Direct current slice imaging

We report a new variation of the velocity map ion imaging method that allows the central section of the photofragment ion cloud to be recorded exclusively. The relevant speed and angular distributions for a molecular photodissociation or scattering event may therefore be obtained without need to utilize inversion methods such as the inverse Abel transform. In contrast to the recently reported “slicing” technique of Kitsopoulos and co-workers C. R. Gebhardt et al., Rev. Sci. Instrum. 72, 3848 (2001), our method makes no use of grids or pulsed electric fields which can distort the photofragment cloud and therefore compromise the resolution of velocity mapping. We find that by operating a multilens velocity mapping assembly at low voltages, the ion cloud stretches in the acceleration region owing to the kinetic energy release in the fragments. Furthermore, this inherent stretching is sufficient to allow the central section of the distribution to be recorded exclusively by application of a narrow time gate (∼40 ns) to a position sensitive detector. We have performed extensive ion trajectory simulations to understand this “direct current (dc) slice imaging” technique, and experimentally we have applied it to the 355 nm dissociation of Cl2 and NO2 as well-understood test cases. In the Cl2 studies the velocity resolution obtained for the Cl35 fragments is on the order of Δν/ν=2.8% and for the first time we are able to directly observe dissociation via the weak B 3Π0u+ state channel whilst imaging the ground state Cl(2P3/2)-atom distribution. For the case of NO2 dissociation the internal state distributions of the NO fragment are extracted more cleanly using slicing than is possible with the Abel inversion and our resolution is sufficient to resolve some of the NO rotational structure in the kinetic energy release for the first time. Overall, we find our data to compare very favorably with previously reported results and conclude that dc slice imaging offers an important, easily implemented refinement to the velocity mapping approach. We also demonstrate a second dc slice imaging method that records only the central section of an expanded photofragment distribution by using a probe laser displaced off-axis from the molecular beam. This approach, which we term “raster imaging,” may be particularly advantageous in two-color experiments where the probe laser also makes a significant contribution to the initial photolysis of the molecular species under investigation.