Adsorption and Reactions of Small Molecules at Grain

“Adsorption and Reactions of Small Molecules at Grain and Ice Surfaces” CO Helen Jane Fraser Raymond & Beverley Sackler Laboratory for Astrophysics at Leiden Observatory Klaus Pontoppidan Leiden Observatory

Acknowledgements Prof. E. F. van Dishoeck Fleur van Broekhuizen Suzanne Bisschop Klaus Pontoppidan Dr. M. R. S. Mc. Coustra Dr. M. P. Collings John Dever Prof. D. A. Williams Ewie de Kuyper & all the technical staff at UL €€€€ Prof. X. Tielens ££££ VLT ISSAC TEAM!!

Models Laboratory Charnley et. al. , A&A, 378, 1024 (2001) Observations NGC 3324 Keyhole Nebula The dark Keyhole Nebula is superimposed on the bright Eta Carina Nebula, NGC 3372 The nebula is a star forming region ©AAT (Anglo-Australian Observatory)

A gallery of interstellar ice • A thousand laboratory experiment to explain a few astronomical spectra? • A few laboratory experiments to explain a thousand astronomical spectra? Most comparisons depend on very, very few extremely biased astronomical sources: W 33 A, RAFGL 7009 S, Galactic Center etc. We need better statistics for “typical” lines of sight in space! Unfortunately, there will be no space telescope optimized for ice in the nearest future… We’re stuck with ground-based facilities.

Ground-based observations Atmospheric windows allow ground-based spectroscopy of H 2 O, CH 3 OH, OCN-, CO, OCS, NH 3 and silicates.

A universal CO band? Three components in a CO ice band: Broad red (Lorentz), narrow middle (Gauss), narrow blue (Gauss).

13 CO ice 13 CO is not dependent on grain shape Pure CO O 2 rich N 2 rich Breaks degeneracy between CO environment and grain shape

How can the lab help? • Understand spectroscopic origin of ‘ 3’ bands • Understand behaviour of CO ices • Understand reactivity of CO ices

Spectroscopy. . (see Wassim’s poster): • CO in ‘pure’ & CH 3 OH / H 2 O / CH 4 / HCOOH / CO 2 matrices has multi-component features in spectrum • Spectra of CO OVER / UNDER / MIXED with above NOT EQUIVALENT • Components in very similar positions to those used in astronomical phenomenological fit

adsorption sticking PHYSICAL BEHAVIOUR desorption

10 L CO n-porous IASW 10 L CO m-porous IASW 10 L 45 K CO m-porous IASW 10 L 70 K 30 K CO m-porous IASW M. P. Collings, H. J. Fraser, J. W. Dever and M. R. S. Mc. Coustra Ap. J. , 538, no. 2, 2003 10 L 8 K 8 K CO

Dever, Collings, Fraser & Mc. Coustra, A&SS, (2003) in press

CO CO CO & IASW m-porous IASW n-porous IASW M. P. Collings, H. J. Fraser, J. W. Dever and M. R. S. Mc. Coustra, Ap. J. , 538, no. 2, 2003,

CO on H 2 O ice 160 K Temperature 135 - 140 K 30 - 70 K 10 - 20 K < 10 K M. P. Collings, H. J. Fraser, J. W. Dever and M. R. S. Mc. Coustra Ap. J. , 538, no. 2, (2003)

= + To simplify the system: Collings et. al Ap. J, 583, no. 2, (2003) Collings et. al Ap&SS, (2003) Collings et. al NASA LAW (2002) CH 4 HCOOH ? CH 3 OH NH 3 TRAPPING H 2 O H-bonding capabilities - trapping CO 2 Permanent dipole -both? No permanent dipole - no trapping Fraser et al A&A 2003, in prep

CO desorbing from CO CO desorbing from HCOOH surface & No desorption CO desorbing during b to a phase change in HCOOH desorbs No CO Bisschop, Fraser, van Dishoeck, A&A, (2003) in prep

