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-ionization yields as a function of absolute intensity. In contrast to the method recently described by Walker et al. [Phys. Rev. A 57, R701 (1998)], our method does not require any assumption on the intensity distribution inside a laser focus, nor does it use any mathematical procedure such as deconvolution that would be based on such an assumption. In this sense, our method is self-calibrating. The proposed method immediately gives ion yields as a function of absolute intensity. Furthermore, it allows the intensity distribution inside the focal volume to be measured with a spatial resolution of a few μm. The proposed method uses a five-grid high-resolution reflecting time-of-flight ion spectrometer, in combination with an electron spectrometer. The advanced design of the ion spectrometer allows detection of ions originating exclusively from a well-defined source volume with μm-size dimensions, thus enabling absolute measurements of ionization probabilities and saturation intensities. By moving the source volume of the ion spectrometer through the focal region, we can quantitatively measure local ion densities inside the focus. The corresponding spatial absolute intensity distribution is measured by electron-ion coincidence measurements via the ponderomotive shifts in the electron spectrum of a suitable target gas, e.g., He. Both aspects of the proposed method (ion measurements from a confined volume and intensity measurements based on ponderomotive shifts) have been successfully applied in the past.