Low-Energy Electron-Induced Chemistry of CF₂Cl₂:  Implications for the Ozone Hole?

Abstract

We report on the first direct investigation of the low-energy electron-induced production of neutral species from the chlorofluorocarbon CF2Cl2, commonly known as Freon-12 or CFC-12. Our experiments were motivated by a newly proposed hypothesis, which suggests that low-energy electrons produced by cosmic rays, in addition to UV−vis photons from the sun, interact with chlorofluorocarbons to produce chlorine atoms that subsequently destroy ozone in the Antarctic. Our experimental procedure involves low-energy (5−100 eV) electron irradiation of nanoscale thin films (∼10 Å thickness) of CF2Cl2 grown at 100 K on a molybdenum single crystal in an ultrahigh vacuum chamber (p ∼ 1 × 10-10 Torr). Post-irradiation temperature-programmed desorption experiments were used to identify C2F4Cl2, C2F3Cl3, C2F2Cl4, C2F3Cl, C2F2Cl2, and C2F4 as electron-induced radiolysis products of CF2Cl2. In contrast to previous studies of photon-induced dissociation, our studies of electron-induced dissociation demonstrate facile C−F bond cleavage in CF2Cl2. This finding may have implications for understanding the partitioning of Cl and F among source, sink, and reservoir gases in the stratosphere.

Electron stimulated reactions of methyl iodide coadsorbed with amorphous solid water

“…8,[14][15][16][17][18][19][20][21][22][23] In addition to serving as a model adsorbate, CH 3 I adsorbed even transiently on the surface of water droplets or water ͑ice͒ particles in the upper atmosphere has the potential to interact with solar photons and other energetic sub-atomic particles and generate environmentally important species. [26][27][28][29][32][33][34][35][36] The present study focuses on the electron-initiated decomposition of methyl iodide adsorbed on and suspended in water ͑ice͒, and examines how the presence of water influences the electron-driven chemistry. [24][25][26] These partially halogenated species may undergo subsequent photodissociation in the upper atmosphere, yielding haloformyl radicals ͑XCO͒ and halogen atoms ͑X͒.…”

Correlation between Cosmic Rays and Ozone Depletion

“…CRs are the only electron source in the stratosphere, while halogen (Cl, Br)-containing molecules are long known to have extremely large cross sections of dissociative attachments of low-energy electrons [9]. The latter reaction will be greatly enhanced when halogenated molecules are adsorbed or buried at the surfaces of polar molecular ice, relevant to polar stratospheric cloud (PSC) ice in the winter polar stratosphere, as firstly discovered by Lu and Madey [6,7] and subsequently confirmed by others in experiments and theoretical calculations [10][11][12][13][14][15][16]. For example, the dissociative attachment cross section at $0 eV electrons for CF 2 Cl 2 adsorbed on the surface of water ice has been measured to be $1 Â 10 À14 cm 2 , which is about 1 Â 10 6 times the photolysis cross section of CF 2 Cl 2 [6,10].…”

“…There is interest in studying the effects of galactic cosmic rays (CRs) on Earth's climate and environment, particularly on global cloud cover in low atmosphere ( 3 km) [1-5] and ozone depletion in the stratosphere [6][7][8][9][10][11][12][13][14][15][16]. The former has led to a different scenario for global warming, while the latter has provided an unrecognized mechanism for the formation of the O 3 hole.…”

Revisiting the electron stimulated desorption of anions from thin films of CF2Cl2

“…Additional measurements (see supplemental materials) performed in our laboratory show that the desorbed yield of each anion is proportional to the incident current (in the range 0.02-5 nA) and indicate that even the weak anion signals are generated by single electron scattering events and should not be attributed to the dissociation of any product synthesised in the film by electron irradiation [15]. Moreover, measurements as a function of CF 2 Cl 2 coverage on a 3-ML thick Kr film (supplemental material), reveal that all anion signals increase linearly with coverage (at low coverage) and gradually approach saturation at about 3-4 ML, as is commonly seen in ESD experiments [16,17].…”