Bisschop, Alsindi, Fraser, A&A, (2003) in prep

Subset of molecules behaving the same way ABLE TO MAKE EDUCATED PREDICTIONS ON BEHAVIOUR & DATA VALUES OF OTHER SIMILAR MOLECULES CH 4 HCOOH ? CH 3 OH H 2 O H-bonding capabilities - trapping Permanent dipole -both? No permanent dipole - no trapping

Astronomical Implications • We are able to empirically measure • sticking probabilities • binding energies Ea CO-CO < Ea CO-ice surface (typically up to 10 k. J mol-1) • kinetics for data needs in astrochemical modeling • We can generalise about volatile gas trapping in hydrogenated ices • CO can be in the solid state at higher T than previously thought (through trapping and surface binding) • CO will be highly mobile in the ice matrix – able to react

Migration and CO in water 16% CO in water

Evolutionary tracks for the CO components

CO 2 -ice = ubiquitous P. A. GERAKINES Ap. J, 522, 357 -377, 1999

C H C (2) O O (1) O O O T S H H O O ’S R IE R (3) C R AO B N O I T C A E R O C O

= 18 O 2 = Ar X 25 K 10 K WARM : 1 = 13 C 16 O : 1

No thermal reactions Significant energy barrier to the CO + O reaction which lies beyond the sublimation energy of CO Fraser, Tielens, van Dishoeck, Ap. J, (2003) in prep

1000: 1: 1 Ar: CO: O 2 // 13 C 16 O 18 O 10: 1: 1 Ar: CO: O 2 // Fraser, Tielens, van Dishoeck, Ap. J, (2003) submitted

+ + + hn hn + + hn + + hn + + + hn

100: 1: 1 Ar: CO: O 2 Fraser, Tielens, van Dishoeck, Ap. J, (2003), in prep

Fraser, Tielens, van Dishoeck, NASA LAW Proceedings, (2002)

HV experiments on CO + O show: • CO 2 isotopic yield is highly dependent on the reagent concentrations in the initial ice mixture, and H 2 O contamination from the vacuum. If H 2 O is present the OH pathway dominates CO 2 production • Significant energy barrier to the reaction which lies beyond the sublimation energy of CO • In the solid state CO 2 is more readily produced from the reaction between CO + OH than CO + O QUALITATIVE NOT QUANTITATIVE METHOD

TPD results • In absence of H 2 O no detectable levels of CO 2 produced (Therefore conclude Eley Rideal reaction is not efficient in this case) • With water ice cap present CO 2 yield is roughly proportional to the O-dose (rate limiting factor in experiment) • Estimate Ea = 35 k. J mol-1 Joe E. Roser et. al. Astrophysical Journal, 555: L 61–L 64, 2001

Astronomical Implications • Modelers can assume that in photon dominated regions • CO + OH is more efficient than CO + O • unless CO is trapped it desorbs BEFORE reacting with O • Does this help us explain ubiquitous observations of CO 2 in H 2 O rich ices? • water is a key catalyst or ‘support’ media for the reaction • CO 2 is predominantly produced from CO + H 2 O reactions • should we also consider OH provision from the gas phase • If CO 2 can also be produced in non UV photon mediated processes, then are they efficient enough to reproduce the CO 2 observed?

Summary…. or answers to some perennial questions Why don’t we see -1 the 2152 cm band?

No 2152 cm-1 band!

CO on IASW @ 8 K CO on IASW @ 80 K CO on Ic CO / ice mixture Fraser et al. MNRAS, 2003, in prep

Summary…. or answers to some perennial questions • Is the underlying ice structure key? • Does this tell us something about processing? • Are the binding sites blocked or inaccessible? • e. g. through reactions of CO on the dangling OH to form CO 2, CH 3 OH etc? • e. g. through accretion of other species onto to H 2 O surface BEFORE CO itself adsorbs / DURING H 2 O formation?