Study of the mechanism of the negative secondary ionic emission by selective superficial oxidation and adsorption

Abstract

With the aid of a speed analyser mass spectrograph able to function in an ultra vacuum, it was shown in previous work by conducting a study of the speed of ejection of secondary ions O− emitted under the impact of ions K+ by copper target, that under certain experimental conditions at least two different origins of this emission could be demonstrated. The hypothesis that we thus adopted was that the O− ions having the greatest ejection speed could originate from the layer adsorbed on the surface, whereas the O− ions having the weakest ejection speed could originate from the oxide. In order to confirm this hypothesis, we conducted the following experiments. We oxidized a copper target with a natural isotope 188O of oxygen. The spectrogram of the negative secondary emission involved then, in addition to the ions of the adsorbed layers (H−, C−, CH−, 168O−, 168O−,...) the ion 168OH−, which could only originate from a chemical compound formed at the time of the reaction of the oxidation. In a second series of experiments, we introduced molecular oxygen 188O2 into the system, beginning with an initial pressure of 10−8 torr up to a pressure of 10−5 torr. The mass spectrogram again contained, over and above the aforementioned ions, the ion 188O−. Here this ion could only originate from the adsorbed layers, since the experimental conditions favored the chemical and physical adsorptions. In addition, the energy analysis of the ions 188O− obtained in the two instances allows us to say that the ion O− originating from the rupture of the chemical junction Cu2O has a weaker speed of ejection than the ion O− originating from the adsorbed layer of gas. In conclusion, the experiment shows that the energy transfer from the incident particle to the ejected particle, by means of the atoms in the target, leads to a speed of ejection which is discrete but weak, in the case of the particle originating from the rupture of a chemical junction; whereas in the case of particles originating from the adsorbed layers of gas, the speeds of ejection are greater and have a widespread distribution.