In this project supported by the Chemical Structure, Dynamics and Mechanisms Program of the Chemistry Division, Professor Christopher Arumainayagam of Wellesley College and his undergraduate students will employ infrared spectroscopy, temperature programmed desorption (TPD), and isothermal electron stimulated desorption (ESD) to characterize the effects of low-energy electrons on thin films of halomethanes, methanol and molybdenum hexacarbonyl. The underlying hypothesis of the research is that much of the damage that occurs in materials exposed to ionizing radiation or high energy particles is due to secondary electrons which, while lower in energy per particle, are generated in much greater abundance relative to the primary ionizing photon or particle.
The results of this research will not only advance the specific chemistry field of electron-induced reactions, but will be relevant to other areas of science and technology, including our understanding of radiation damage in biological systems, electron beam lithography for microelectronics technology, new environmental remediation techniques, and atmospheric reaction cycles. Importantly, these investigations will also provide a vehicle for research training of undergraduate scientists.
In the interstellar medium, UV photolysis of ice mantles surrounding dust grains is thought to be the mechanism that drives the formation of "complex" molecules. The source of this reaction-initiating UV light is assumed to be local because externally sourced UV radiation cannot penetrate the ice-containing dark, dense molecular clouds. Specifically, exceedingly penetrative high-energy cosmic rays generate secondary electrons within the clouds through molecular ionizations. These cosmic-ray induced secondary electrons excite hydrogen molecules present within the above-mentioned dense molecular clouds. It is the UV light, emitted by these electronically excited hydrogen molecules, that is thought to photoprocess interstellar icy grain mantles to generate "complex" molecules. In addition to producing UV light, the large numbers of low-energy (< 20 eV) secondary electrons, produced by cosmic rays, can also directly initiate radiolysis reactions in the condensed phase. We hypothesize that cosmic-ray induced low-energy electron processing of interstellar ices may occur via three mechanisms:(1) the interaction of cosmic rays with gaseous molecular hydrogen produces low-energy electrons that can interact with the surface (top few molecular layers) of cosmic ices, (2) the interaction of cosmic rays with molecules within cosmic ices generates a cascade of low-energy electrons that can interact with the surface and the bulk of the ice mantles, (3) the interactions of the cosmic rays with the dust grain beneath the ice mantle engenders low-energy electrons that can interact with the bottom ice layers in contact with the dust grain. The goal of our studies has been to understand the low-energy, electron-induced processes that occur when high-energy cosmic rays interact with cosmic ices. Using post-irradiation temperature-programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS), we have investigated the radiolysis initiated by low-energy (5 – 20 eV) electrons in condensed methanol at ~ 90 K under ultrahigh vacuum (1×10−9 Torr) conditions. Our experimental results suggest that low-energy electron-induced condensed phase reactions may contribute to the interstellar synthesis of "complex" molecules previously thought to form exclusively via UV photons. This NSF-RUI grant to the PI supported the research efforts of 40 undergraduate/high school students. Typically, ten to fifteen students per semester did research in the PI’s laboratory. During the past three years, four students who worked in the PI’s research lab won five of the nine Katharine Malone prizes, the highest award given by Wellesley College. During the last five years, in addition to four community college students, the PI mentored five high school students, two of whom are coauthors on two manuscripts, one published and one under review.
