All Figures

$\begin{figure} \par\includegraphics[width=7.3cm,clip]{3306_f1.ps}\end{figure}$ Figure 1: Emission intensity spectrum, in arbitrary units, of the microwave stimulated hydrogen flow discharge lamp for a MgF₂ window ( top, solid line) and a quartz window ( bottom, solid line), corresponding to the hard and soft UV spectrum, respectively. Spectra were offset for clarity. For comparison, the radiation field of the diffuse interstellar medium ( $\times$ 5 $\times$ 10⁷) is included (Jenniskens 1993, and ref. therein).
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In the text

$\begin{figure} \par\includegraphics[width=16.8cm,clip]{3306_f2.eps}\end{figure}$ Figure 2: IR spectrum of the residue of the standard experiment, Exp. OR1. Feature identifications are given in Table 2.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{3306_f3.ps}\end{figure}$ Figure 3: IR spectrum of the residue of the standard experiment (OR1; solid line, bottom) compared to the reference spectra of possible products. The fit (dashed line) is obtained by addition of spectra 2 and 3 at 240 K, corresponding to ammonium glycolate and HMT.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{3306_f4.ps}\end{figure}$ Figure 4: Comparison of IR spectra of irradiated standard ice mixture at 12 K for the standard experiment (OR1, dose of 0.25 photon molec^-1, hard UV spectrum), an experiment with a higher UV dose (OR4, dose of 3.33 photon molec^-1, hard UV spectrum), and the only experiment performed with the soft UV spectrum (OR7, dose of 0.66 photon molec^-1). For peak assignments see Schutte et al. (1999), Grim et al. (1989).
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{3306_f5.ps}\end{figure}$ Figure 5: IR spectra of residues for the same starting ice composition and different irradiation dose, or different UV spectrum.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{3306_f6.ps}\end{figure}$ Figure 6: IR spectra of 12 K irradiated ice of standard (OR1) and H₂O:NH₃:CO = 2:1:1 experiment.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{3306_f7.ps}\end{figure}$ Figure 7: IR spectra of residues for different starting ice composition to study the effect of excluding CH₃OH and CO₂. Dashed lines indicate the position of the amides and HMT features.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{3306_f8.ps}\end{figure}$ Figure 8: IR spectra of residues for different ice samples to study the effect of low concentration of organic molecules with respect to water and the result of excluding the CO and CO₂ ices.
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In the text

$\begin{figure} \par\includegraphics[width=7.9cm,clip]{3306_f9.ps}\end{figure}$ Figure 9: IR spectra of the residue of the standard experiment (OR1), with deposition and irradiation at 12 K, compared to the residue of the same ice mixture with deposition and irradiation at 80 K (OR6).
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{3306_f10.ps}\end{figure}$ Figure 10: Blank experiments to find the effect of irradiation and to constrain the influence of possible organic contaminants in the setup.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{3306_f11.ps}\end{figure}$ Figure 11: Time evolution of the residue spectrum (OR3) at room temperature (T = 298 K). The dashed lines indicate the main features of NH₄⁺ and HMT.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{3306_f12.ps}\end{figure}$ Figure 12: Time evolution of the column densities of NH₄⁺ and HMT (OR3) at room temperature (T = 298 K).
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In the text

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{3306_f13.eps}\end{figure}$ Figure 13: Chemical pathway for the formation of HMT, after Smolin & Rapoport (1959), Walker (1964), Bernstein et al. (1995).
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In the text

$\begin{figure} \par\includegraphics[width=16.8cm,clip]{3306_f2.eps}\end{figure}$	Figure 2: IR spectrum of the residue of the standard experiment, Exp. OR1. Feature identifications are given in Table 2.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{3306_f3.ps}\end{figure}$	Figure 3: IR spectrum of the residue of the standard experiment (OR1; solid line, bottom) compared to the reference spectra of possible products. The fit (dashed line) is obtained by addition of spectra 2 and 3 at 240 K, corresponding to ammonium glycolate and HMT.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{3306_f5.ps}\end{figure}$	Figure 5: IR spectra of residues for the same starting ice composition and different irradiation dose, or different UV spectrum.
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{3306_f6.ps}\end{figure}$	Figure 6: IR spectra of 12 K irradiated ice of standard (OR1) and H₂O:NH₃:CO = 2:1:1 experiment.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{3306_f7.ps}\end{figure}$	Figure 7: IR spectra of residues for different starting ice composition to study the effect of excluding CH₃OH and CO₂. Dashed lines indicate the position of the amides and HMT features.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{3306_f8.ps}\end{figure}$	Figure 8: IR spectra of residues for different ice samples to study the effect of low concentration of organic molecules with respect to water and the result of excluding the CO and CO₂ ices.
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$\begin{figure} \par\includegraphics[width=7.9cm,clip]{3306_f9.ps}\end{figure}$	Figure 9: IR spectra of the residue of the standard experiment (OR1), with deposition and irradiation at 12 K, compared to the residue of the same ice mixture with deposition and irradiation at 80 K (OR6).
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$\begin{figure} \par\includegraphics[width=8cm,clip]{3306_f10.ps}\end{figure}$	Figure 10: Blank experiments to find the effect of irradiation and to constrain the influence of possible organic contaminants in the setup.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{3306_f11.ps}\end{figure}$	Figure 11: Time evolution of the residue spectrum (OR3) at room temperature (T = 298 K). The dashed lines indicate the main features of NH₄⁺ and HMT.
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{3306_f12.ps}\end{figure}$	Figure 12: Time evolution of the column densities of NH₄⁺ and HMT (OR3) at room temperature (T = 298 K).
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$\begin{figure} \par\includegraphics[width=8.5cm,clip]{3306_f13.eps}\end{figure}$	Figure 13: Chemical pathway for the formation of HMT, after Smolin & Rapoport (1959), Walker (1964), Bernstein et al. (1995).
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