A Study of a Positron Lifetime in a Rare Gas Adsobed on a HOPG Surface

我们一直在使用能量有价值的脉冲正电子束测量正电子寿命谱。它使我们能够在正电子停留在表面附近的条件下测量反映表面电子密度的正电子寿命。我们发现在HOPG（高取向热解石墨）表面比在体中有一个特定的长寿命。当样品被加热时，它变得更短，并且观察到一个正电子从表面发射到真空中。因此，我们曾经认为正电子处于表面俘获态，可以通过加热的方式射出，最终以正电子捕获电子的形式发射到真空中。然而，通过在特高压条件下引入正电子俄歇谱的研究，我们新发现上述假设是由表面吸附的杂质引起的。即使从电子俄歇能谱上我们认为是干净的样品，在正电子俄歇能谱上我们也可以观察到更多的氧原子和碳原子（HOPG的组成部分）的核信号。它利用一个正电子与表面组成原子的一个核心电子对湮灭所产生的真空。认为正电子本身倾向于接近表面的h2o或碳氢化合物的氧原子。当样品被加热时，这些氧信号减少，正电子寿命变短，发射的正电子增加。从这些明显的相关性中，我们得出结论，正电子在表面上的行为没有被很好地理解的原因是意想不到的吸附剂，其数量低于普通技术的检测极限。只有脉冲束的正电子俄歇光谱才能使我们探测到原子序数更小的原子，这对实际应用很重要，因为束的强度，这在世界上是第一次做到的。此外，我们可以以非常高的灵敏度检测到普通技术无法检测到的极少量的氧气。这表明，它是一个强大的工具，表面诊断杂质的非常少量。少

We have been measuring positron lifetime spectra using an energy valuable pulsed positron beam. It enabled us to measure a positron lifetime reflecting a surface electron density under the condition the positron stops in the neighborhood of the surface. We had found out a specific long lifetime on the HOPG (highly oriented pyrolytic graphite) surface compared with that in the bulk. When the sample was heated, it had become shorter and a positronium emitted from the surface into the vacuum was observed. Thereby, we had once considered that a positron occupies the surface trapped state, and it can be exited by heating and finally emitted into the vacuum as positronium capturing an electron.However, from the present study introducing a positron Auger spectroscopy under the UHV condition, we have newly found out that the above hypothesis resulted from adsorbed impurities on the surface. Even for the sample which we regarded as clean from an electron Auger spectroscopy, We could observe a c … More lear signal from oxygen atoms as well as carbon atoms which is constituent of HOPG by positron Auger spectroscopy. It utilizes a vacany produced by a pair annihilation of a positron with a core electron of the constiuent atoms of the surface. It is regarded that a positron itself tends to approach an oxygen atom of H_2O or hydrocarbons on the surface. When the sample was heated, these oxygen signals reduced, the positron lifetime became shorter, and an emitted positronium increased. From these apparent correlations, we concluded that the reason why the positron's behavior on the surface was not so well understood is the unexpected adsorbants whose quantities are less than the detection limit of the ordinary technique.Only a positron Auger spectroscopy with the pulsed beam enables us to detect atoms with smaller atomic numbers which are important for practical applications, because of the beam intensity, and such was done for the first time in the world. Moreover, we can detect with very high sensitivity the oxygen of a very small amount which is undetectable by the ordinary technique. This suggests that it is a powerful tool for the surface diagnosis of impurities of a very small amount. Less