““Adsorption and Reactions of Adsorption and Reactions of Small Molecules at Grain and Small Molecules at Grain and
Ice Surfaces”Ice Surfaces”
Helen Jane FraserRaymond & Beverley Sackler Laboratory
for Astrophysics at Leiden Observatory
Klaus PontoppidanLeiden Observatory
CO
AcknowledgementsAcknowledgementsProf. E.F. van Dishoeck
Fleur van BroekhuizenSuzanne BisschopKlaus Pontoppidan
Ewie de Kuyper & all the technical staff at UL
Prof. X. Tielens
VLT ISSAC TEAM!!
Dr. M.R.S. McCoustraDr. M. P. CollingsJohn Dever
Prof. D.A.Williams
€€€€
££££
NGC 3324 Keyhole NebulaThe dark Keyhole Nebula is superimposedon the bright Eta Carina Nebula, NGC 3372The nebula is a star forming region©AAT (Anglo-Australian Observatory)
Charnley et. al., A&A, 378, 1024 (2001)
Laboratory
Observations
Models
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 inthe nearest future… We’re stuck with ground-based facilities.
Ground-based observations
Atmospheric windows allow ground-based spectroscopy of H2O, CH3OH, OCN-, CO, OCS, NH3 and silicates.
A universal CO band?
Three components in a CO ice band:
Broad red (Lorentz), narrow middle (Gauss),
narrow blue (Gauss).
13CO ice
Pure CO
O2 rich
N2 rich
13CO is not dependent ongrain shape
Breaks degeneracy between CO environment and grain shape
How can the lab help?How can the lab help?
•Understand spectroscopic origin of ‘3’ bands
•Understand behaviour of CO ices
•Understand reactivity of CO ices
Spectroscopy..Spectroscopy..
(see Wassim’s poster):
2150 2148 2146 2144 2142 2140 2138 2136 2134 2132 2130
0.0
5.0x10-2
1.0x10-1
1.5x10-1
2.0x10-1
2.5x10-1
3.0x10-1
3.5x10-1
4.0x10-1
Wavenumber / cm-1
2128 2130 2132 2134 2136 2138 2140 2142 2144 2146 2148 2150 2152
0.0
5.0x10-2
1.0x10-1
1.5x10-1
2.0x10-1
2.5x10-1
3.0x10-1
CO and Methane in a 1:1 Solid Mixture:at 30 K
Ab
sorb
an
ce /
Arb
. u
nits
Wavenumber / cm-1
2150 2148 2146 2144 2142 2140 2138 2136 2134 2132 2130
0.0
5.0x10-2
1.0x10-1
1.5x10-1
2.0x10-1
2.5x10-1
3.0x10-1 CO and Methane in a 1:1 Solid Mixture:at 25 K
Wavenumber / cm-1
• 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
2150 2145 2140 2135 2130
0.00
0.05
0.10
0.15
0.20
Ab
sorb
ance
/ A
rb.
