A high sensitivity, high resolution tandem mass spectrometer to research low-energy, reactive ion–surface interactions

Abstract

The device described is the combination of two mass spectrometers, with a surface sample placed between them. Its aim is to allow for detailed research on low-energy ion–surface interactions, involving and triggering surface chemistry. This task is fulfilled by a carefully chosen geometry: Projectile ions from an electron impact source are mass-per-charge selected using a quadrupole. Such continuous bombardment allows for good control of the surface condition. Species emerging from the collisions are focused onto a beam and analyzed using a purpose-built orthogonal pulsing time-of-flight mass spectrometer. Neutral species can be post-ionized using a second electron impact source. Neutral gases can be adsorbed to the surface from the gas phase in a controlled manner, using a feedback-controlled pressure regulator. In order to minimize the discrimination of secondary ions, the distance from the surface to the analyzing mass spectrometer system was kept as short as possible and the acceptance angle of the lens system as large as possible. This increased the sensitivity five orders of magnitude compared to its predecessor. The rigorous use of computer aided design software is responsible for the successful commissioning of the new device. This article describes first which parameters can be measured or controlled. Then, these are linked to the physical processes that occur in reactive ion–surface interactions. Next, the design goal and the design implementation are presented. In the end, a performance comparison, measurements of hydrogen surface chemistry with extensive use of isotope labeling, and measurements of post-ionized beryllium are presented.

Formation of HCN+ in collisions of N+ and N2+ with a self-assembled propanethiol surface on gold

“…The new apparatus SurfTOF and its performance were in detail described elsewhere. 33 Briefly, projectile ions from an electron impact source are mass-per-charge selected using a quadrupole Pfeiffer QMA 400 filter. The projectile beam then hits the surface (in the experiments described here under an angle of 451) with an incident energy adjustable from a few eV to several hundred eV.…”

“…In the meantime, however, BESTOF, the apparatus for studying reactive ion-surface collisions, was replaced by a considerably more advanced machine, designated SurfTOF (Surface Time-of-flight Mass Spectrometer). 33 The main advantages of this new apparatus are higher sensitivity, higher resolution, better vacuum conditions, and much higher intensity of the incident projectile beam then the apparatus BESTOF could provide (by several orders of magnitude).…”