An ion optical system for the detection of charged fragments with high acceptance in a time-of-flight mass spectrometer

Production of negatively charged fragments in collisions of swift positive hydrogen ions with molecules

“…These extreme operating conditions led in turn to an enhanced production of positive fragments (statistically one per projectile pulse and frequently accompanied by emitted electrons) and were responsible for the large continuous background leading to an estimated error of about 10% for the experimental results, which can be larger for the yields extracted from the lines of very small intensity. Another reason for the increased background in the negative mode of the spectrometer is that electrons, knocked out of the interiour walls of the drift tube by magnetically deflected primary electrons or field emitted electrons from the ion optical elements, are caught by the strong fields of the "Doppelring" ion optics [9] of the final focusing stage and accelerated towards the detector as well.…”

“…1. Basic scheme of the time-of-flight mass spectrometer action region) at whose exit they are post-accelerated by an additional ion optical system [9] and focused on an ion-converter dynode, which they hit with a kinetic energy of 12 keV/q. The emitted secondary electrons are collected and multiplied by a channeltron facing the dynode.…”

Formation of O−2 in charge transfer collisions of fast molecular hydrogen ions with O2 molecules

“…This was controlled by performing measurements under reversed polarity of the spectrometer with methyl chloride as the target gas. Here, the fragment ion H~ always appeared with a probability of more than 80%, irrespective of the projectile's type and energy, accompanied by the partner fragment C1 ÷ [18]; this pair of strongly correlated fragments resulting from the breakup of a single CH3C1 parent molecule from a well-defined doubly ionized state. The two fragments share a mean dissociation energy of approximately 5 eV inversely proportional to their masses according to momentum conservation.…”

“…The negative ions produced in the interaction region are mass analysed through the time-of-flight mass spectrometer, in which their time of production is fixed by pulsing the projectile beam by means of fast high voltage switches (with a width of several nanoseconds and a repetition rate of 20 kHz) while the signal released by the ion reaching the detector stops the multihit time-to-digital converter (LeCroy model 4208) that measures the flight time. Extraction of the product ions is effected under an homogeneous electric field of about 14 V/mm; the ions are then accelerated to a kinetic enery of 3.5 keV and pre-focused by an ion optical system using einzel lenses and "Doppelring" elements [18]; after passage through a drift tube being one meter in length the ions are post-accelerated through the focussing fields of the final "Doppelring" ion optical system and directed through an aperture with a diameter of 1 cm before hitting with a kinetic energy of 15.5 keV on an ion-electron converter dynode [6]. The secondary electrons released are conducted to a channeltron that generates the stop signal.…”

“…Details of the ion source and accelerator stage are given in [17] while the basic design of the time-of-flight mass spectrometer is described in [18], the actual modifications carried out to analyse negative ions being outlined in [6].…”