Un
its
Wavenumber / cm-1
No deposition, 14.6 K 240s deposition, 14.6 K Warm-Up, 14.6 K 18.0 K 20.0 K 22.8 K 25.1 K 28.0 K 29.9 K 35.0 K 40.0 K 50.0 K
Deposition and Warm-Up Of CH4, CO (1:1) Ice Mixture: 12CO Feature
-200 -150 -100 -50 0 50 100 150 200
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Nor
mal
ise
d In
tens
ity o
f Gas
Pha
se C
O
Kinetic Temperature / K
adsorption
stickingdesorption
PHYSICALBEHAVIOUR
M. P. Collings, H. J. Fraser, J. W. Dever and M. R. S. McCoustraAp. J., 538, no.2, 2003
n-porous IASW
CO10 L
70 K
-porous IASW
CO10 L
45 K
-porous IASW
CO10 L
8 K
-porous IASW
CO10 L
30 K
-porous IASW
CO10 L
8 K
Dever, Collings, Fraser & McCoustra, A&SS, (2003) in press
2180 2160 2140 2120 2180 2160 2140 21202180 2160 2140 2120
absorbance= 0.005
absorbance= 0.005
(b)
4.7174.6734.6304.587Wavelength / m
50 K35 K
25 K
20 K
15 K
8 K
Frequency / cm-1
absorbance= 0.02
(c)
4.7174.6734.630
130 K
70 K
50 K
30 K
20 K
15 K
8 K
Wavelength / m4.587
Frequency / cm-1
Wavelength / m4.7174.6734.6304.587
135 K
80 K
45 K
20 K
15 K
8 K
(a)
Frequency / cm-1
M. P. Collings, H. J. Fraser, J. W. Dever and M. R. S. McCoustra, Ap. J., 538, no.2, 2003,
-porous IASW
CO
n-porous IASW
CO
CO & IASW
< 10 K
Tem
pera
ture
10 - 20 K
30 - 70 K
135 - 140 K
160 K
M. P. Collings, H. J. Fraser, J. W. Dever and M. R. S. McCoustraAp. J., 538, no.2, (2003)
CO on H2O ice
++ == TRAPPING
To simplify the system:
H2ONH3
CH3OH
HCOOHCH4
CO2
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
0 20 40 60 80 100 120 140 160 180 200
0.0
0.2
0.4
0.6
0.8
1.0
1.2
No
rma
lise
d In
teg
rate
d In
ten
sit
y
of
CO
str
etc
hin
g b
an
d
Temperature / K
CO alone CO above HCOOH CO mixed with HCOOH CO below HCOOH
&
CO desorbing from COCO desorbing from HCOOH surface
Nodesorption
CO desorbing during to phase change
in HCOOH
No CO
HCOOHdesorbs
Bisschop, Fraser, van Dishoeck, A&A, (2003) in prep
20 40 60 80 100 120 140 160 180 200
0.0
0.2
0.4
0.6
0.8
1.0
No
rma
lise
d in
ten
sity
of c
om
ple
te C
O
stre
tch
ing
vib
ratio
n b
an
d in
21
39
cm
-1 r
eg
ion
Temperature / K
CO alone 5% CO in H
2O
5% CO in HCOOH 5% CO in CH
3OH
5% CO in CH4
Bisschop, Alsindi, Fraser, A&A, (2003) in prep
H2O
CH3OH
HCOOH
H-bonding capabilities- trapping
No permanent dipole- no trapping
?
Permanent dipole-both?
Subset of moleculesbehaving the same way
ABLE TO MAKE EDUCATEDPREDICTIONS ON BEHAVIOUR
& DATA VALUESOF OTHER SIMILAR MOLECULES
CH4
Astronomical ImplicationsAstronomical Implications
•We are able to empirically measure
• sticking probabilities
• binding energies Ea CO-CO < Ea 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
Migration and CO in water
16% CO in water
Evolutionary tracks for the CO components
COCO22-ice = ubiquitous-ice = ubiquitous
P. A. GERAKINES ApJ, 522, 357-377, 1999
O C OO C O
O CO C OO
OO
O CO CHH OO
O CO C HH
(1)
(2)
(3)
BARRIER’S TO REACTIO
N
BARRIER’S TO REACTIO
N
= 13C16O= Ar = 18O2
X : 1 : 1
10 K10 KWARM25 K
No thermal reactionsNo thermal reactionsba
ckgr
ound
depo
sitio
n1
min
UV
3 m
in U
V5
min
UV
10 m
in U
V12
K15
K18
K20
K22
K25
K28
K30
K35
K40
K45
K
-0.01
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0.11
0.12
0.13
Bending Modes
635 cm-1
O3
984 cm-1
CO Region
2041 cm-1
2092 cm-1
CO2 Region
2245 cm-1
2263 cm-1
2281 cm-1
Inte
gra
ted
Inte
nsi
ty (
no
t co
rre
cte
d f
