M.H. Shapiro

Simulation of isotopic mass effects in sputtering

The multiple interaction, molecular dynamics code SPUT1 has been used to simulate the effects of isotopic mass differences on atoms sputtered from single crystal Cu targets by normally incident Ar ions. Calculations were carried out for 1 keV and 5 keV ions incident on natural Cu targets (69.1% ^(63)Cu, 30.9% ^(65)Cu). and for 5 keV ions incident on pseudo-Cu targets composed of mixtures of natural Cu (63.546 amu) and “very light” Cu (50.837 amu) in the abundance ratios 1:3, 1:1, and 3:1. In all cases the sputtered ejecta showed an overall enrichment in the light isotope relative to the isotopic composition of the target. Preferential enrichment of the light isotope in the normal direction was pronounced. Material ejected at oblique angles was either depleted in the light isotope or had a much lower enrichment of the light isotope compared to material ejected normal to the target. Studies with the pseudo-Cu targets showed that smaller enrichments were obtained when the incident ion recoiled immediately back through the first layer of the target, while larger enrichments were associated with deeper penetration of the incident ion into the target crystallite. In both cases, the average energy of the light atoms in the collision cascade was found to be higher than that of the heavy atoms. However, this effect was enhanced with deeper penetration of the incident ion into the target. The preferential enrichment of the light ejected atoms normal to the target is largely the result of a strong momentum asymmetry in the collision cascades. Light atoms in the cascades, on average, carry far greater momentum towards the surface of the target than do the heavy atoms. A limited number of simulation runs also were carried out with heavy ions (74 amu) incident on pseudo-Cu targets. Overall enrichment of the light atoms in the sputtered material was reduced, but the angular variation of the isotopic yields persisted.

A molecular dynamics simulation of collisional excitation in sputtering from Al

Abstract The molecular dynamics sputtering code SPUT2 was modified to permit investigation of core excitation in Al following bombardment with 5 keV Ar + ions. This code was used to investigate sputtering mechanisms responsible for the ejection of core-excited atoms from solid surfaces. Simulations were carried out with the ion incident along both low- and high-index directions. In contrast to most previous studies, essentially all ejection of core-excited atoms resulted from asymmetric collisions (i.e. collisions between the incident ion and a target atom). When the ion is incident along a low-index direction, core-excited atoms arise almost exclusively from the first layer of the target. When the ion is incident along a high-index direction, core-excited atoms are found to eject from deeper layers as well.

A molecular dynamics study of Cu dimer sputtering mechanisms

Molecular dynamics simulations were used to investigate the mechanisms responsible for the sputtering of dimers from Cu(100) and Cu(111) surfaces following bombardment by normally incident, 5 keV Ar^+ ions. Simulations were carried out using both a pair-potential and a many-body, embedded-atom potential to describe the Cu-Cu interaction. Computer animation techniques which allowed visual inspection of individual dimer trajectories were used to identify the mechanisms responsible for dimer ejection. In the pair-potential simulations dimers accounted for about 5% of the sputtering yield from both the (100) and (111) surfaces, while in the embedded-atom simulations dimers accounted for approximately 2% of the yield from the (100) and (111) surfaces. Three mechanisms were found to be responsible for the bulk of the dimer ejection events. Direct ejection of intact dimers and recombination in or near the surface were the most prevalent mechanisms observed. Less frequent, but still significant numbers of “push-stick” events were also seen. The simulations suggest that the sputtering of dimers is the result of competing mechanisms that take place preferentially towards the later phase of collision cascades that produce relatively large numbers of sputtered atoms.

Amolecular dynamics simulation of collisional excitation mechanisms in Al

A modified version of the SPUT2 molecular dynamics sputtering code was used to reinvestigate core excitation in Al atoms following bombardment with 1–5 keV Ar+ ions. For all bombarding energies, asymmetric collisions between the incoming ion and target atoms yielded smaller minimum distances-of-closest-approach between the collision partners for hard collisions than did symmetric collisions between pairs of target atoms. Simple critical distance-of-closest-approach models were used to estimate core excitation for both asymmetric and symmetric collisions. A single value of Rc (0.367 A) was used for asymmetric Ar-Al collisions, while two choices of Rc were used for symmetric Al-Al collisions (0.442 and 0.530 A). With the smaller Rc value for Al-Al collisions, we find that core excitation proceeds predominantly by asymmetric collisions at all bombarding energies above threshold. At 5 keV bombarding energy the percentage of sputtered, core-excited atoms originating from asymmetric collisions ranged from 89 to 95% depending on the incident direction of the projectile. With the larger Rc value, core excitation proceeds predominantly by asymmetric collisions at bombarding energies above approximately 3 keV; and at 5 keV asymmetric collisions accounted for ∼ 60 to ∼ 84% of sputtered, core-excited atoms. Lifetime corrections and corrections for Auger neutralization near the target surface had little effect on the ratio of asymmetric to symmetric collisions responsible for atomic-like Auger emission. These simulation results suggest that simultaneous multiple collisions are very important in the initial energy- and momentum-transfer stage which initiates the cascade.

