Samarin S N, Petrov V N, Sudarshan K, Guagliardo P, Baraban A P, Williams J F
Department of Physics, The University of Western Australia, Perth 6009, Australia.
St. Petersburg State Polytechnical University, St. Petersburg 195251, Russia.
J Phys Condens Matter. 2023 Oct 12;36(2). doi: 10.1088/1361-648X/ad0016.
The energy distributions of scattered and re-emitted low-energy positrons from a W(100) surface were measured as a function of incident positron energy from 0 to 25 eV. Given that tungsten has a negative work function of about -3 eV for positrons, one can envisage three scenarios of very low-energy positron scattering from such a surface. First, a positron approaching the sample surface with energy say 1 eV above the vacuum level will see a potential barrier of about 2 eV height and will be reflected back to the vacuum. Second, when the energy of incident positrons increases up to the top of the surface potential barrier (positron work function), they start entering the solid and, therefore, the reflectivity of positrons from the surface reduces. Positrons entering the solid are thermalised within few picoseconds and have a chance to escape back to the vacuum with kinetic energy about 3 eV above the vacuum level undergoing so-called. Third, coherent scattering of low-energy positrons may occur on the crystal surface, i.e. positron diffraction. All the three scenarios of low-energy positrons scattering are studied here experimentally. Measured spectra are very sensitive to the surface conditions of the sample: they change dramatically after surface oxidation or thin film deposition.
测量了从W(100)表面散射和重新发射的低能正电子的能量分布，作为入射正电子能量从0到25 eV的函数。鉴于钨对正电子的功函数约为-3 eV，人们可以设想从这样一个表面进行的极低能正电子散射的三种情况。首先，一个能量比方说比真空能级高1 eV的正电子接近样品表面时，会看到一个高度约为2 eV的势垒，并会被反射回真空。其次，当入射正电子的能量增加到表面势垒顶部（正电子功函数）时，它们开始进入固体，因此，正电子从表面的反射率降低。进入固体的正电子在几皮秒内被热化，并有机会以比真空能级高约3 eV的动能逃回真空，这一过程称为……第三，低能正电子的相干散射可能发生在晶体表面，即正电子衍射。这里对低能正电子散射的所有三种情况都进行了实验研究。测量的光谱对样品的表面条件非常敏感：在表面氧化或薄膜沉积后，它们会发生显著变化。
