Use of positive ion appearance curves for energy scale calibration in electron beam experiments

Linearly extrapolated ion appearance curves are frequently used to obtain a corrected electron energy scale in electron beam experiments. The relative merits of the rare gases are critically examined using the published total ion collection data of Rapp and Englander‐Golden. For each gas the error in the linearly extrapolated threshold has been calculated as a function of the extent of the data included in the linear regression fit. The results clearly indicate that He is the best choice in this context, allowing the linear fit to be extended to 8 eV above threshold without introducing any significant error. With Xe the error is less than 0.07 eV, provided the fitted data does not extend beyond 7 eV above the threshold. The worst choice is argon, for which the error increases rapidly with the range of the fit, and can exceed 0.3 eV.