5.2 Dependence of the dispersion coefficient on the hade of bombarding particles
Let's consider influence of a hade of ions on the dispersion coefficient. The first experimental proof of existence of angular dependence of dispersion of metals with low energies by ions has been received by Fets [23], but the regular studying of this phenomenon began only 15 years later. In particular, almost in all experiments executed till now, it was not possible to determine a hade precisely. It spoke not strict parallelism and uniformity of bunches of falling ions, and also poor quality of a surface of a target (uncertainty macro -and microrelief in the result of both preliminary processing of a target, and ionic bombardment during experiment). To the exact definition of angular dependence also interfere:
а) Reflection of ions, which, according to Molchanov and coll.. [55], appreciablly increases at the big hades (for example, in the case of falling of a bunch of ions of argon with energy 27 keV on a copper target under the corner 84 ° to its normal, 22 % of a bunch is reflected);
b) Secondary electronic emission, which also depends on the hade of ions on a target and gives the undesirable contribution to the current on a target.
On fig. 5.6 dependence of the dispersion coefficient on the hade α of ions of argon with different energies on a copper target is shown. Apparently, that the dispersion coefficient appreciablly grows at increase of the hade. So, size S for ions of argon with energy 27 keV (Molchanov and coll. [55]) increases up to 280 % in an interval of corners of falling 0 ° <α <70 °. However this angular dependence becomes weaker at transition to small energies of ions as it is possible to see, comparing with curves for energies 27 keV (Molchanov and coll. [55]), 20 keV (Rol and coll. [69]) and 0,5 keV (Verner [45]).
Fig. 5.6. dependence of factor of dispersion S on a corner of falling of ions of argon α on a copper target.
1- Molchanov and сoll. data [557];
2- Rol and coll. data [699];
3- Verner data [876].
Molchanov and coll. [55] have made the conclusion, that dependence experimentally found by them S(α) at corners α down to 70 ° is precisely described by the formula
S=S0/cosα, (5.1)
where S0 – factor of dispersion at normal falling ions on the target.
At the same time Rol and coll. [69] have found, that their results (a curve for ions Ar + with energy 20 keV) down to corners of falling 45 ° are precisely described by dependence
S=S0(2-cosα)/cosα. (5.2)
At the big corners of falling (α≤70°) function S(α) passes through the maximum as it is visible from the curves corresponding to ions of argon with energies 27 and 0,5 keV. Such behavior can be partly caused by appreciable increase in reflection of primary ions at the big hades of ions on the target.
Molchanov and coll. [55] have established, that at α =70 ° there is no appreciable reflection of fast ions of argon from the surface of a copper target, but at α =78 ° 6 %, at 82 ° - 17 % and at 84 ° - 22 % of energy of a primary ion bunch are reflected. Quantitative specification of dependence S (α) for ions of argon with energy 27 keV, bombarding a copper target (Molchanov and coll. [55]), in view of a share of the reflected ions cannot explain completely behaviour of functions S (α) at α> 70 °).
On fig. 5.7 values S (α), received at dispersion of various targets by ions of inert gases: Хе + at 22 keV (Pitkin [66]), Kg + at 45 keV (Almen and Brook [14]), Аг + at 20 keV (Roll. and coll [69]) and Ne + at 45 keV (Almen and Brook [14]) are resulted. Comparison of the results for a copper target shows, that function S (α) starts to increase at that smaller hades, than bigger is nuclear weight of bombarding ions. On the other hand, in case of an irradiation of target ions of inert gases at the big hade of ions on the target ratio S (α)/S (0 °) varies only insignificantly with increase in nuclear weight of ions. At bombardment of a copper target by ions Т1 + and Аг + with energy 20 keV (Roll and coll. [69]) appeared, that to heavier ions Т1 + there corresponds stronger angular dependence (fig. 5.8) and that ratio S (50 °)/S (0 °) is higher for ions Т1 +, than for Аг + (see tab. 5.3). Almen and coll. [14] have found, that in a range of hades from 0 up to 60 ° curve for ions of Kg + and Ne +, bombarding different targets, can be described by ratio S Sa= S0(cos a)3/2; only the curve for a silver target can not be described by this ratio.
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