Grain-surface chemistry

The most abundant molecule in the ISM isH2, which can be formed in the gas phase via the radiative association reaction of two H atoms: H + H→H₂+ hν. However, this process is extremely slow (≤ 10⁻²³cm³s⁻¹), as the radiative decay in a homonuclear molecule like H2 is highly forbidden. This is a clear indication that the only way for H2 to form efficiently in the ISM, is on the surface of dust grains: the excessive energy that is released during the H₂ formation is absorbed by dust grains that act as a third body. This makes the presence of dust grains vital to the interstellar chemistry. Once H2 is formed, a large number of reactions is triggered through the formation of H⁺₃, as mentioned already in Section 1.3.1. But how are dust particles formed in the first place? When an old, dying star reaches the end of its hydrogen fusion stage, it begins to eject its outer layers, that consist of heavy particles such as SiO, SiC and TiO. These molecules eventually condense in the cool atmosphere of the evolved star and become the seeds of dust grains. As time evolves, the grains become larger and convert into amorphous structures made of sillicate and/or carbonaceous material. Their typical size is ∼ 0.1µm, although they can grow

up to a few cm towards dense cores and circumstellar disks. At the low temperatures of dense cores, gas-phase molecules freeze-out on the grain-surfaces forming layers of ice that contain predominantly water, CO, CO₂, CH₃OH and H₂CO. Fig. 1.3 shows an infrared spectrum taken towards the W33A massive young star by Gibb et al. [2000]. Here, several absorption features are visible that originate either from the silicate core of the grain (broad absorption lines), or the icy species on the grain-surface (like H₂O, CO and etc.).

Figure 1.3: Infrared spectrum of the dust-embedded W33A young stellar object [Gibb et al., 2000].

Dust-grain chemistry mainly involves the deposition of species on grain-surfaces, known as accretion, the surface mobility of the accreted species and finally the ejection of the grains-species back to the gas-phase, called desorption (see Fig. 1.4). In the following paragraphs we will discuss these mechanisms in more detail.

When a gas-phase species is coming close to a grain-surface it experiences attractive Van-der-Waals forces, that originate from the mutually induced dipole moment between the approaching molecule and the atoms of the grain-surface. The probability of a species to accrete onto the surface (also known as sticking coefficient) depends on the thermal energy of the accreting molecule, the phonon energy of the grain-lattice as well as the interaction energy between molecule and grain. At typical temperatures of pre-stellar cores (10− 20 K) the sticking probability of most species has been estimated to be 1, except for atomic hydrogen, whose sticking coefficient amounts to 0.8 at 10 K. The location of the accreted species on a dust grain is called adsorption or surface site. Depending on the binding/desorption energy E_D, i.e. depth of the potential well, surface sites are distinguished between the so-called physisorbed sites with well depths of 0.01-0.2 eV and the chemisorbed sites with an energy depth of∼1 eV. Chemisorbed sites have strongly bound molecules that share electrons with the atoms of the grain-lattice. Based on experimental

studies done on olivine dust grains ⁸, the surface density has been estimated to be 2× 10¹⁴sites cm⁻², which results to ≈3×10⁵ sites per grain [Biham et al., 2001].

Figure 1.4: Depiction of the main chemical processes (accretion, desorption, surface mi-gration) taking place on grain-surfaces.

As depicted in Fig. 1.4, reactions on grains can happen in two ways: (1) adsorbed species move through the surface, until they meet and eventually react with each other, known as the Langmuir-Hinshelwood mechanism or (2) gas-phase species impact the dust grain and react directly with already adsorbed molecules without being accreted on the surface first; this is the so-called Eley-Rideal mechanism. For the first mechanism to occur at low temperatures, at least one of the reactants has to be a physisorbed species, or in other words, weakly bound to the surface. A chemisorbed species on the other hand, is not capable of diffusive motion through the surface and can therefore react with another species only through the Eley-Rideal mechanism.

The Langmuir-Hinshelwood process is considered to be the most probable mechanism for surface chemistry. Especially towards diffuse clouds, where the dust grains are not covered by icy mantels, the probability of a gas-phase H-atom to find and react with another H-atom already accreted on a grain-surface is very small. According to the formalism proposed by Hasegawa et al. [1992], diffusive surface chemistry can take place either via quantum tunneling of light species through potential barriers or via thermal hopping. If we assume a rectangular potential barrier of height E_b and width a, light species (H, D and H2) can tunnel through that energy barrier in a time τqt:

8olivine is a polycrystalline silicate thats consists ofMg₂SiO₄ andFe₂SiO₄.

