**2. Experimental**

The measurements of differential energy spectra and angular distributions of scattered ions were performed in experimental equipment (energy analyzer of the spherical deflector type) with high angular (Δ*ψ* ≅ 0.6o ) and energetic (Δ*E*/*E* ≅ 1/125) resolutions and with the capability to analyze the secondary ion masses by means of time-of-flight technique [24, 25]. The experimental setup includes a UHV scattering chamber with the oil-free pumping system and a base pressure in the 10−9 Torr range. During the measurements, the working pressure rises to about 5 × 10−9 Torr. Ions of alkaline metals were obtained in a thermal ion source with a target density of current *J* <sup>0</sup> = 5 ⋅ 10−7 A∙cm−2 under operating conditions. The repeated cycles of an electron bombardment was used for cleaning the target surface. Beams of *Е* 0 = 50–500 eV of Cs+ and K+ ions, with a current density *J* <sup>0</sup> = 5 ⋅ 10−7 A∙cm−2, scattered from clean Ta, W, Re and Ti, V, Cr polycrystal surfaces under an incidence angle of *ψ* = 55°. At the registration of spectra from contaminated targets, the peak of the straight flight was constantly observed. This peak corresponded to the energy of primary ions *E* o, that is, the reflection from oxide film which behaved as a screen. After cleaning by electron bombardment, this peak disappeared and bell-shaped spectra were observed. The incident and scattering beams were laid in the same plane, perpendicular to the surface of the target in the point of incidence of ions. The size of ion spot on the sample at normal incidence of ions on the surface was 2 mm, and the scatter of angles of incidence of the ion beam did not exceed Δ*ψ* = ±2°. The backscattered ions are collected at a scattering angle θ = 70°. The angular resolution of the device is Δ*θ* = ±2°. Usually, within the method of ion-scattering spectroscopy, the interpretation of both angular and energy distribution of the scattered heavy alkali ions (K, Cs) is based on the differential cross section of scattering only, the ionization degree *η* <sup>+</sup> being 100% [26]. The measured energy spectra were processed by computers for averaging of statistical fluctuations of impulse registration with the use of low-frequency digital filter of Spencer. Repeating the deflection of the voltage on the plate of energy analyzer with alteration of cleaning of the target, the impulse analyzer was functioning in the mode of accumulation, and summed spectra were registered on the display and digitally printed [24, 25].
