Abstract: In the multi-photon ionization experiment, the parent ion peak of time-of-flight (TOF) was observed to become broader with increasing laser power. Briefly, this phenomenon is caused by the space-charge effect of ions. A qualitative theoretical model, that the ion cloud created by the laser has a spherical form and the initial charge distribution is Gaussian, is used to evaluate the full width at half maximum (FWHM), which depends on the laser intensity, the pressure of the sample gas, the extraction electric field, the ionic mass, and the focal length of the lens employed. The proposed model agrees well with the experimental data.

Key words: TOF, Coulomb expansion, Space-charge effect