ABSTRACT
We present work detailing the oxidative destruction of the nerve agent simulant diisopropyl methylphosphonate (DIMP) with O(3P) using time-resolved, in situ reflection absorption infrared spectroscopy (RAIRS) and X-ray photoelectron spectroscopy (XPS). Thermally annealed DIMP films deposited on Au(111) are observed to react upon exposure to a supersonic beam containing O(3P) with average translational energies of 0.12 eV. The reaction is initiated by a hydrogen abstraction from one of three possible sites on DIMP, and then progresses through various secondary reactions with resultant hydroxyl radicals, carbon-centered DIMP-derived radicals, and nondissociated O2 in the beam. These reactions are accompanied by uptake of oxygen into the film, leading to new hydrogen bonding with the DIMP phosphoryl group. The generated product also presents greater thermal stability than pristine DIMP, suggesting the formation of a distribution of oligomeric and polymeric products. As reactivity is observed to decrease upon continued O(3P) exposure, this product likely forms a protective layer at the vacuum-film interface, hindering destruction of thicker films. Importantly, the rate of reaction and general reactivity trends are the same between DIMP and the smaller simulant dimethyl methylphosphonate (DMMP). The comparable reaction rates of the two molecules coupled with oxygen's inability to erode thick films all the way down to the substrate have specific implications for the development of oxidation-based decontamination strategies for these and other organophosphates in the solid phase. The findings presented in this paper add significant new fundamental understanding of the oxidative chemistry of such species, knowledge needed in order to develop efficacious nerve agent decontamination strategies as well as the refinement of existing models for the dispersal, adsorption, persistence, and destruction of organophosphates in the environment.
ABSTRACT
We present the first study detailing the capture and aggregation of hyperthermal CO2 molecules by amorphous solid water (ASW) under ultra-high vacuum conditions at 125 K, near the amorphous/crystalline transition. Using time-resolved in situ reflection-absorption infrared spectroscopy (RAIRS), CO2 molecules with translational energies above 3.0 eV are observed to directly embed underneath the vacuum-solid interface to become absorbed within the ice films despite an inability to adsorb at 125 K; this behavior is not observed for crystalline films. Upon embedding, the mobility of CO2 within 125 K amorphous ice and the strength of its intermolecular interactions result in its segregation into clusters within the ice films. Tracing the kinetics of CO2 embedding events under different energetic conditions allows for elucidation of the underlying dynamics, and we draw comparison with other projectiles we have studied to promote generalized conclusions in regard to empirical prediction of a projectile's embedding probability. Through application of a classical model of the entrance barrier for projectiles colliding with amorphous ice, we provide direct evidence for a unified connection between embedding probability and projectile momentum; an account of all embedding data measured by our group traces a unified barrier model. This work highlights the interplay between translational energy and momentum accommodation during collisions with ice in high speed gas flows.
ABSTRACT
The interaction of atomic and molecular species with water and ice is of fundamental importance for chemistry. In a previous series of publications, we demonstrated that translational energy activates the embedding of Xe and Kr atoms in the near surface region of ice surfaces. In this paper, we show that inert molecular species may be absorbed in a similar fashion. We also revisit Xe embedding, and further probe the nature of the absorption into the selvedge. CF4 molecules with high translational energies (≥3 eV) were observed to embed in amorphous solid water. Just as with Xe, the initial adsorption rate is strongly activated by translational energy, but the CF4 embedding probability is much less than for Xe. In addition, a larger molecule, SF6, did not embed at the same translational energies that both CF4 and Xe embedded. The embedding rate for a given energy thus goes in the order Xe > CF4 > SF6. We do not have as much data for Kr, but it appears to have a rate that is between that of Xe and CF4. Tentatively, this order suggests that for Xe and CF4, which have similar van der Waals radii, the momentum is the key factor in determining whether the incident atom or molecule can penetrate deeply enough below the surface to embed. The more massive SF6 molecule also has a larger van der Waals radius, which appears to prevent it from stably embedding in the selvedge. We also determined that the maximum depth of embedding is less than the equivalent of four layers of hexagonal ice, while some of the atoms just below the ice surface can escape before ice desorption begins. These results show that energetic ballistic embedding in ice is a general phenomenon, and represents a significant new channel by which incident species can be trapped under conditions where they would otherwise not be bound stably as surface adsorbates. These findings have implications for many fields including environmental science, trace gas collection and release, and the chemical composition of astrophysical icy bodies in space.