Simulations of core excitation in energetic cluster impacts on metallic surfaces

A modified version of the SPUT2 molecular dynamics sputtering code was used to investigate core excitation in Al atoms following the impact of pure Al clusters and composite Al-Au clusters on Au surfaces. Information was obtained on distances-of closest-approach times-of-closest-approach, and ejection properties of cluster-atoms undergoing hard collisions for 32-, 63-, and 108-atom Al clusters impacting Au(100) targets with energies/atom ranging from 0.2 keV to 1.0 keV. Similar information was obtained for composite Al-Au clusters (38 Al atoms, 25 Au atoms) with comparable total cluster energies. Using a simple critical distance model (R_c = 0.44 A) for L-shell core excitation in Al, the threshold for core excitation in pure Al clusters was found to be at approximately 0.4 keV/atom. For the composite clusters, the threshold was at approximately 0.11 keV/Al-atom. Core excitation was most probable during the early, compressional phase of the cluster impacts. A significant fraction (typically ∼40%) of core-excited Al atoms were found to eject from the cluster-target system. Elapsed times between excitation and ejection typically were less than 80 fs, and significant numbers of Al atoms ejected within 20–30 fs after excitation. Our results suggest that line spectra of Auger electrons from sputtered, excited cluster-atoms can be used as a diagnostic tool to investigate the early stages of energetic cluster collisions with surfaces.

Simulation of sputtering induced by high energy gold clusters

Recently Andersen et al. reported exceptionally large nonlinear sputtering yields following the bombardment of gold targets with small, high energy gold clusters (H.H. Andersen, A. Brunelle, S. Della-Negra, J. Depauw, Y. LeBeyec, Phys. Rev. Lett. 80 (1998) 5433). Nonlinearities up to ∼1000% were observed for Au_5 clusters with a total energy of 800 keV. We have carried out molecular dynamics simulations on a massively parallel computer to model the Au_n → Au system. The results suggest that both collisional and thermal spike mechanisms contribute to the large nonlinear yields observed by Andersen et al. Our simulations also indicate that sputtering statistics are very important at high cluster bombarding energies. A substantial fraction of the simulated impacts produce no sputtered atoms, while individual sputtering events producing very large numbers of sputtered atoms contribute significantly to the total sputtering yield at high energy.

A cusp in the 54Cr(p, γ)55Mn reaction

Abstract Absolute cross sections for the reactions 54 Cr ( p , γ) 55 Mn and 54 Cr ( p, n ) 54 Mn are presented for effective bombarding energies E p from 0.830 to 3.606 MeV. A substantial cusp is observed in the 54 Cr ( p , γ) 55 Mn excitation function. The data are compared with the predictions of global Hauser-Feshbach models in order to evaluate their applicability to nucleosynthesis calculations.

Animated molecular dynamics simulations of energetic mixed cluster impacts

Molecular dynamics and computer animation techniques have been used to investigate atom ejection and damage caused by the impact of energetic, uniformly mixed Al_32Au_31 clusters on 12-layer, 3750-atom Au(100) targets at total cluster energies of 5, 10, 50, 100, and 200 keV. At all incident energies we find that a significant fraction of the aluminum is ejected from the cluster on impact. At the two lowest bombarding energies damage to the target is limited generally to the primary impact zone, and crystallinity is retained throughout most of the the target. At the higher bombarding energies significant damage occurs throughout the central region of the target, and crystallinity is lost throughout the entire target. In contrast to collision cascades initiated by single ions, those initiated by energetic clusters are observed to be highly nonlinear (essentially all the atoms in the cascade region are in motion) and to propagate through the target with relatively uniform wavefronts.

Molecular dynamics simulation of nonlinear effects in sputtering — Cu(100) targets

Nonlinear effects in sputtering from Cu(100) targets have been simulated with the multiple-interaction molecular dynamics code SPUT2. Both dimer and single-ion impacts were studied for Ar, Cu, Kr, and Xe ions normally incident on Cu(100) with energies of 5 keV/atom. Statistically significant nonlinear yields were found for bombardment with Kr and Xe dimers, but not for Ar and Cu dimer bombardment. Even for those cases not exhibiting nonlinearities in yield, differences were found in the energy and angular distributions of sputtered atoms for dimer impacts compared to the sum of individual ion impacts. For both dimer and single-ion impacts most sputtering was found to occur within 200 fs of impact; however, atom ejection tended to be shifted to later times for dimer impacts. Likewise, energy degradation in the collision cascades was much slower for dimer impacts than for single-ion impacts. These results suggest that a thermal spike begins to develop relatively early in collision cascades initiated by dimer impacts on Cu. This developing spike together with collisional disruption of the surface contributes to the excess yield found with dimer impacts.

Theoretical studies of ion bombardment: Many‐body interactions

Many‐body forces obtained by the embedded‐atom method are incorporated into the description of low‐energy collisions and surface ejection processes in molecular dynamics simulations of sputtering from metal targets. Bombardment of small, single‐crystal Cu targets (400–500 atoms) in three different orientations ({100}, {110}, {111}) by 5‐keV Ar+ ions have been simulated. The results are compared to simulations using purely pairwise additive interactions. Significant differences in the spectra of ejected atoms are found.