Coulomb expansion of an Ar ῌ and Xe ῌ ion mixture in a Time-of-flight (TOF) spectrometer was studied using N-body simulations. We found that Ar ῌ ions form a shell structure around a Xe ῌ ball, a result which was not observed for the case of one ion species. The time evolution of the Coulomb expansion of the Ar ῌ ion shell as well as the Xe ῌ ion ball has been evaluated. The simulations successfully reproduced a distorted double peak in the TOF spectra, caused by the initial turnaround time,

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... ch is known to appear for mixed gas samples when the applied extraction voltage is low. Our preliminary experiments convincingly explained the simulation results. The space-charge e#ect has also been estimated quantitatively when more than one ion species is present. These results are useful for understanding the dynamical behavior of high-field laser ionization experiments in which multi-component ions are produced.