This research will address the question of the balance between molecules in the gas and on dust grains in dark interstellar molecular clouds, one of the central unsolved problems in the evolution of interstellar matter.
The problem consists of the following: due to the low temperature of molecular clouds and high condensation coefficients, all gaseous species, with the exception of H2 and He would be depleted by condensation on grains, in times of typically 100,000 years. Yet a typical molecular cloud lives much longer, millions of years, and contains a relatively high number of molecules in the gas phase. Therefore, an efficient desorption mechanisms must operate. However, none of the proposed mechanisms can convincingly maintain the gas population observed.
For this reason, the laboratory studies to be undertaken here perform novel experiments to determine desorption rates due to: a) photodesorption by ultraviolet Lyman-alpha photons, b) thermal desorption by recombination of H atoms into H2 molecules adsorbed on ices, c) desorption from dissociative electron attachment caused by low energy electrons. In addition, since the main interstellar gas in molecular clouds is molecular hydrogen, which can be readily absorbed in microporous water ice, the experiments will simulate those conditions, studying H2 adsorption, replacement of H2 by adsorption of other gases such as CO, and the effect of adsorbed hydrogen on the photodesorption and molecular photosynthesis in water ice.
The research will use highly sensitive techniques of microgravimetry and mass spectrometry to measure desorption of ices at cryogenic temperatures in ultrahigh vacuum. The research group has demonstrated expertise in the research area and promote innovation by applying knowledge and techniques from other areas of science into astronomical problems, such as the new concept that desorption by low energy electrons will occur in molecular clouds.
In addition to the astronomical applications of the research, the understanding of radiation processing of materials developed here has applications in semiconductor, thin film and chemical industries. The Dr. Baragiola is collaborating with scientists on some of these applications. Students will be active participants in this research which will provide training for students that will prepare them for industrial, government and academic career paths. Some of the research will also be incorporated into undergraduate introductory astronomy courses in part to help students understand the role laboratory astrophysics plays in our ongoing quest to understand our universe.
The project was concerned with how molecules are formed and destroyed on cosmic dust, before the dust collapses by gravity into stars and planetary systems. A long standing question has been the lack of a mechanism for making carbon dioxide at very low temperatures (less than 20 K). We were able to quantify the reaction of atomic oxygen with solid carbon monoxide and found that, although it leads to CO2, the main product formed is ozone from the accumulation of the oxygen atoms and molecules. We also determined the process of removing CO2 from dust grains by ultraviolet radiation from stars, and found to be a much more efficient process than anticipated. This actually makes it harder to explain how CO2 can accumulate in grains in the observed quantities. We discovered a new method for making CO2 by shining ultraviolet light on carbon coated with oxygen. This process is efficient but produces small amounts of CO2.It is likely more important in producing carbon dioxide on planetary surfaces in the outer solar system. Another discovery was that ultraviolet light can enhance the trapping of gases in water ice. We used oxygen as an example and found that the enhanced trapping was due to the breakup of the O2 molecule and the reaction of the resulting oxygen atoms with water to produce hydrogen peroxide and ozone. Since ultraviolet light can penetrare deeper in ice than most ionizing radiation, we conclude that this process dominates chemical reactions at icy satellites below 1 mm from the surface. Such ultraviolet exposure would likely destroy molecules that could be precursors for life. The project also led to the training of graduate students who obtained Ph. D. degrees and to the training of post-doctoral fellows.
