Helium and neon implantation and memory observed in a quadrupole mass spectrometer

High accuracy static quadrupole mass spectrometer (QMS) measurement of heli um and neon may be impaired by implantation and long lasting thermal desorp tion effects of the previously trapped atoms. A QMS produces only moderatel y accelerated ions of 100-200 eV. The ions decelerate at stainless steel su rfaces somewhere in the QMS and a part of them will be trapped in surface n ear sites. For helium and neon most of these trapped atoms are able to deso rb with time constants of several hours. This causes an memory effect which hinders high precision measurements. The trapping and desorption experimen ts reported here indicate that these time constants for the desorption are about 90-477 min for temperatures between 330 and 470 K. 20-60% (330 K) and 450% (470 K) of the atoms which have been trapped desorb subsequently. Thi s could be explained by assuming self-sputtering processes. From the desorp tion behaviour a binding energy of 1.1 and 1.6 eV for these atoms has been estimated. (C) 1999 Elsevier Science B.V. All rights reserved.