We report on measurements of electron emission spectra from surfaces of highly oriented pyrolytic graphite (HOPG) excited by 1-5 keV He+ and Li+ which, for He+, exhibit a previously unreported high-energy structure. Through a full quantum dynamic description that allows for the calculation of neutralization and electron-hole pair excitation, we show that these high-energy electrons can arise from autoionization of excitons formed by electron promotion to conduction band states close to the vacuum level. The same calculation explains the observed absence of high-energy excitons for Li+ on HOPG.
The relative intensities of the Ne 2p4(3P)3s23P and 2p4('D)3s2 'D autoionization lines produced by 700 eV Ne+ ion impact on Na deposited on polycrystalline Al and Mo show a sensitive dependence on Na coverage. We show that the collisionally excited Ne 2p4 singlet core configuration can be efficiently converted to a triplet at the immediate vicinity of the excitation site, and that its rate depends strongly on the local electronic properties around the target atomic site. These findings point to a technique to obtain site-specific information of the electronic structure and potentials outside surfaces. PACS numbers: 79.20.Rf, 32.80.Dz, 32.80.Hd, 79.20.Nc Since the observation of the singlet to triplet conversion of He* ls2s metastable atoms at metal surfaces [1,2], various physical models and theoretical calculations on the conversion rate have been reported [3 -8]. Three mechanisms have been proposed: (1) resonant surface ionization of the 2s electron of He* 2'S to form a He+ ion and a subsequent resonant neutralization to He* 235 [1]; (2) Auger deexcitation in which a valence electron with opposite spin drops into the He 2s level and the other 2s electron is simultaneously transferred to the solid [2]; and (3) the transient formation of a negative He ion (on low work function surfaces) followed by a tunneling of the 2s electron with reversed spin into the substrate [3]. A rearrangement mechanism similar to (2) was invoked previously by Zampieri, Meier, and Baragiola [9] to interpret the observation of an intense Ne** 2p4(3P)3s2 3P autoionization line when bombarding Mg, Al, and Si surfaces with Ne+ ions.In this Letter we present a study on the Ne autoionization spectra obtained by Ne+ impact on Al and Mo surfaces covered with an increasing amount of Na. We show that the collisionally excited Ne 2p4('D) singlet core configuration can be converted efficiently into a 2p (3P) triplet and that the conversion rate depends strongly on the local electronic properties around the target Na or Al sites.This conversion process involves valence electrons of the solid close to the Fermi level through an Auger rearrangement mechanism and occurs in the immediate vicinity of the excitation sites with two colliding atoms still in a coupled molecular state. Thus, autoionization spectroscopy in ion-surface collisions constitutes a novel site-specific probe of the electronic structure and provides information on the electrostatic potentials outside a macroscopically limited surface region. Our results indicate a continuous variation in the charge transfer from the Na atoms to the substrate (or polarization of the 3s orbital toward the substrate) without an abrupt metallization of the overlayer.The experiments were made on polycrystalline Al and Mo samples at a base pressure of 5 X 10 ' Torr. The 700 eV Ne ion beam was incident at 70 relative to the surface normal and was rastered to minimize sputtering. Electrons were collected with a hemispherical energy analyzer whose axis was normal to the surface, operating at 0.2 eV energy reso...
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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