We have observed that a weak applied external electric field dramatically affects the secondary electron emission, luminescence, and electronic sputtering from thin Ar films bombarded by MeV H+, He+, and Ne+. For MeV H+ and an external field of only 70 V/cm, the electron yield is equal to the number of electron-hole pairs created, the luminescence from the electron-hole recombinations is eliminated, and the sputtering is reduced by 45%. These effects decrease for heavier projectiles. For the first time, the relative contributions of ionizations and excitations to sputtering can be separated. PACS numbers: 79.20.Nc, 71.35.+z, 78.60.b, 79.20.HxIonization is an important phenomenon in insulators like electronic materials and biological systems subject to ionizing radiation or high electric fields. The rare gas solids are useful model systems for studying these effects because their electron states are well known, and because aspects of these monatomic van der Waals solids can be treated as dense gases with generally negligible chemical changes [1 -3].When solid Ar is exposed to MeV particles, electronic energy is deposited in the form of ionization (electronhole pairs) and direct excitation (excitons). These give rise to luminescence and sputtering (desorption). The sequence of events is described by the following wellknown model [4,5]. The atomic holes and excitons diffuse primarily by resonant processes. A hole can strongly attract a ground state atom, trap by interacting with lattice vibrations, and form the Ar2 dimer hole in -10 " s. Recombination of the Ar2+ with a thermalized electron produces Ar', a ground state Ar atom, and kinetic energy; if this recombination occurs near the surface, it can produce sputtering of the Ar or Ar* involved, or even of neighboring atoms struck by the separating pair.Ar* can also be produced directly by the projectile or by its associated electronic collision cascade. Regardless of how an Ar* is formed, it can pair with a neighboring ground state atom in an attractive or repulsive state. If it is in the repulsive state and is at the surface, it can desorb by cavity ejection [6]. If it is in the attractive state, it
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