COMPLETO PARA LA PROPUESTA 4

A summary of all the kinematic factor (K-factor) assessments for Ne+ and Ar+ scattering off the four targets (Ge, Ag, Au, and Pb) is given in Fig. 6.9 as a ″master curve″ of K-factor versus target-to-projectile mass ratio. It is hard to say what the proper way to calculate the overall K-factor for the experimental data would be, given possible inelastic losses. In addition, some of the experimental data show deviations from a linear dependence of exit energy on incident energy which makes it difficult to establish an overall K-factor. In an effort to summarize the data, we have chosen to allow for

inelasticity in the hard encounter and ignore electron friction losses on the incoming and outgoing trajectory paths. Therefore, the experimental K-factor is just the slope of the best fit regression line put through the versus data for each projectile-target combination. All regression lines for the K-factor fits to experimental data have correlation coefficients (r

exit

E E₀

2_{) greater than 0.978. The BCA model prediction is shown in}

the figure as the solid curve. This data presentation method is somewhat misleading because it removes any K-factor dependency on the incident energy for different energy ranges (if there is one). The reader should reference each of the individual versus

plots. The conclusion from such an analysis is that the scattered projectile energy most certainly varies in a linear way with respect to incident energy. In addition, if we lump any inelasticities into the hard collision, the BCA model predicts the energy transfer dependency reasonably well at low impact energies (< 500 eV) for the Ge, Ag, Au, and Pb targets tested.

exit E 0

6.5 Summary

One of the main driving forces behind the Ne+ and Ar+ scattering studies presented in this chapter was to demonstrate the performance of our ion beamline and scattered product detector system in its entirety. As well, a cursory goal was to perhaps add to the discussion of single collision phenomena at low impact energy where

scattering phenomena are more nebulous. Most certainly, a detailed evaluation of BCA model validity with inert ions should be conducted in an ISS system with high resolution energy filter that can be rotated around the target. In this way, the scattering angle can be varied to probe multiple bouncing phenomena and ion neutralization channels which depend on the particle trajectory length in the near surface region. Our ion beamline and scattering system was designed for an entirely different purpose in mind. However, several conclusions about the scattering studies mention in this chapter can be made.

As a general rule, the lighter the projectile, the more closely the experimental data follows BCA predictions under 500 eV. We know that He+ fits the BCA model

exceptionally well (Czanderna op. cit.; Nielhus op. cit.). Our Ne+ studies show experimental data with less scatter, more linearity, and better consistency with BCA single collision predictions than Ar+. This statement is consistent with the picture that Ar+ damages the target surface more than Ne+ from momentum arguments alone, making it more probable that the incoming projectile will suffer losses in traversing the upper atomic layers of the target. In addition, Ar+ and Ar0 have a much richer distribution of lower energy excited states than Ne+ or Ne0, which may favor small losses to electronic excitation in Ar more so than Ne. From a simplistic point of view, the kinetic energy of the projectile seems like it should be more easily accommodated into electronic

excitations of Ar0 or Ar+ due to the availability of more energetically lower electron states.

Next, the measured exit energies for single collision scattering events are quite close, but slightly less than BCA values in most cases. This implies that the collisions in the 50-500 eV range are indeed quite binary in nature for ion exit channels. Multiple bouncing phenomena, giving a higher than BCA exit energy, were not seen in any of our scattering experiments at 90° lab angle for the Ge, Ag, Au, and Pb targets. Also, the

scattered ion yield at 90° for Ne+ was usually much greater than Ar+ for most impact energies and targets tested.

Finally, we have seen mixed results for Ar+ scattering which puts our scattering work somewhere in the middle of the debate on BCA validity for Ar+. As mentioned earlier, both textbook-like agreement as well as significant deviations from single collision phenomena have been documented for Ar+ below 500 eV. In some cases, like Au and Ge (with inelasticity offset), the BCA predictions are right on for our

experimental data, while other targets give more data scatter and less than BCA values on average. Perhaps single collision events with Ar+ are mediated by damage to the target surface which affects the incoming or outgoing ion trajectory. This phenomenon would, of course, depend on the specific target material and how easy the target is to sputter.

From an equipment standpoint, the entire ion beamline and product detector system have demonstrated their performance in an actual ion scattering experiment. Noble gas ion scattering on metals at low energy is dominated by high neutralization rates (at least Ar+) for the projectile ion because Auger transition rates depend

exponentially on the ion approach or exit velocity perpendicular to the surface (Neilhus op. cit.). Low impact energy greatly increases the probability for ion neutralization. Our product detector system has shown clean spectra, easily identifiable BCA-like scattering peaks, and high signal to noise for these low ion survival rate surface collisions. The system has also shown that mass-filtering, in addition to energy analysis of the ion flux leaving the target surface, can significantly clean up the scattered ion spectrum so that peak assignments are easy and subtle features like sub-surface scattering can be seen.