Determining accurate laser intensity dependence despite the intensity-volume effect with z-scanning intensity-difference spectra*

A focal spot or z-scanning intensity-difference spectrum method is developed to allow the determination of the intensity dependence of laser-produced features while improving experimental statistics. This method is applicable to a focused Gaussian beam interacting with an approximately uniform planar target. We apply this method to the angularly resolved kinetic energy release spectra of laser-induced dissociation of O$_{2}^{+}$ and H$_{2}^{+}$ so as to keep the exact contribution from a predetermined intensity range and eliminate the contributions from outside this range.