Maximal kinetic energy and angular distribution analysis of spatial map imaging: Application to photoelectrons from a single quantum state of H2O

Abstract

Dynamical or spatial properties of charged species can be obtained using electrostatic lenses by velocity map imaging (VMI) or spatial map imaging (SMI), respectively. Here, we report an approach for extracting dynamical and spatial information from patterns in SMI images that map the initial coordinates, velocity vectors, and angular distributions of charged particles onto the detector, using the same apparatus as in VMI. Deciphering these patterns required analysis and modeling, involving both their predictions from convolved spatial and velocity distributions and fitting observed images to kinetic energies (KEs) and anisotropy parameters (βs). As the first demonstration of this capability of SMI, the ensuing photoelectrons resulting from (2 + 1) resonant ionization of water in a selected rotational state were chosen to provide a rigorous basis for comparison to VMI. Operation with low acceleration voltages led to a measured SMI pattern with a unique vertical intensity profile that could be least-squares fitted to yield KE and β, in good agreement with VMI measurement. Due to the potential for improved resolution and the extended KE range achievable by this new technique, we expect that it might augment VMI in applications that require the analysis of charged particles and particularly in processes with high KE release.

Kinetic Energy-Broadened Spatial Map Imaging for Recovering Dynamical Information

“…The KESMIs exhibit different widths with high-intensity horizontal lines in their centers and slightly changing intensity in the vertical direction until the occurrence of a steep decrease. As we have shown very recently, the different widths in the measured KESMIs of Figure (a–d) and the sharp intensity decrease in the SST vertical profiles along the y axis, Figure (e–h), exhibited by the blue dots, are related to changes in the KE SST of the ensuing PEs (see below), following the above excitations.…”

“…KESMI and VMI measurements are performed by the TOFMS used in our previous studies. ,, This TOFMS includes a source and a differentially pumped detection chamber, separated by a skimmer. The AB/MB are produced by either neat Ar buffer gas at a stagnation pressure of 2 bar or by passing it through H 2 O vapor.…”

“…Charged particle trajectories were simulated using the 3D SIMION package, incorporating the TOFMS geometry and the voltage settings of the electrostatic lens under different KESMI operational modes, i.e., SST and DST. This was done by using the initial conditions of the charged particles and the spherical and Cartesian coordinate systems, Figure (a), where φ is clockwise relative to the y axis instead of counterclockwise in SIMION and ref . The patterns formed on the detector by the trajectory end points were analyzed and modeled via the described MATLAB code.…”

“…Intriguingly, in addition to the use of SMI for monitoring suppressed velocity patterns of probed parent ions, we have shown that by employing a larger V E / V R ratio, with a steeper field gradient between V E and V G (for a similar V R as in VMI), narrow stripes form. For photofragments and PEs released with higher KEs, vertically broadened SMIs appear, which can provide additional information at inherently improved resolution and extended dynamic range. We derived these properties by considering the primary advantage of SMI, providing linear energy mapping compared to velocity in VMI, and suggested that these benefits may offer particular significance in measurements where high KE particles are released while applying lower electrode voltages …”

“…For photofragments and PEs released with higher KEs, vertically broadened SMIs appear, which can provide additional information at inherently improved resolution and extended dynamic range. We derived these properties by considering the primary advantage of SMI, providing linear energy mapping compared to velocity in VMI, and suggested that these benefits may offer particular significance in measurements where high KE particles are released while applying lower electrode voltages …”