SELF-CALIBRATING METHOD FOR MEASURING LOCAL MULTIPHOTON-IONIZATION YIELDS AS A FUNCTION OF ABSOLUTE INTENSITY

We present a self-calibrating method for measuring local multiphoton-i onization yields as a function of absolute intensity. In contrast to t he method recently described by Walker et al. [Phys. Rev. A 57, R701 ( 1998)], our method does not require any assumption on the intensity di stribution inside a laser focus, nor does it use any mathematical proc edure such as deconvolution that would be based on such an assumption. In this sense, our method is self-calibrating. The proposed method im mediately gives ion yields as a function of absolute intensity. Furthe rmore, it allows the intensity distribution inside the focal volume to be measured with a spatial resolution of a few mu m. The proposed met hod uses a five-grid high-resolution reflecting time-of-flight ion spe ctrometer, in combination with an electron spectrometer. The advanced design of the ion spectrometer allows detection of ions originating ex clusively from a well-defined source volume with mu m-size dimensions, thus enabling absolute measurements of ionization probabilities and s aturation intensities. By moving the source volume of the ion spectrom eter through the focal region, we can quantitatively measure local ion densities inside the focus. The corresponding spatial absolute intens ity distribution is measured by electron-ion coincidence measurements via the ponderomotive shifts in the electron spectrum of a suitable ta rget gas, e.g., He. Both aspects of the proposed method (ion measureme nts from a confined volume and intensity measurements based on pondero motive shifts) have been successfully applied in the past. [S1050-2937 (98)09408-6].