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A new photoelectron/photoion spectrometer for the characterisation of molecules and clusters using XUV and UV radiation Forysinski, Piotr Wojciech


A new photoelectron/photoion spectrometer with a light source tuneable from 17.9 eV to 1.5 eV (bandwidth ≲ 0.7 cm⁻¹) for the characterisation of molecules, clusters and ultrafine aerosol particles is described. The first vibrationally resolved PFI-ZEKE (pulsed field ionisation-zero kinetic energy) photoelectron spectra of acetic acid and difluoromethane are presented, which in the case of acetic acid leads to a revision of earlier values for the adiabatic ionisation energy. For the difluoromethane cation a long-standing mystery regarding its vibrational progression previously observed in low resolution photoelectron spectra is unravelled, and the vibrationally resolved PFI-ZEKE photoelectron spectrum is assigned based on full dimensional anharmonic calculations. A sodium oven was incorporated into the spectrometer to dope clusters and ultrafine aerosol particles with a single atom of sodium. Doped species are subsequently ionised with a single UV (ultraviolet) photon. The composition of the molecular beam can be assessed independently using XUV (extreme ultraviolet) radiation. The results of comparing these two techniques for the first time on the case study of small acetic acid clusters are presented. Fragmentation processes which occur following sodium capture and subsequent photoionisation are strongly dependent on the particular cluster. For the small acetic acid clusters studied (n ≤ 8) the total number of monomer units which evaporate from the sodium doped cluster is determined to be ≲ 4. This amount of evaporation is insignificant for larger clusters and ultrafine aerosol particles. An almost “fragmentation free” particle sizer for volatile ultrafine aerosol particles is thus proposed based on the sodium doping technique. As a first step towards this goal it is shown that the sodium doping technique can indeed be applied to ultrafine aerosol particles. On the examples of large clusters of acetic acid and dimethyl ether, the technique is demonstrated to preserve the size and chemical composition of the clusters. This is in contrast to other ionisation techniques such as XUV ionisation, for which substantial cluster fragmentation is observed.

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