ABSTRACT
Dense interstellar clouds are the birthplaces of stars of all masses and their planetary systems. Interstellarmoleculesanddustbecomethebuildingblocksforprotostellardisks,fromwhich planets,comets,asteroids,andothermacroscopicbodieseventuallyform.IthasbeenrecognizedthatdustisubiquitousintheGalaxy.Lightessentiallycannotpenetratedenseinterstellar clouds,andtemperatureinsidedarkcloudsremainsat∼10K.About120interstellarmoleculeshave been observed by millimeter and submillimeter spectroscopy in such dark clouds. It is becoming
29.1 Introduction ..........................................................................................................................805 29.2 Simulation of the Chemical Evolution Taking Place on the Dust
Grains in the Dark Clouds ...............................................................................................806 29.3 Reactions of H and D Atoms with Solid C2H2, C2H4, and C2H6
at Cryogenic Temperatures .............................................................................................. 808 29.3.1In Situ and Real-Time Observation of Products Formed from Reactions
of H Atoms with Solid C2H4 and C2H2 .....................................................................808 29.3.2Temperature Dependence on the Yield of Reaction Products .................................. 810 29.3.3Reactions of D Atoms with Solid C2H4, C2H2, and C2H6: Isotope Effect ................ 811
29.4Formation of Formaldehyde by the Tunneling Reactions of H with Solid CO at 10K ........ 815 29.4.1Formation of H2CO from the Reaction of H with Solid CO .................................... 815 29.4.2Temperature Dependence of the Yield of H2CO from the Reaction H + CO........... 816 29.4.3Does the van der Waals Solid Film Erode during the H-Atom Spray? .................... 816
29.5Reaction of H and D Atoms with Solid Formaldehyde and Methanol at Cryogenic Temperatures ................................................................................................... 817 29.5.1Reaction of H with Solid H2CO ................................................................................ 818 29.5.2Reaction of D with Solid H2CO ................................................................................824 29.5.3Reactions of H and D with Solid CH3OH ................................................................. 825
29.6Methanol Formation from Electron-Irradiated Mixed H2O/CH4 Ice at 10 K ....................... 827 29.7 Solid-Phase Reactions of D with CN to Form DNC and DCN
at Cryogenic Temperatures .............................................................................................. 832 29.8Is NH3 Formed in Solid-Phase Reactions? ........................................................................... 835 29.9Conclusion ............................................................................................................................ 837 References ...................................................................................................................................... 837
increasingly clear that gas-phase ion-molecule reactions-the dominant processes in diffuse interstellarclouds-arelessimportantinthedenseclouds.Instead,chemicalreactionsoncosmicdust grainsplayanimportantrole(MillarandWilliams,1993).Interstellarices,whichgrowonsolid dustparticlesincold,denseclouds,becomethesubstratesforvariouschemicalreactions.The InfraredSpaceObservatory(ISO)hasprovidedmuchinformationaboutinterstellarandcometary ices (Ehrenfreund and Schutte, 2000).
