A molecular dynamics study of collisional excitation/deexcitation at ion-bombarded surfaces

Shiang, Keh-Dong; Cai, Jihua; Shapiro, M.H.; Fine, Joseph; Tombrello, T. A.

doi:10.1016/0168-583x(95)00871-3

Cited by 4 publications

(4 citation statements)

References 28 publications

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“…(1) The electron signals of the three Na ‫ء0‬ transitions are very intense with an estimated yield for NaCl of 2 3 10 23 electrons/projectile ion [5]. This very high yield is consistent with the molecular dynamics (MD) simulation of the Na ‫ء1‬ excitation or neutralization (by electron-transfer collisions with Cl 2 ) process.…”

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confidence: 73%

“…He suggests that the neutralization process involves Na ‫ء1‬ and weakly bound surface electrons and explicitly rules out any electron transfer from halogen ions. We believe that his neutralization concept [3,4] is not consistent with our experimental data [1] for the following two reasons.(1) The electron signals of the three Na ‫ء0‬ transitions are very intense with an estimated yield for NaCl of 2 3 10 23 electrons/projectile ion [5]. This very high yield is consistent with the molecular dynamics (MD) simulation of the Na ‫ء1‬ excitation or neutralization (by electron-transfer collisions with Cl 2 ) process.…”

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confidence: 61%

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Fineet al.Reply:

et al. 1996

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Fine et al. Reply: In our Letter [1], we reported on the formation of three autoionizing states of neutral, 2p core-excited Na ‫ء0‬ 2p 5 3s3l ͑l s, p, d͒ observed in the electron spectra of ion-bombarded sodium halide surfaces. We proposed that the formation of these Na ‫ء0‬ states, due to electron capture by collisionally excited moving excitons Na ‫ء1‬ 2p 5 3s, 3p, 3d [2], involves Landau or Zener-type resonant electron transfer in Na ‫ء1‬ collisions with lattice halogen ions. Evidence for such a mechanism comes from measurements obtained on slightly halogen-depleted NaCl surfaces (Fig. 1) which show that near surface Cl 2 is involved in the neutralization of the above excitonic states [1]. Such moving Na ‫ء0‬ atoms can, clearly, autoionize inside the solid or, if they are sputtered, outside.In the preceding Comment [3] and in [4], Baragiola acknowledges that the formation of Na ‫ء0‬ does involve electron capture by Na ‫ء1‬ but disputes our collisional electron-capture model. He suggests that the neutralization process involves Na ‫ء1‬ and weakly bound surface electrons and explicitly rules out any electron transfer from halogen ions. We believe that his neutralization concept [3,4] is not consistent with our experimental data [1] for the following two reasons.(1) The electron signals of the three Na ‫ء0‬ transitions are very intense with an estimated yield for NaCl of 2 3 10 23 electrons/projectile ion [5]. This very high yield is consistent with the molecular dynamics (MD) simulation of the Na ‫ء1‬ excitation or neutralization (by electron-transfer collisions with Cl 2 ) process. The MD yield is 10 24 [5]. It is difficult to imagine that such a high

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et al. 1996

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“…It follows that excited, sputtered Na + * ions can decay, as we have noted, only radiatively and therefore not contribute to the electron spectra observed. According to some recent molecular dynamics simulations [ 8 ], collisionally excited particles that do not undergo subsequent energetic (small impact parameter) collisions with target atoms appear to dominate the number of collisionally excited particles that are sputtered. It would therefore seem reasonable to assume that for sodium halide crystals most of the ejected, excited particles which subsequently deexcite outside the solid are Na + * and do not contribute to the electron spectra.…”

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“…11 . The projectile P represents either Ar or Ne particles; the initial Na + excitation step is assumed to occur in a projectile-sodium collision (P 0,+ + Na + ) [ 8 ] but the specific type of Na + excitation collision is not critical to the sequence that follows.…”

Section: Collisional Kinetics In a Sodium Halide Latticementioning

confidence: 99%

Bombardment induced electron-capture processes at sodium halide surfaces

Fine¹,

Szymoński²,

Kołodziej³

et al. 1996

J. Res. Natl. Inst. Stand. Technol.

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Discrete features observed in the energy distribution of electrons emitted from ion-bombarded sodium halide surfaces can be attributed to a new type of collisional deexcitation mechanism. Such a mechanism involves sodium atoms in bombardment-excited autoionizing states that are the result of cascade collisions within the crystal lattice. This deexcitation process, in contrast to that for a metal, is not simply a consequence of the inner-shell lifetime of the initial collisionally excited sodium Na+* ion. Rather, the deexcitation consists of a sequence of lattice collisions during which the excited Na+* ion captures an electron to form the inner-shell-excited Na0* states responsible for the observed transitions. The formation of such autoionizing Na0* states is described within the framework of a new model in which excitation processes and localized collisional electron-transfer mechanisms are taken into account. These localized electron-transfer processes make possible new channels for electronic deexcitation, chemical dissociation, and defect production; they are critical for understanding inelastic ion-surface collisions in solids.

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What do we want from computer simulation of SIMS using clusters?

Webb

2008

Applied Surface Science

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A molecular dynamics study of collisional excitation/deexcitation at ion-bombarded surfaces

Cited by 4 publications

References 28 publications

Fineet al.Reply:

Fineet al.Reply:

Bombardment induced electron-capture processes at sodium halide surfaces

What do we want from computer simulation of SIMS using clusters?

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