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Table A.1

Main grain reactions involved in the formation and destruction of H2.

Reaction Comment
1 H → H# Physisorption, barrierless
                 
2 H + * → *H# = = 900 K
                 
3 H# + * → *H# Epc =
4 H# → H = 600 K
5 H# + hν → H Photodesorption
6 H# + CR → H
7 *H# → H + * = 10 000 K
8 *H# + hν → H + *
9 *H# + CR → H + * Via CR induced UV
10 H + H# → H2 Eley-Rideal (ER) mechanism, barrierless
11 H# + H# → H2 = 0 K (Navarro-Ruiz et al. 2014)
12 H + *H# → H2 + * ER mechanism, barrierless
13 H# + *H# → H2 + * =
14 *H# + *H# → H2 + 2* =
15 H2 + * → *H# + H# = 3481 K (Diño et al. 2004)
16 PAH → PAH# Physisorption
17 HPAH → HPAH# –
18 PAH# → PAH Desorption
19 HPAH# → HPAH –
20 PAH# + hν → PAH Photodesorption
21 HPAH# + hν → HPAH –
22 PAH# + CR → PAH Cosmic ray induced photodesorption
23 HPAH# + CR → HPAH –
24 H + PAH → HPAH = 692 K (Rauls & Hornekær 2008)
25 HPAH → H + PAH Thermal H-detachement with = 16 250 K
26 HPAH + hν → H + PAH Photodetachment, E(C–H) = 4.45 eV (51640 K)
27 HPAH + H → H2 + PAH Cross-section σ = 0.06 Å2/C atom, Eact = 0 K (Mennella et al. 2012)
28 H + PAH+ → HPAH+ = 116 K
29 HPAH+ + e → PAH + H Dissociative recombination
30 HPAH+ + H → PAH+ + H2 Langevin rate (Montillaud et al. 2013)
31 H2 + PAH → HPAH + H = 3481 K
32 PAH + hν → PAH+ + e Photoionisation
33 PAH + e → PAH− Electron attachment
34 PAH− + hν → PAH + e Photodetachement
35 PAH+ + e → PAH Electron recombination
36 PAH+ + X → PAH + X+ Charge exchange with species X
37 PAH + X+ → PAH+ + X –
38 H + e− → H− hν Radiative attachment
39 H + H− → H2 + e− Associative detachment
40 H + H + H → H2 + H Three-body reactions
41 H + H + H2 → H2 + H2 –
42 H2 + hν → H + H Photodissociation including self-shielding
43 H2 + CR → H + H By secondary electrons and CR-generated UV photons

Notes. The energies are expressed in units of Kelvin. is the energy barrier for chemisorption of a gas-phase hydrogen atom on the grain surface. Epc is the energy barrier for physisorbed H-atom to chemisorb on the grain surface. is the adsorption energy of a physisorbed H-atom. We assumed no activation barrier for H2 recombination between two physisorbed H-atoms (= 0 K). and are activation energy for H2 recombination involving one or two chemisorbed H-atom(s). is the H2 dissociative adsorption. * and ! are pseudo-species as defined in Table 1.

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