Laboratoire d’Études du Rayonnement et de la Matière en Astrophysique et Atmosphères
Hydrogenation of CO-bearing species on cold graphite surfaces
Marco. Minissale2, Henda. Chaabouni1, Audrey. Moudens1,3 , Emanuele. Congiu1, Saoud. Baouche1 and François. Dulieu1
1 LERMA,
2
Université de Cergy Pontoise, 5 mail Gay Lussac 95000 Cergy Pontois. Observatoire de Paris, UMR 8112 CNRS
Aix Marseille Université, CNRS, PIIM UMR 7345, 13397 Marseille, France ; Aix-Marseille Université, CNRS, Centrale
Marseille, Institut Fresnel UMR 7249, 13013 Marseille.
3 Université de Bordeaux
EXPERIMENTS
INTRODUCTION
Surface hydrogenation reactions play an important role in the evolution of molecules
on interstellar ices, especially at low temperatures in dense molecular clouds where
the secondary photon field is very weak and where hydrogen atoms have an
important residence time on the surface.
The amount of methanol in the gas phase and the CO depletion from the gas phase
are still open problems in astrophysics. We investigate solid state hydrogenation
of CO-bearing species via H-exposure of carbon monoxide CO, formaldehyde H2CO,
and methanol CH3OH thin films deposited on cold graphite surfaces.
The products are probed via infrared spectroscopy (RAIRS), and two types of mass
spectroscopy techniques: temperature-programmed desorption (TPD),
and during-exposure desorption (DED)[1].
CO-bearing species present a see-saw mechanism between CO and H2CO balanced
by the competition of H-addition and H2 abstraction that enhances the CO chemical
desorption. The methanol formation on the surface of interstellar dust grain is still
possible through CO+H reaction; nevertheless its consumption by adsorbed H atoms
should be higher than previously expected.
Solid state CO hydrogenation has been the subject of study by many theoretical [2],
and astrphysical experimental groups [3,4].
The experiments were performed with the FORMOLISM (FORmation of MOLecules
in the ISM) set-up located in LERMA laboratory at the University of Cergy.
The DED (during exposure desorption)
technique consists in monitoring with
the QMS, the species released in the gas
phase during the deposition phase of
the reactive species (CO+H) or (H 2CO+H)
and (CH3OH+H).
 Low flux of H : 10x 1012 atoms.cm-2.s-1
 Substrate: oxidized high-oriented pyrolytic graphite (HOPG).
 Deposition temperature of the surface is 10 K.
Depolymerization process of paraformaldehyde
and preparation of the formaldehyde beam.
RESULTS
2.5 ML of CO at 10 K
2.5 ML (CO) + 6 ML (H)
1.8 ML (H2CO) at 10 K
1.8 ML (H2CO)+3.6 ML (H), 10K
— 1.8 ML (H3COH) at 10 K
--- 1.8 ML (H3COH)+3.6 ML (H), 10K
- After H atoms addition, there is no detection of H2CO (M30) and
CH3OH (M32) by RAIRS and TPD at 100 K, and 140 K, respectively.
- Decrease of CO by TPD and RAIRS, CO consumed by H atoms.
The Reativity of [CO+H] is not efficient.
The studies of the [H2CO +H] reactive system show a strong
competition between all surface processes:
 H-addition of H2CO and formation of saturated CH3OH.
 H-abstractions of H2CO and formation of CO.
CONCLUSIONS
We have shown that the direct hydrogenation of CO to methanol
is a too simple description of the chemical network.
Results
Reactivity of CO+H is not efficient on HOPG surface
The [CH3OH + H] seems to be a nonreactive system and chemical desorption of
methanol CH3OH is not efficient
- The hydrogenation of CO and H2CO is probably better
described by taking account the chemical equilibrium between
H addition and H2 abstraction.
- In the case of the [CO+H] reactive system, we have found that
the chemical desorption of CO is more efficient than H-addition
reactions and HCO and H2CO formation.
- In the case of the [H2CO+H] reactive system, there is competition
between H-addition, H2 abstraction and chemical desorption.
- On rigid HOPG graphite surface, with low CO coverage, and
low H flux, the chemical desorption is important.
- In experiments chemical desorption CD strongly inhibits the
hydrogenation of CO molecules.
- In space, the chemical desorption should slow the CO
depletion, and delay its hydrogenation to methanol CH3OH.
REFERENCES: [1] M. Minissale, A. Moudens, S. Baouhe, H. Chaabouni, and F. Dulieu. MNRAS, 458 (2016) 2953-2961.
[2] Rimola, A., Taquet, V., Ugliengo, P., Balucani, N., & Ceccarelli, C. (2014), Astron & Astrophys, 572, A70.
[3] Watanabe, N., Nagaoka, A., Shiraki, T., & Kouchi, A., Astrophysical Journal, (2004) 616, 638
[4] Pirim, C. & Krim, L. 2011, Chemical Physics, (2011) 380, 67