Absolute density-profile tomography of molecular beams using multiphoton ionization

We describe an approach for the absolute density measurement of rotationally symmetric molecular beams via multiphoton ionization. This simple single-projection tomographic technique requires only knowledge of the spatial intensity profile and ionization characteristics of the focused laser beam that probes the pulsed molecular jet. Multiphoton ionization (MPI) of a xenon beam allowed tomographic reconstruction of a two-dimensional density profile with a peak density of (4.2±0.4)x1018 m-3, which was compared with the theoretical predictions of the sudden freeze model. An analytic solution to the Abel transform is derived for Gaussian projected density profiles which greatly simplifies the reconstruction of the absolute radial density. MPI is sufficiently general that this technique can be readily applied to atomic beams with a broad range of chemistries.