τ_qt=ν₀⁻¹exp 2a

~

(2mE_b)^1/2

, (1.10)

where m is the mass of the accreted species ν0 is the vibrational frequency of the species within the grain lattice. Typical timescales for atomic hydrogen scanning with a barrier width of a= 1 Å, Eq. 1.10 areτ_qt∼10⁻⁴s. If the temperatures are high enough, accreted species can overcome the energy barrier by “hopping” from one surface site to another. The thermal hopping timescale is expressed as:

τ_th =ν₀⁻¹exp(E_b/k_BT_dust). (1.11) At a dust temperature of 10 K the thermal hopping timescale for H can be as short as τ_th ∼ 10⁻³s, suggesting that quantum tunneling happens faster at low temperatures.

The decisive factor for the surface mobility is determined by the energy ratio E_b/E_D. A large potential height E_b will decrease the probability of quantum tunneling and thus only thermal hopping will play an essential role. Based on previous work [Hasegawa et al., 1992, Ruffle and Herbst, 2000, Vasyunin and Herbst, 2013a] the most common values adopted for the E_b/E_D ratio are: 0.30, 0.50 and 0.77. For the latter two cases the quantum tunneling is expected to be negligible due to the long resulting timescales (see Eq. 1.10).

One way for grain-species to be ejected back to the gas phase is via thermal desorption.

This process happens when the dust temperature reaches the sublimation temperature of the accreted species, which mostly depends on its desorption energy E_D. The thermal desorption rate is described as

kev =ν0exp(−ED/kBTdust). (1.12) Due to the low existing temperatures towards dense molecular clouds, the observed abun-dances of gas-phase molecules can only be explained if non-thermal desorption processes are taken into account. One of these happens upon irradiation of the grains by interstellar UV photons, also known as photodesorption. Recent experimental studies [Fayolle et al., 2013, Dupuy et al., 2017] have shown that photodesorption occurs through the electronic excitation of the accreted molecules, which can lead to two possible outcomes: (1) the excited species release the UV radiation when returning to the ground state, which then causes other molecules of the grain-lattice to rearrange and subsequently desorb. In this case the photodesorption yield (defined as number of desorbed molecules per UV pho-ton) strongly correlates with the efficiency of the energy redistribution between the excited species and the grain surface [Fayolle et al., 2013]; (2) the excited molecules dissociate upon UV irradiation, which introduces new ways of ejecting grain species to the gas phase. For example, in case of the O₂ photodissociation, the resulting oxygen atoms can meet on the grain and react with each other to form molecular oxygen, which releases its excessive en-ergy via desorption (exothermic recombination). Another option is that molecular oxygen

is ejected to the gas phase following its collision with a mobile oxygen atom (kick-out by atomic oxygen). Finally, an oxygen atom can react with accreted O₂ and form O₃, which can lead to the formation of excitedO₂ upon photolysis (photoinduced dissociation ofO₃).

Typical values for photodesorption yields range from 10⁻² to 10⁻⁵ molecules/UV photon.

Good estimates have been done for H₂O, O₂, CH₄ and CO ices with a photodesorption efficiency of ∼10⁻³ molecules/UV photon.

Non-thermal desorption can also be induced by the irradiation of cosmic rays. In this case, cosmic rays will transfer energy to a dust grain, which will in turn heat up, leading to the desorption of volatile accreted species. A study by Hasegawa and Herbst [1993]

has shown that the impact by a heavy cosmic ray (like Fe) of 20-70 MeV per nucleon will increase the dust temperature up to ∼ 70 K, causing some molecules to desorb, but not complex ones likeCH₃OH that are more strongly bound to the surface. Nevertheless, this kind of non-thermal desorption has proven to be very slow and is relevant only on large timescales (t≥10⁶yr) for species like CO [Hasegawa and Herbst, 1993].

Finally, another non-thermal desorption process essential for interstellar chemistry is the reactive desorption, in which the excited product of an exothermic reaction is desorbed from the surface due to its excessive energy. The reactive desorption rate is only poorly constrained, as it has been experimentally determined just forH₂ so far [Katz et al., 1999].

Recent theoretical studies [Garrod et al., 2007, Vasyunin and Herbst, 2013b] have adopted for the reactive desorption efficiency values of 1% and 10%, that were able to reproduce the observed abundances of several complex organic molecules (like CH₃OH, CH₃OCH₃) towards cold cores.