1. Li, A. & Greenberg, J. M., in Solid State Astrochemistry, ed. V. Pirronello J. Krelowski, (Dordrect: Kluwer), in press (2002).
2. de Graauw, T. et al., Astron. Astrophys. 315, L49–L54 (1996).
3. Clegg, P. E. et al., Astron. Astrophys. 315, L38–L42 (1996).
4. Kessler, M. F. et al., Astron. Astrophys. 315, L27–L31 (1996).
5. Bradley, J. P. et al., Science 285, 1716–1718 (1999).