or
colu
mn
de
nsi
ty)
back
grou
nd
5 m
in d
epos
ition
10 m
in d
epos
ition
12 K
15 K
18 K
20 K
22 K
25 K
28 K
30 K
35 K
40 K
45 K
0.00
0.02
0.04
0.06
0.08
Inte
grat
ed In
tens
ity (
CO
n 3 ba
nd)
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
2380 2360 2340 2320 2300 2280 2260 2240 2220
2242 cm-1
13C18O18O
2262 cm-1
13C18O18O
2280 cm-1
13C16O16O
experimental data Lorentzian fit
2140 2120 2100 2080 2060 2040
2100 cm-1
13C16O-H2O
2092 cm-1
13C16O
Wavenumber/ cm-1
Fraser, Tielens, van Dishoeck, Ap.J, (2003) submitted
2140 2120 2100 2080 2060 2040
experimental data Lorentzian Fit
2092 cm-1
CO
2102 cm-1
CO-H2O
Wavenumber / cm-1
2380 2360 2340 2320 2300 2280 2260 2240 2220
2280 cm-1
13C16O16O2275 cm-1
13C16O16O
2338 cm-1
12C16O18O or 12C16O16O
2260 cm-1
13C16O18O
Wavenumber / cm-1
//
1000:1:1Ar:CO:O2
//
13C16O18O
10:1:1Ar:CO:O2
hn+ +
hn+ +
hn+ + hn+ +
hn+ +
hn+ +
hn+ +
hn+ +
2380 2360 2340 2320 2300 2280 2260 2240 2220
2309 cm-1
2329 cm-1
2341 cm-1
CO2 Region
2241 cm-1
2280 cm-12262 cm-1
Wavenumber / cm-1
background deposition 1 min UV 3 min UV 5 min UV 10 min UV 12 K 15 K 18 K 20 K 22 K 25 K 28 K 30 K 35 K 40 K 45 K
2140 2120 2100 2080 2060 2040 2020
CO Region
2040 cm-1
2100 cm-1
2092 cm-1
Wavenumber / cm-1
background deposition 1 min UV 3 min UV 5 min UV 10 min UV 12 K 15 K 18 K 20 K 22 K 25 K 28 K 30 K 35 K 40 K 45 K
100:1:1 Ar:CO:O2
Fraser, Tielens, van Dishoeck, Ap.J, (2003), in prep
0 2 4 6 8 10 12
0.0
0.2
0.4
0.6
0.8
1.0
No
rma
lise
d In
teg
rate
d In
ten
sity
Irradiation Time / secFraser, Tielens, van Dishoeck, NASA LAW Proceedings, (2002)
HV experiments on CO + O show:
• CO2 isotopic yield is highly dependent on the reagent
concentrations in the initial ice mixture, and H2O contamination
from the vacuum. If H2O is present the OH pathway dominates
CO2 production
• Significant energy barrier to the reaction which lies beyond
the sublimation energy of CO
• In the solid state CO2 is more readily produced from the
reaction between CO + OH than CO + OQUALITATIVE NOT QUANTITATIVE METHOD
TPD resultsTPD results
•In absence of H2O no detectable levels of CO2 produced(Therefore conclude Eley Rideal reaction is not efficient in this case)•With water ice cap present CO2 yield is roughly proportional to the O-dose (rate limiting factor in experiment)•Estimate Ea = 35 kJ mol-1
Joe E. Roser et. al. Astrophysical Journal, 555:L61–L64, 2001
Astronomical ImplicationsAstronomical 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 CO2 in
H2O rich ices?
• water is a key catalyst or ‘support’ media for the reaction
• CO2 is predominantly produced from CO + H2O reactions
• should we also consider OH provision from the gas phase
•If CO2 can also be produced in non UV photon mediated processes,
then are they efficient enough to reproduce the CO2 observed?
Summary….or answers to Summary….or answers to some perennial questionssome perennial questions
Why don’t we see the 2152 cm-1 band?
No 2152 cm-1 band!
Fraser et al. MNRAS, 2003, in prep
CO on IASW @ 8 K CO on IASW @ 80 K CO on Ic
CO / ice mixture
Summary….or answers to Summary….or answers to some perennial questionssome 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 CO2, CH3OH etc?
•e.g. through accretion of other species onto to H2O surface BEFORE CO itself adsorbs / DURING H2O formation?