Presentation on theme: "“Adsorption and Reactions of Small Molecules at Grain and Ice Surfaces” Helen Jane Fraser Raymond & Beverley Sackler Laboratory for Astrophysics at Leiden."— Presentation transcript:
“Adsorption and Reactions of Small Molecules at Grain and Ice Surfaces” Helen Jane Fraser Raymond & Beverley Sackler Laboratory for Astrophysics at Leiden Observatory Klaus Pontoppidan Leiden Observatory CO
Acknowledgements Prof. E.F. van Dishoeck Fleur van Broekhuizen Suzanne Bisschop Klaus Pontoppidan Ewie de Kuyper & all the technical staff at UL Prof. X. Tielens VLT ISSAC TEAM!! Dr. M.R.S. McCoustra Dr. M. P. Collings John Dever Prof. D.A.Williams €€€€ ££££
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: W33A, RAFGL7009S, 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 Pure CO O 2 rich N 2 rich 13 CO is not dependent on grain shape 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’ & CH3OH / H2O / CH4 / HCOOH / CO2 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
M. P. Collings, H. J. Fraser, J. W. Dever and M. R. S. McCoustra Ap. J., 538, no.2, 2003 n-porous I ASW CO 10 L 70 K -porous I ASW CO 10 L 45 K -porous I ASW CO 10 L 8 K -porous I ASW CO 10 L 30 K -porous I ASW CO 10 L 8 K
Dever, Collings, Fraser & McCoustra, A&SS, (2003) in press
M. P. Collings, H. J. Fraser, J. W. Dever and M. R. S. McCoustra, Ap. J., 538, no.2, 2003, -porous I ASW CO n-porous I ASW CO CO & I ASW
< 10 K Temperature 10 - 20 K 30 - 70 K 135 - 140 K 160 K M. P. Collings, H. J. Fraser, J. W. Dever and M. R. S. McCoustra Ap. J., 538, no.2, (2003) CO on H 2 O ice
+ = TRAPPING To simplify the system: H2OH2O NH 3 CH 3 OH HCOOH CH 4 CO 2 H-bonding capabilities - trapping No permanent dipole - no trapping ? Permanent dipole -both? Collings et. al ApJ, 583,no. 2, (2003) Collings et. al Ap&SS, (2003) Collings et. al NASA LAW (2002) Fraser et al A&A 2003, in prep
& CO desorbing from CO CO desorbing from HCOOH surface No desorption CO desorbing during to phase change in HCOOH No CO HCOOH desorbs Bisschop, Fraser, van Dishoeck, A&A, (2003) in prep
H2OH2O CH 3 OH HCOOH H-bonding capabilities - trapping No permanent dipole - no trapping ? Permanent dipole -both? Subset of molecules behaving the same way ABLE TO MAKE EDUCATED PREDICTIONS ON BEHAVIOUR & DATA VALUES OF OTHER SIMILAR MOLECULES CH 4
Astronomical Implications We are able to empirically measure sticking probabilities binding energies E a CO-CO < E a CO-ice surface (typically up to 10 kJ 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
CO 2 -ice = ubiquitous P. A. GERAKINES ApJ, 522, 357-377, 1999
O C O O C O O H O H (1) (2) (3) BARRIER’S TO REACTION
= 13 C 16 O= Ar= 18 O 2 X:1:1X:1:1 10 K WARM 25 K
No thermal reactions Fraser, Tielens, van Dishoeck, Ap.J, (2003) in prep Significant energy barrier to the CO + O reaction which lies beyond the sublimation energy of CO
Fraser, Tielens, van Dishoeck, Ap.J, (2003) submitted // 1000:1:1 Ar:CO:O 2 // 13 C 16 O 18 O 10:1:1 Ar:CO:O 2
h ++ h ++ h ++ h ++ h ++ h ++ h ++ h ++
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 E a = 35 kJ mol -1 Joe E. Roser et. al. Astrophysical Journal, 555:L61–L64, 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 the 2152 cm -1 band?
Fraser et al. MNRAS, 2003, in prep CO on I ASW @ 8 K CO on I ASW @ 80 K CO on I c CO / ice mixture
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?