Calculating time-of-flight spectra of post-ionized sputtered neutrals

Abstract

We have developed a computational approach to estimate the three-dimensional number density distribution of sputtered atoms above a solid target surface. This distribution is a function of pulsed primary ion beam characteristics, such as the time profile of the ion pulse, and the size, shape and current profile of the ion beam spot. The model is also a function of laser post-ionization conditions, namely, the delay time between the end of the ion pulse and the laser shot. The calculations, based on the collision cascade theory of sputtering, are designed to represent accurately the ion cloud produced when sputtered neutrals are photo-ionized. This mathematical framework is important for modeling analytical instruments based on ion sputtering, particularly for pulsed draw out time-of-flight (TOF) mass spectrometry instruments. The output of the calculations is structured for easy input of the derived 3D distribution into ion optics simulation packages. To test the accuracy of the approach, we calculated the number density distribution of Ca atoms sputtered by normal incident 4 keV Ar + ions from a pure Ca target and used the results as input for a SIMION 3D © computer model of an existing TOF instrument. The model was then used to simulate useful yield estimates and TOF mass spectra and to compare them to actual experimental measurements. Good agreement was observed between the model calculations and the experiments. The developed modeling technique has been used to aid in the design of a new TOF MS instrument at Argonne National Laboratory.