$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig1.eps} \end{figure}$ Figure 1: Sample result for our reference model ( $n(\rm {H}_2) = 10^6$ cm^-3, T = 10 K, $a_{\rm g}$ = 0.1 $\mu$ m, $\zeta = 3\times 10^{-17}$ s^-1) showing the approach to steady-state conditions for ortho/para H₂, D and H.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig2.eps}\end{figure}$ Figure 2: Abundances of major ions and electrons in our standard model ( $a_{\rm g}$ = 0.1 $\mu$ m, T = 10 K, $\zeta = 3\times 10^{-17}~\rm s^{-1}$ ) as functions of the density of molecular hydrogen.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig3.eps}\end{figure}$ Figure 3: Ortho/para ratios of H₂ (multiplied by 10⁴), H₃⁺, and H₂D⁺ as functions of density for our reference model (cf. Fig. 2). The approximate expression of Gerlich et al. (2002) for H₂D⁺ is shown for comparison.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig4.eps}\end{figure}$ Figure 4: Abundances of major ions and electrons in a model with a smaller grain size of 0.025 $\mu$ m.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig5.eps}\end{figure}$ Figure 5: The free electron abundance computed as a function of density for grain sizes between 0.025 and 0.2 $\mu$ m. The result of McKee (1989) is shown for comparison (broken curve). Note the decrease in the level of ionization for small grain sizes (large grain surface areas).
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig6.eps} % \end{figure}$ Figure 6: Ratio of ambipolar diffusion time $\tau _{\rm AD}$ (Eq. (5)) to free fall time $\tau _{\rm ff}$ as function of molecular hydrogen density for values of the grain radius between 0.025 and 0.2 $\mu$ m.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig7.eps}\end{figure}$ Figure 7: Ortho/para ratios of H₃⁺ and H₂D⁺ as functions of the molecular hydrogen ortho/para ratio. The conditions are those of our reference model: n(H₂) = 10⁶ cm^-3, T = 10 K, $a_{\rm g}$ = 0.1 $\mu$ m, $\zeta = 3\times 10^{-17}$ s^-1.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig8.eps}\end{figure}$ Figure 8: The computed ortho-H₂D⁺ abundance from our reference model (T = 10 K and $\zeta = 3\times 10^{-17}$ s^-1) as a function of density for several values of the assumed grain radius. The values observed by Caselli et al. (2003) are shown for comparison.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig9.eps}\end{figure}$ Figure 9: Atomic H (full lines) and atomic D (broken lines) densities for four values of the grain radius (increasing from bottom to top) for densities between 10⁵ and 10⁷ cm^-3.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig1.eps} \end{figure}$	Figure 1: Sample result for our reference model ( $n(\rm {H}_2) = 10^6$ cm^-3, T = 10 K, $a_{\rm g}$ = 0.1 $\mu$ m, $\zeta = 3\times 10^{-17}$ s^-1) showing the approach to steady-state conditions for ortho/para H₂, D and H.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig2.eps}\end{figure}$	Figure 2: Abundances of major ions and electrons in our standard model ( $a_{\rm g}$ = 0.1 $\mu$ m, T = 10 K, $\zeta = 3\times 10^{-17}~\rm s^{-1}$ ) as functions of the density of molecular hydrogen.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig3.eps}\end{figure}$	Figure 3: Ortho/para ratios of H₂ (multiplied by 10⁴), H₃⁺, and H₂D⁺ as functions of density for our reference model (cf. Fig. 2). The approximate expression of Gerlich et al. (2002) for H₂D⁺ is shown for comparison.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig4.eps}\end{figure}$	Figure 4: Abundances of major ions and electrons in a model with a smaller grain size of 0.025 $\mu$ m.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig5.eps}\end{figure}$	Figure 5: The free electron abundance computed as a function of density for grain sizes between 0.025 and 0.2 $\mu$ m. The result of McKee (1989) is shown for comparison (broken curve). Note the decrease in the level of ionization for small grain sizes (large grain surface areas).
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$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig6.eps} % \end{figure}$	Figure 6: Ratio of ambipolar diffusion time $\tau _{\rm AD}$ (Eq. (5)) to free fall time $\tau _{\rm ff}$ as function of molecular hydrogen density for values of the grain radius between 0.025 and 0.2 $\mu$ m.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig7.eps}\end{figure}$	Figure 7: Ortho/para ratios of H₃⁺ and H₂D⁺ as functions of the molecular hydrogen ortho/para ratio. The conditions are those of our reference model: n(H₂) = 10⁶ cm^-3, T = 10 K, $a_{\rm g}$ = 0.1 $\mu$ m, $\zeta = 3\times 10^{-17}$ s^-1.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig8.eps}\end{figure}$	Figure 8: The computed ortho-H₂D⁺ abundance from our reference model (T = 10 K and $\zeta = 3\times 10^{-17}$ s^-1) as a function of density for several values of the assumed grain radius. The values observed by Caselli et al. (2003) are shown for comparison.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{0718fig9.eps}\end{figure}$	Figure 9: Atomic H (full lines) and atomic D (broken lines) densities for four values of the grain radius (increasing from bottom to top) for densities between 10⁵ and 10⁷ cm^-3.
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