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UBC Theses and Dissertations

Simulation of drifting gas-phase ions of arbitrary shape Chen, Xiu


This thesis is mainly devoted to the description of the first attempt to extend the simulation of drifting ions from the treatment of linear ions to that of arbitrary-shaped ones. The simulation is based on molecular dynamics using classical equations of motion. The simulation results for NO⁺ drifting in argon are presented. The accuracy of an approximate distribution function is examined in detail. It is found to give results that are generally qualitatively correct, and for many properties, semiquantitatively correct as well. The agreement, however, is not as good as that found for systems with ion-bath gas ratios greatly exceeding unity. The ion-bath gas mass ratio for NO⁺-Ar is close to but less than unity, thus increasing the importance of inelastic collisions. The perpendicular velocity coupling term [formula not included] has a minimum which displays a general behaviour of drifting ions missed in the study of NO⁺-He. Strong velocity-angular momentum coupling is found, and in particular the quadrupolar alignment parameter as a function of the velocity parallel with the field takes on, with decreasing velocities, values that start as negative, become positive, and subsequently decay toward negative. To the best of our knowledge, this is the first report of the decay of this alignment towards negative values at the low end of the velocity distribution. In addition to NO⁺-Ar results, simulations of H₂O⁺-He are also presented. This system was used to test modifications of the code to simulate ions of arbitrary shape. An ab initio potential energy surface for H₂O⁺-He was calculated. The mobilities at different field strengths is reported.

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