Interaction of ammonia with deuterium atoms on oxidized graphite surface
Henda Chaabouni1, Marco Minissale2, Saoud Baouche1, François Dulieu1
1
Université de Cergy Pontoise, UMR 8112, 5 Mail Gay Lussac, 95000 Cergy Pontoise Cedex France
LERMA Laboratoire d’Etudes du Rayonnement et de la Matière en Astrophysique
Observatoire de Paris, UPMC, ENS, CNRS. France
2
Université Pierre et Marie Curie - Paris 6. France
25-29 th May, Prague
Répubilique chèque
Introduction Singly and multiply deuterated ammonia species NH D, NHD , and ND have been observed in many astrophysical sources, such as cold
2
2
3
dense cores (10-20 K, 106 cm-3) and cold dense interstellar clouds (10 K, 104 cm-3) [1]. The abundances of these species in gas phase were found to be higher than the
cosmic elemental D/H ratio ( 2 10-5) [2]. In order to know if surface chemistry may contribute as well as gas phase chemistry for the deuteration of ammonia
molecules, we have investigated laboratory experiments for the reaction between solid NH3 and D atoms on cold surfaces of dust grains analogs. The experiments
were performed in the sub-monolayer regime of solid ammonia [3]. For comparison, identical control experiments were performed with CH 3OH + D.
Experiments
 Experiments : - The experiments were performed with the FORMOLISM (FORmation of MOLecules in the ISM)
set-up located in Cergy Pontoise. It is composed of an ultra high vacuum chamber (<10-10 mbar),
two atomic and molecular differentially pumped beam-lines, a quadrupole masse spectrometer
(QMS) and a Fourier Transform Infrared Spectrometer (IR-TF).
 Sample holder: - The sample is made with an oxidized graphite slab of Highly Ordered Pyrolytic Graphite (HOPG).
 Exposure:
- About 0.8 ML of solid ammonia was pre-deposited on the oxidized graphite surface held at 10 K.
The film is then exposed to D atoms for different deposition times.
Triply differentially
Pumped beamlines
- NH3 and D species were deposited successively by using the same beam line aimed at the surface.
- D atoms are produced by microwave dissociation of D 2 molecules with an efficiency of 85 %.
The same exposure experiments were realized with CH3OH and D atoms.
 TPD:
NH3
- After the deposition phase, we apply the Temperature Programmed Desorption (TPD) Technique,
which consists to heat the films of NH3+D or (CH3OH +D) from 10 K to 210 K using a linear
heating rate of 10 K/min. The desorbed species into the gas phase are detected with the QMS.
Kinetic evolutions of NH3, NH2D, NHD2, ND3 species in
Results : NH3 + D
reaction
m/z=17
m/z=18
solid phase as a function of D atoms exposure doses.
1,0
b)
a)
Counts/s
400
94 K
104 K
0.8 ML (NH3) + 0 min D c)
m/z=19
m/z=20
d)
0.8 ML (NH3) + 30 min D
NH2D
NH3
500
Surface density in (ML)
500
Microwave discharge
0.8 ML (NH3) + 70 min D
0.8 ML (NH3) + 140 min D
300
136 K
98 K
0.8 ML (NH3) + 240 min D 450
145 K
ND2H
200
100
145 K
98 K
0
60
90
120
150
180
60
210
90
120
150
180
210
60
90
150
180
210
ND3
145 K
0,6 Data
Fits
m/z=17: NH3
m/z=18: NH2D
0,4
m/z=19: NHD2
m/z=20: ND3
0,2
98 K
0,0
60
90
Temperature(K)
Temperature(K)
Temperature(K)
120
80
60
40
20
0
0,8
120
150
180
210
0
50
Temperature(K)
100
150
200
250
D-atoms irradiation times (min)
TPD experimental results showed a decrease in the desorption peak of NH3 (m/z=17)
at ~ 94 K, and an increase in the desorption peaks at ~ 98 K, attributed to ammonia
isotopologue species: NH2D (m/z=18), NHD2 (m/z=19), and ND3 (m/z=20).
These species are formed by direct H-D substitution exothermic
surface reactions between adsorbed ammonia species and D atoms.
60
CH3OH
Count/s)
80
Kinetic evolutions of CH3OH, CH2DOH, CHD2OH and CD3OH3
in solid phase as a function of D atoms exposure doses.
1,0
0.8 ML CH3OH
0.8 ML CH3OH
0.8 ML CH3OH + 30 min D
0.8 ML CH3OH + 70 min D
0.8 ML CH3OH + 30 min D
CH2DOH
0.8 ML CH3OH + 70 min D
m/z=34
CHD2OH
0.8 ML CH3OH
0.8 ML CH3OH + 30 min D
0.8 ML CH3OH + 70 min D
m/z=35
CD3OH
0.8 ML CH3OH
0.8 ML CH3OH + 30 min D
0.8 ML CH3OH + 70 min D
50
40
30
20
Surface density in (ML)
70
Results : CH3OH + D
reaction
m/z=33
m/z=32
Fits
Data
m/=32: CH3OH
m/z=33: CH2DOH
0,8
m/z=34: CHD2OH
m/z=35: CD3OH
0,6
0,4
0,2
10
0
100
0,0
120
140
160
Temperature (K)
180
200100
120
140
160
Temperature (K)
180
200100
120
140
160
180
200100
120
Temperature (K)
140
160
180
200
Temperature (K)
0
10
20
30
40
50
60
70
80
D-atoms irradiation (min)
The TPD experimental results showed a rapid decrease in the desorption peak of CH 3OH
(m/z=32) at ~ 140 K, and the increase of three desorption peaks at ~ 140 K, attributed to
methanol isotopologue species: CH2DOH (m/z=33), CHD2OH (m/z=34), and CD3OH3
(m/z=35).
These deuteration species are formed by surface process, which is ruled by quantum
tunneling H abstraction and D addition exothermic surface reactions [4].
Reaction rates and activation barriers
Conclusions
- Contribution of grain surface chemistry in the deuterium enrichment of NH3
A simple kinetic model is developed to
estimate the relative rate constants ki, ki’ and
the activation energy barriers Eai Eai’ (i=2-4)
of the three successive deuteration reactions
of solid ammonia and (methanol) species by
D atoms on cold graphite surface.
The deuteration processes are dominated by
Eley-Readel (ER) mechanism, which occur
between an adsorbed ammonia (or methanol)
species on the cold surface and an impinging
D atom.
molecules.
- The deuteration process of ammonia by D atoms addition on cold grain surfaces is
slower than that of methanol species, and occurs with higher activation energy
barriers.
References
[1] Roueff. E., Tine. S., Coudert, L.H., et al, Astronomy  Astrophys, 388, L53 (2002).
[2] J. L. Linsky, Space Sci. Rev. 106, 49 (2003).
[3] Chaabouni. H, Minissale. M, Baouche. S, Dulieu.F., submitted
[4] Nagaoka. A., Watanabe. N., and Kouchi. A, Journal of Physical Chemistry A, 111, 3016 (2007)