The chemical model

We take into account gas-phase reactions, gas-dust interaction, and dust surface reactions. The species set and gas-phase reaction rates are taken from the UMIST $95$ database (Millar et al. [4]). Dust surface reactions are adopted from Hasegawa et al. [8] with corrections of the activation energies for some species from Hasegawa & Herbst [9]. The gas-dust interaction includes the following physical processes: accretion of neutral species onto dust grains, their desorption due to the thermal evaporation and cosmic ray heating, and dissociative recombination of ions on grain surfaces (for more detail, see Hasegawa et al. [8]).

The probability of species to stick on the dust surfaces is assumed to be $0.3$ for all ions and all neutral species except for H, He, and H$_2$. The sticking coefficient of the atomic hydrogen is estimated from the expression given in Hollenbach & McKee [10], equation (3.7). Sticking probabilities for helium and molecular hydrogen are assumed to be zero.

Two chemical networks are investigated, namely the pure gas-phase network ``GAS'' consists of electron, $12$ atoms, $137$ molecules, and $245$ ions (in total $395$ species) involved in $3864$ gas-phase reactions and the gas-grain network ``DUST'' having additional $148$ surface species, $729$ gas-grain and $192$ dust surface reactions.

Table: Model designations

Notation	Meaning
DIFF	$n_{\rm H}=10^3$ cm$^{-3}$
DENS	$n_{\rm H}=10^7$ cm$^{-3}$
GAS	gas-phase network
	( $395\times3864^a$)
DUST	gas-grain network
	($543\times4785$)
HM	``High metals''
LM	``Low metals''

$^a$The entry ``$395\times3864$'' means that the network has $395$ species involved in $3864$ reactions.

We consider a range of physical conditions, listed in Table 1 along with their designations. Gas and dust temperatures are fixed to $10$ K. No photoprocessing by UV radiation is assumed, imply the conditions, typical of obscured regions, like the cores of molecular clouds, where the visual extinction A $_{\rm V}\geq10$. Dust grains are considered as silicate-like spheres having a uniform size $0.1{\mu}\rm {m}$ and density $3$ g cm$^{-3}$. Dust is assumed to constitute $1$% of the gas density by mass.

In our calculations we use well-known "high metal" and "low metal" elemental abundances (e.g. Lee et al. [11]) quoted in Table 2. The "high metal" means standard solar elemental composition with a modest depletion of $2$ for S and stronger depletions of $10$ for Na, $50$ for Si, $60$ for Mg, and $110$ for Fe. The "low metal" values contain additional depletion factors of $100$ for each of these elements. The abundances of all elements but P and Cl are taken from Aikawa et al. [12]. For P and Cl we take the values from Grevesse & Sauval [13] and use the same depletion factors as for Fe.

We suppose that only these atomic neutral species are present at initial time $t=0$. The only exception is hydrogen assume to be completely in molecular form.

Table: Adopted elemental abundances.

Element	``High metals''	``Low metals''
He	$1.95(-1)^a$	$1.95(-1)$
C	$1.57(-4)$	$1.57(-4)$
N	$4.94(-5)$	$4.94(-5)$
O	$3.60(-4)$	$3.60(-4)$
S	$1.83(-5)$	$1.83(-7)$
Si	$1.95(-6)$	$1.95(-8)$
Na	$4.50(-7)$	$4.50(-9)$
Mg	$2.18(-6)$	$2.18(-8)$
Fe	$5.48(-7)$	$5.48(-9)$
P	$4.32(-8)$	$4.32(-10)$
Cl	$2.00(-7)$	$2.00(-9)$

$^a$The entry ``$1.95(-1)$'' means that abundance of the element is equal to $1.95\times 10^{-1}$.