Experimental study of H2 formation on grainsoverview and recent results

F. Dulieu

Experimental study of H2 formation on grainsoverview and recent results

J. H. Fillion !

H2 formation on grains ?

T ?Composition ?Morphology ?

H

H

H

Accretion (flux, sticking…)H

Mobility

HDesorption

Formation Desorption

Rrecombibation efficiency

H2 formation in ISMDiffuse clouds Dense clouds

H2 formation in ISMDiffuse clouds Dense clouds

• High destruction rate (UV…) • Low destruction rate (cosmic…)

H2 formation in ISMDiffuse clouds Dense clouds

• High destruction rate (UV…) • Low destruction rate (cosmic…)R ~ 0.3 !

H2 formation in ISMDiffuse clouds Dense clouds

• High destruction rate (UV…)

• Bare grains

• T >20 K 1 Chemisorbed

• Low destruction rate (cosmic…)

• Ice mantles (H2O, CO…)

• 10 K 2 Physisorbed atoms

• Presence of H2 adsorbed

New experiments on H2 formation

Diffuse clouds Dense clouds

• UCL:Graphite, excited moleculesCreighan et al (2006) • U. Aarhus :Graphite, chemisorbed sitesHornekaer, poster 14

• UCP Water ice surfacesAmiaud et al (2006)

previous work on water ice surfacesCalculations- Hollenbach and Salpeter (1971)- Buch et al (1991)- Takahashi et al (2000)- Al Halabi et al (2002)

Experiments- Govers et al (1980)- Vidali, Pirronello, Biham et al (5 refs)- Hornekaer et al (2003)

18 20 22 24 26 28 30 32 34

Manico et.al. APJ 548 : L253 2001

Roser et.al. APJ 581: 276 2002

HD

Des

orpt

ion

rate

(arb

ritra

ry U

nits

)

Temperature (K)

Hornekaer et.al. Science 302, 1943 2003

UCP experimental strategy

• Compare D2 vs D+D, during and after irradiation (thermal ramp = TPD)

• Use ice as template of physisorption study(Porosity and adsorption sites can be changed)

• Caracterize as much as possible differentexperimental parameters

• And not H+D studies (Vidali, Pirronello, Biham et al, Hornekaer et al)

UCPJ. L. LemaireJ. H. FillionA. Momeni

S. BaoucheL. AmiaudL. Kristensen

E. SomsonJ.J. BousquetS. Diana

FORMOLISMJ. L. LemaireJ. H. FillionA. Momeni

S. BaoucheL. Amiaud

E. SomsonJ.J. Bousquet

D + D or D2 Water ice sample (8 - 30 K)

QMS detection

Ice samplesGrowth at 10 K Growth at 120 K

© Guillot & Guissani 2004

High effective surface areaProportionnal to thickness

X ML

© Kimmel et al 2001

Surface area ~ geometric area

IIIIVD + D

IIID2

Non Porous IcePorous Ice

During and after irradiation !

D2 desorption from porous ASW

10 11 12 13 140,5

1,0

1,5

2,0

10 15 20 25 30 35

f'

D2 d

esor

ptio

n R

ate

(arb

. uni

ts)

Temperature (K)

Figure 4

c

j

i

h

g

d

ef

χ2 (arb

.uni

ts)

Log(Α)

- Profiles depend on coverage

- Filling behaviour (Kimmelet al 2001)

10 ML

New model energy distribution

• Assume a distribution of adsorptoin sites g(E)

• Assume Thermal equilibrium (Fermi Dirac statistic)

• 4 fitting parameters

New model energy distribution

• Assume a distribution of adsorptoin sites g(E)

• Assume Thermal equilibrium (Fermi Dirac statistics)

• 4 fitting parameters

Residence time on a sitevs adsorption energy (Ea)

> 101315674.80.01tres (day)

60403530Ea (meV)

T =10 K

aEkT

des op eυ−

=1 1 aE

kTres

des o

t ep υ

= =

Residence time on a sitevs adsorption energy (Ea)

> 101315674.80.01tres (day)

60403530Ea (meV)

T =10 K

aEkT

desp Ae−

= 1 1 aEkT

resdes

t ep A

= =

There is a non negligible amount of H2adsorbed on ice mantles

Isotopic segregation effect

10 15 20 250

5

1010 15 20 250

5

10

15

20

25

30

35

40

45

50

0.14 ML of H2 in presence of various D2 doses

Various D2 doses in presence of 0.14 ML of H2

0 ML 0.1 ML 0.5 ML 2.0 ML 4.0 ML

H2

Temperature (K)

D2

10 ML

Isotopic segregation effect10 ML

- Enrichment of deuterated moleculeson ice mantles (see poster 18)- TPD profiles of H2 HD and D (seebelow) are very sensitive to thepresence D2 (even 10%)

Summary I D2 interaction with porous ASW

• Interaction is coverage dependant• There is a very large energy distribution (in

aggrement with calculations by Hixson et al1992)

