2 The models

The gas-grain models are those described in Ruffle & Herbst (2000, 2001a; hereafter RHI and RHII), in which gas-phase and grain surface chemistries are linked through accretion and desorption processes. These models were developed in response to new experimental data on the surface mobility of hydrogen atoms on olivine and amorphous carbon (Katz et al. 1999) and so incorporate slower rates of diffusion of species across grain surfaces than was previously assumed. The kinetic equations used for surface diffusion are taken from the modified rate equation approach of Caselli et al. (1998) and Shalabiea et al. (1998), although the need for modifications is greatly reduced owing to the low diffusion rates employed (see RHI). We are using the versions of the models which incorporate surface photochemistry (RHII).

There are two sets of gas-grain models. In the first case, atomic H is the only species with reduced mobility relative to that used in previous models, and the diffusion of H is not fully slowed to the extent measured by Katz et al. (1999), since that would mean it moves more slowly than other, heavier, species. In the second case, therefore, where H is slowed to the measured rate, the diffusion rates of all other species are slowed proportionally. In the discussion which follows, models where only H atoms are slowed are referred to as P1 (for an olivine surface) and P1/ac (for amorphous carbon). Models P2 and P2/ac represent models on olivine and amorphous carbon surfaces, respectively, where all diffusion rates have been slowed.

Although it is becoming increasingly clear that simple gas-phase chemistries are unrealistic even in quiescent clouds, they have been used in the past. Therefore, for completeness, we also present some results from a pure gas-phase chemistry, where the only surface reaction included is for the formation of H₂ (Bettens et al. 1995; Lee et al. 1996a).

Throughout, the grains are assumed to have radii of 0.1 $\mu$m and 10⁶ binding sites. These canonical values (Tielens & Allamandola 1987) represent the peak of the grain size distribution; in future models we hope to consider both smaller and larger grains as well. The cosmic ray ionisation rate is $1.3~\times~10^{-17}$ s^-1. Unless otherwise stated, gas-phase species other than hydrogen (H₂) are initially atomic with the "low-metal'' abundances listed in RHI and Lee et al. (1996a).