The new design incorporates the negative ion source and the mass analyzer, both constructed from cylindrical electrodes. The ion source is formed by three gridded cylindrical electrodes: a pulsed grid, the intermediate grid and the final accelerating grid. During a first time lapse, the electrons penetrate through the pulsed grid into the retarding field between this grid and the intermediate grid. The electrons are turning at some depth inside this intergrid space, where the attachment to neutral molecules most probably occurs. Next, the pulsed grid becoming strongly negative and ions are extracted towards the final acceleration grid. The ions from the cylindrical surface where they were created concentrate on the common axis of the electrodes (lateral focusing). The source lateral and time focus are coincident. A cylindrical electrostatic mirror is fitted to the source. The design, with a single stage, ensures also lateral focusing of the ions diverging from the common axis of the electrodes. The mirror electric and geometric parameters were selected to ensure both lateral and time focusing on the final detector with subsequent high luminosity. The basic parameters of the specific negative ion source time-of-flight mass analyzer design proposed here, are ion source final acceleration, intermediate, pulsed cylindrical grid radii 10, 20 and 30 mm, respectively, electrostatic mirror earthed grid and ion turning points surface radii 0.6 and 0.8 m, respectively. Ion packet smearing by the ion energy spread (resulting from the initial electron energy spread as electrons are turning at different depths inside the ionization region, from the moment when ions were created, being accelerated towards the pulsed grid during ionization) and by the turnaround time inside the cylindrical field was accounted for. Maintaining very high sensitivity, a resolution of the order of 100 is expected.

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The detected ion peak shape results from the convolution of the initial ion distributions in space, time and energy, transformed from the source to the detector by the time-of-flight (TOF) analyzer.' While the initial distributions are dependent on the action of the ionizing factor and on the conditions in the ion source, the transformation is determined by the mass spectrometer ion optical properties.Detected peak shapes were calculated for time-offlight mass spectrometers incorporating reflectrons, for rectangular time pulses of the ionizing factor and for various initial energy distributions of ions generated from solid plane surfaces. CALCULATION OF THE DETECTED ION CURRENT DISTRIBUTIONSIt is assumed that ionization occurs on a plane surface, or very close to it, during some time interval when the intensity of the primary ionizing current or photon radiation remains unchanged. For ionic processes such as plasma desorption, where ions are individually counted, the errors in the start signal are perceived as an initial time spread.The main peak structure is described by first-order packet components, aberrations being involved in higher-order details.' As reflectrons correct for the first-or even second-order energy spread, the remaining first-order packet components are those due to the time of generation t and to the longitudinal emission velocity component u (similar to half that inducing the 'turn around' time). So the arrival time t at the detector in a reflectron TOF mass spectrometer is given by the simple relation:where u = 2d/uz, d the length of the accelerating space of a single-field ion source, us the longitudinal velocity gained at the source exit grid by a reference ion having no initial velocity. The constant in (1) vanishes if the arrival time origin is taken as the time when the reference ion produced at the beginning of the ionization process is detected.The detected ion charge, q, is proportional to the (1)* Author to whom correspondence should be addressed.number of ions that reached the detector during a pre-determined time period, ions having higher longitudinal velocities arriving first. To sum their number, we integrate in the to, u plane over the domain depicted in Fig. 1. It is limited at the left by the condition (l), by the u axis and the to= T line. The ionization lasts from to = 0 to to = T, u, being the lowest initial velocity of the emitted ions. The detected charge, disregarding a multiplicative constant, can be expressed by the following combination of integralsTo determine the detected charge for 0 < t < T, only the first lower limit of integration, -t/a, must be replaced by zero in the preceding formula.I Figure 1. Integration domain in the to--v plane, that determines the number of detected ions.

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