• There is a strong isotopic effect• We have modeled it tool for ice

caracterization• Our model permit us to study the effect of ice

thickness, non uniform ice layer and non uniformdeposition of D2 that damatically affect theshapes of the TPD curves

10 12 14 16 18 20 220

2000

4000

6000

8000

A

B

0,0 0,1 0,2 0,3 0,4 0,5 0,60

10000

20000

30000

40000

50000

Inte

grat

ed A

rea

(cps

K)

Exposure Dose (ML)

D2 S

igna

l (c

ps)

Surface temperature (K)

A

B

D2 desorption from Non porousASW

Similar behaviour than porous ASW, not the same energy distribution

10 12 14 16 18 20 220

2000

4000

6000

8000

A

B

0,0 0,1 0,2 0,3 0,4 0,5 0,60

10000

20000

30000

40000

50000

Inte

grat

ed A

rea

(cps

K)

Exposure Dose (ML)

D2 S

igna

l (c

ps)

Surface temperature (K)

A

B

D2 desorption from Non porousASW

Similar behaviour than porous ASW, not the same energy distribution

D2 during irradiation

0 20 40 60 80 1001500

2000

2500

3000

D2 S

igna

l (cp

s)

Exposure Time (s)

D2 during irradiation

0 20 40 60 80 1001500

2000

2500

3000

D2 S

igna

l (cp

s)

Exposure Time (s)0,0 0,1 0,2 0,3 0,4

0,0

0,1

0,2

0,3

0,4

0,5

0,6

This study Govers et al

Hornekaer et al

Stic

king

coe

ffici

ent

Exposition Dose (ML)

D2 accomodates more efficiently when some D2 is already adsorbed

Summary II:D2 interaction with non porous ice

• Follow the « filling behaviour »• Distribution of adsorption energy is

significantly shifted to lower bindingenergies

• Sticking coefficient is dependant on thepresence of already adsorbed molecules

D + D during irradiatiton

0 100 200 300 400 500 6000

400

800

1200

D+D irradiation Non dissociated part

D2 S

igna

l (cp

s)

Exposure time (s) 0,0 0,2 0,4 0,6 0,80,00

0,02

0,04

0,06

0,08

0,10

0,12

Frac

tion

of a

tom

s re

actin

g

D2 Exposed (ML)

Ts = 10 K

- Prompt reaction occurs- Recombination efficiency depends on stiking that isenhanced by adsorbed molecules- Ea(D) ~ 0.6 Ea(D2) (as expected from polarisability)

Excited moleculesD + D

13 14 15 16 170

1

012345

ACDEF

D2+ S

igna

l (A

. U.)

Electron energy (eV)

B

v = 6

Up to v = 4 is detected

D+D at 8 KBeam QMS head

Summary III D2 formation on Non porous ice

• Prompt reaction that leads to excitedmolecules at 10 K

• Ea(D2) > Ea(D) > Ediff(D) in agreementwith calculations

• Molecules already adsorbed enhance therecombination efficiency (see also Goverset al)

Formation on porous water ice surface

10 15 20 25 30 350

200

400

600

800

1000

Sig

nal D

2 (cp

s)

Surface temperature (K)

Desorption after 100 s irradiation of D+D Desorption after 51 s irradiation of D2

<150 ML

TPD profiles provide no information about formationNo track of formation beetween 10 K and 20 K

(in case of porous surfaces !)

T= 10 KR ~ 0.35

Effect of porous structure

ASW Sample Gas phase

Sout

Sin

0

aEout kT

desout in

SP eS S

υ−

=+

Has been verified for molecules(see also Hornekaer et al 2005)

Ea(D2) > Ea(D) >Ediff(D)

Molecules are mobile enough to enter the porous network

They react and are recaptured by the porous structure

It happens during the irradiation

R vs T during D+D irradiation

10 12 14 16 18 20 220,0

0,2

0,4

0,6

0,8

1,0

1,2

Ref

f

Temperature de la surface lors du dépôt

Expérience Ea= 44 meV; Emob=22 meV Ea= 39 meV; Emob=22 meV Ea= 49 meV; Emob=22 meV

Model: Katz et al, µ =100%, A = 1013 /sEmob = 22 meV; 3 ms between 2 hops at 10 K.

but solution is not unique

<150 ML

Outline of D+D experimentson porous amorphous ice

• Kinetics of formation is blurred by porous structure of the ice, TPDs reveal only molecular desorption

• Ea(D2) > Ea(D) >Ediff(D)• at 10 K diffusion is fast enough to allow atoms to enter

the porous network and to react but the molecules formed are recaptured by the porous network

• The recombination efficiency is 0.35 at 10 K and is mostly governed by the sticking probability

Question to elucidate:• What about the effect of adsorbed molecules ?• Diffusion energy, Adsorption energy should have a

distribution (see Buch et al 1991) need another approach

Astrophysical relevance ?

© Perrets et al 2005

?

H chemistry is effective at 10 KPresence of H2 (and isotopes) could play a role

Future work

• IR characterization of H2 / H interaction with ASW

• Energetic balance (REMPI + imaging)• Interaction with other samples

(carbonaceous)• Formation of water

Thank you !