Exact determination of spatially resolved ion concentrations in focused laser beams

The operation principle and the performance of a variable-volume selective TOF mass spectrometer are discussed in detail. Lateral confinement is achieved with an appropriate entrance aperture. Width and location of the axial confinement are established via the energy filtering capacity of a newly developed five-grid ion mirror. The minimum dimensions are in the μm range. The thus defined detection volume ΔV, can be scanned through a laser beam. Complete ionization of target gases such as SO2 and Xe within ΔV, permits calibration of the overall detection sensitivity. This instrument can be utilized to measure spatially resolved ion concentrations, absolute ionization probabilities and the intensity dependence of ion yields which are not affected by volume expansion. As an example we investigate optical field ionization of water with a Ti:Sa laser at intensities up to 1016 W/cm2. Published by Elsevier Science B.V.