Excitation of volume plasmons in glancing collisions of protons with Al(111) surfaces

Abstract

We studied electron emission from Al͑111͒ surfaces induced by 4-100 keV protons at glancing incidence. The electron energy distributions show a structure due to the decay of volume plasmons. The intensity of the plasmon structure was measured as a function of angle of incidence to the surface. We found that volume plasmons are excited even under conditions where protons do not enter in the solid. The results are consistent with an indirect excitation mechanism by fast secondary electrons produced in binary ion-electron collisions. This mechanism is more important than direct excitations by ions and excitations produced by Auger electrons.

Signature of plasmon excitations in the stopping ratio of fast hydrogen clusters

“…In this method, the energy distribution of excited electrons resulting from the ion impact is measured, and usually, through the derivatives of the energy spectrum, the plasmon excitation contribution is evaluated. 6,[8][9][10] While most of these experiments made use of protons impinging on metals, very few have been carried out using ionic molecules, 11 although early experiments showed the potentialities of channeling experiments with molecules for the observation of plasmon excitation. 12 In this work, we take advantage of the vicinage effect 11,13 observed in the interaction of ionic hydrogen clusters with thin films to enhance the changes in the energy spectrum that take place at the energy threshold for plasmon excitation, since the cross section for plasmon production is very sensitive to the interference arising from the moving fragments.…”

Plasmon excitation in the interaction of protons and electrons with clean and potassium-covered Al surfaces

“…The two collective modes are better discernable in the first derivative dN/dE of the electron spectra. [11][12][13]16 As it could be expected, 25 the surface plasmon contribution presents a stronger attenuation with K coverage, i.e., at 25% monolayer ͑ML͒ it becomes barely observable while the bulk plasmon intensity has decreased slightly. Measurements carried on at different incident directions presented a similar behavior.…”

“…10,12,13,16 The two structures are better discernable in the derivative spectra ͓Figs. 2͑b͒ and 2͑d͔͒.…”

“…The fact that surface and bulk plasmon structures are seen in the whole energy range, starting at 5 keV, cannot be explained by a direct kinetic mechanism, and as discussed previously, 10,12,14,16,17 other processes must be invoked.…”

“…16,17 In the case of plasmons excited by highly charged ions or low-energy ions, potential effects such as plasmon-assisted electron capture and loss must be invoked. 8,10,13,20 For low-energy protons Ritzau et al 10 and later Sánchez et al 12 proposed a kinetic process where the excited secondary electrons of the metal are responsible for the excitation of the volume plasmon. Niehaus et al 18 and Winter et al 17 observed more prominent structures in a direction close to the sample normal which were ascribed to interference effects of Bloch waves.…”