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Electrostatic Lenses
Published in Orloff Jon, Handbook of Charged Particle Optics, 2017
Suitable computer-aided design methods became available for potential computations only after 1970. Now it is easy to calculate the field of electrostatic lenses numerically on personal computers (see Appendix A). For rotationally symmetric lenses, if the correct definition of boundary conditions is used, smooth fields and better than 0.1% accuracy in both potential and its first derivative can be obtained by any of the available numerical methods. The finite difference method (FDM) is traditionally the easiest to use; SIMION is quite a popular program for this and can use square meshes with several million points (Dahl and Delmore, 1994; Dahl, 2000), a significant improvement over the previous standard 16,000 (Dahl and Delmore, 1988; Heddle, 1991, 2000), (Figure 5.6). More freedom in the selection of mesh is provided by the finite element method (FEM). For example, the first-order FEM using meshes with graded step size and with up to 1,000,000 points runs on a fast PC within ~1 min (Lencová, 1995a; Lencová and Zlámal, 2007). Accuracy of the potential or field can be simply determined by comparing the potential values with the results computed in a mesh using twice the number of points in each direction (Lencová, 2002). The second-order FEM allows one to use curved as well as straight lines (Zhu and Munro, 1995). The highest accuracy in potential and field values is provided by the charge density method (Renau et al., 1982; Kasper, 1987; Martinez and Sancho, 1991; Read, 1996). Cubric et al. (1998) compared the accuracy of all three methods on suitably selected examples.
Total electron ionization cross-sections for molecules of astrochemical interest
Published in Molecular Physics, 2019
Weiwei Zhou, Lorna Wilkinson, Jason W. L. Lee, David Heathcote, Claire Vallance
Due to the advanced age of the previous incarnation of our beam-gas instrument, all internal components were redesigned, refabricated, and reoptimised as part of the present work. Extensive SIMION electron and ion trajectory simulations were performed in order to optimise the geometry of the electrostatic lens elements, collision cell, and detector in order to ensure complete detection of the electrons transmitted through the collision cell and the ions formed in the electron-molecule collisions. Simulated electron and ion trajectories for a nominal electron energy of 100 eV are superimposed on the diagram of the instrument shown in Figure 1.
Post extraction inversion slice imaging for 3D velocity map imaging experiments
Published in Molecular Physics, 2021
Felix Allum, Robert Mason, Michael Burt, Craig S. Slater, Eleanor Squires, Benjamin Winter, Mark Brouard
By modifying the delay of the pulsed voltage, conditions can be optimised for a particular m/z value or a small range of values. Figure 6 displays time-of-flight spectra and sliced ion images for the S(D) and CO(J = 38) photofragments recorded around 230.01 nm. In panel (a) no pulsed voltage is applied, in (b) a pulse of 3 kV is applied at of 3.10 μs (optimised for the S ion), in (c) the pulse is applied at of 2.92 μs (optimised for the CO ion). These timings were predicted using SIMION and verified experimentally, and a repeller voltage of 1 kV was used in all cases. Here, significant broadening of the time of flight peak(s) and the resulting improved resolution in the central slice images can be seen clearly. In the S(D) image, more fine-structure can be resolved when the PEISI technique is used to temporally stretch the ion cloud. The observation of many rings within the lower radius feature in panel (b) arises from S(D) fragments paired with different rotational states of the CO cofragment (see Figure 4 for a representative CO rotational distribution). In particular, features at low radius/velocity are sharper than in the DC case. This is due to the fact that the imaged central slice represents a far larger solid angle of the three-dimensional velocity distribution for slower ions in comparison to the fast ions, and so the better slicing resolution of the PEISI approach is of greatest importance here. This can be seen in comparing the kinetic energy distributions of the S(D) fragment extracted from the images in Figure 6(a,b), which are plotted in Figure 7. Evenly spaced peaks, corresponding to different rotational states of the CO cofragment can be more clearly distinguished in the PEISI case. The photofragment anisotropy parameter, β, can be extracted for each feature, which are also plotted. For the majority of the resolved peaks, a β of around 1.1 is observed, in reasonable agreement with previous studies [52,69,70]. This rapidly changes for the two lowest radius features, which have far more isotropic angular distributions. A full description of the dynamics responsible for this is beyond the scope of the current manuscript, however such effects have been recently explored in detail by Janssen and coworkers [54,55]. At higher radius, rings corresponding to individual CO rotational states can no longer be resolved. However, the observed β of averaged over the high S velocity feature is in good agreement with literature values, such as the 0.24 and 0.71 reported by Janssen and coworkers for OCS molecules with zero or one quanta of excitation in the bending mode [49].