The purpose of this project is to look at electron and photon impact ionisation & dissociation of molecules and clusters that are of interest to plasma physics, atmospheric physics and other fields. The experiment consists of a differentially pumped vacuum system, with an expansion chamber to generate a pulsed supersonic beam of molecules or clusters, and a collision chamber where the molecules or clusters are fragmented. In the collision chamber the supersonic beam is crossed with an electron beam and the reaction products (ions and neutral metastable atoms) are detected.
A reflectron time-of-flight mass spectrometer with a microchannel plate detector is used for the mass-resolved detection of ions. Neutral atoms, radicals and cluster fragments in metastable or high-lying Rydberg states are detected using a channeltron. The technique is suitable for states with comparatively long lifetimes (t > 1 ms), and with excitation energies larger than 8 eV (or 5 eV with specially prepared surfaces).
Figure 1. Overview of the vacuum system.
Computer-controlled data-acquisition techniques are employed in order to be able to measure time-of-flight distributions. In the case of the ions the flight time is proportional to the ion mass. In the case of the neutral metastable fragments the flight time is inversely proportional to the kinetic energy the fragment acquired in the fragmentation process. With a simple transformation kinetic energy distributions of the metastable fragments can be obtained from the measured time-of-flight distributions. The time-of-flight spectra are recorded by a fast multichannel scaler (FastComtec 7886S, 1 ns per channel). The timings and delays of the supersonic beam pulse, the electron pulse, and the start pulse for the multichannel scaler are controlled by a digital delay generator (Stanford DG535).
Figure 2. Mass spectra of methanol clusters and adenine molecules obtained by electron impact.
Students with further queries regarding research or applications for M.Sc. or Ph.D. on this topic should contact Dr. Peter van der Burgt.