carbon chain anions
* Future work

2. Boulder Ion Gang

3. Insram
Flowing Afterglow – Selected Ion Flow Tube Reactions of Ions with Atoms

4. ISM

5. eous Pillars

8. DIB line spectrum

9. DIB (spectrum overlaid)

10. Recently Considered DIB Carriers
* PAH cations Composition: Abundant elements, rich structural diversity Accommodate energy without fragmentation Expected to have strong visible absorptions Likely carriers of IR emissions bands
* Carbon chain anions Studies of Maier and co-workers Striking coincidences with several DIBs

11. Why Are PAH Cations of interest?
PAH rationale
Why Are PAH Cations of interest?
* PAHs may be common in the ISM (UIBs)
* PAHs easily satisfy abundance requirements for the DIBs
* PAH cations are known to have rich optical spectra
* In the diffuse ISM where the DIBs form, PAHs are singly ionized
* PAH cations of sufficient size are able to absorb photons without being destroyed

12. PAH+ generation
X X X

13. PAH+ + H2/H

14. Reactivity trends1

15. Timescales

16. PAH modeling

17. Major processes

18. Parameters

19. Modeling Results * State of Ionization [PAH+] > [PAH] > [PAH–]
* Astrophysical Journal Hydrogenation and Charge States of Polycyclic Aromatic Hydrocarbons in Diffuse Clouds, Papers I and II (ApJS, 132, 233; APJ, 584, V. Le Page, T. P. Snow, and V. M. Bierbaum
* State of Ionization [PAH+] > [PAH] > [PAH–]
* State of Hydrogenation Depends strongly on PAH size
* See also: Ruiterkamp et al. (A&A, 432, 515, 2005
(presented at this conference by Nick Cox)

20. Size

21. Summary: PAH Cations in the Diffuse ISM
* PAH cations in diffuse interstellar clouds rapidly add an H atom and become protonated, a trend that we expect will continue to larger PAH+ not yet measured.
* PAH cations in the size range from 50 to 100 carbon atoms appear to be favored (LePage et al 2001, 2003; Ruiterkamp et al. 2005)
* PAH cations are excellent candidates for the DIB carriers!

22. Carbon Chain rationale
Why Are Carbon Chain Anions of interest?
* Carbon chains are the most common molecules in interstellar space
* Carbon chains easily satisfy abundance requirements for the DIBs
* Carbon chain anions are known to have rich optical spectra
* Maier et al. found tantalizing coincidences in wavelength with some DIBs
* Carbon chain anions may lead chemically to larger species observed in molecular clouds

23. Carbon chain generation

24. Carbon Chain Anion Generation Cold Cathode Source

26. Summary: Carbon Chain Anions
* Carbon chain anions are not viable carriers of the DIBs
* Reactions of organic anions with H, N, O occur readily and reveal rich chemistry
* These processes may contribute to formation of observed interstellar species in molecular clouds

27. * Probe reactions of larger PAH+ PAH oven and electrospray ionization
Future Directions
* Model the contribution of negative ion chemistry to molecular synthesis in clouds
* Probe reactions of larger PAH+ PAH oven and electrospray ionization
* Explore structures of product ions incorporating H, N, O

28. Chuck DePuy • Shuji Kato
Acknowledgments
Veronica Bierbaum
Valery Le Page
Momir Stepanovic
Cindy Barckholtz
Brian Eichelberger
Nick Betts
Meredith Drosback
Chuck DePuy • Shuji Kato
NASA