Fs mass spectrometry and photoionization

Time-of-flight mass spectrometry (Tof-MS) is a well established technique for determining ions masses and their relative concentrations. Tof-MS begins with gas-phase ion formation and then involves ion acceleration through an electric field into a field-free drift region. The acceleration voltage, giving all ions the same kinetic energy, propels the ions at different velocities based on their specific mass-to-charge ratios. Subsequently, the ions are allowed to drift in a field free region where they separate spatially as a function of their individual kinetic energies. Lighter ions will move faster than heavier ions. Following ion detection, peaks representing ion-packets each having the same mass comprise the mass spectrum, and the time needed to travel the length of the drift region can be related to the mass of the ion.

Figures: Time of flight fs multiphoton ionization setup and a corresponding ion mass spectrum from an Zn ablation source. The source can be exchanged by either a pyrolytic, a photolytic or a discharge source.

In the context of renewable energy storage processes, the Zn/ZnO redox-cycle gained much attention in the last years. The aim is to store solar thermal energy in the form of metallic zinc by dissociation of zinc oxide in a solar heated reactor. The process inside the reactor comprises in essence two steps: evaporation and dissociation of solid ZnO(s), ZnO(s) --> Zn(g) + O and subsequent rapid cooling with condensation of zinc, Zn(g) --> Zn(s). Due to re-oxidation, possibly accelerated already in the gas phase by the presence of emanating zinc oxide clusters, the efficiency of the overall energy storage yield may be rather poor unless adequately rapid quenching of zinc can be realized.

Neutral ZnO and ZnOH molecules could be produced in a molecular beam by expansion of laser ablated zinc together with H₂O, O₂ or N₂O seeded in a rare gas (Ar, Ne, He). Due to the characteristic Zn isotope distribution, the zinc containing compounds, ionized with a 100 fs laser pulse, could unambiguously be identified with a TOF mass spectrometer. The abundance of ZnOH produced in our experiments exceeds the one of ZnO and ZnN by orders of magnitude if H₂O is present in the system. Small quantities of (ZnO)₂H and Zn₂(OH)₃ compounds could also be observed. To our knowledge this is the first evidence for the occurrence of neutral ZnO and ZnOH molecules in a molecular beam.

D. Cannavo, G. Knopp, P. Radi, P. Beaud, M. Tulej, P. Bodek, T. Gerber, and A. Wokaun, Journal of Molecular Structure, 782, 67-72 (2006).

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