““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?