The determination of accurate size distributions and chemical composition of volatile and semivolatile ultrafine aerosol particles with sizes in the subnanometer to several tens of nanometers range is a problem that plagues many studies in aerosol research. We propose to employ sodium-doping of the aerosol particles with subsequent photoionization in the ultraviolet combined with mass spectrometric detection to solve this problem. Comparison with “soft” EUV ionization demonstrates that this technique can determine size distributions and to some extent the chemical composition of weakly bound ultrafine aerosol particles largely destruction-free. We also discuss how sodium-doping can be turned into a viable quantitative technique for the sizing of ultrafine aerosol particles.

The first vibrationally resolved pulsed-field-ionisation zero-kinetic-energy (PFI-ZEKE) photoelectron spectrum of difluoromethane from its adiabatic ionisation potential (formation of the C(2v) conformer of CH(2)F(2)(+)) to the onset of the first ionic fragmentation channel is presented. Precise values for the adiabatic ionisation potential (12.7252 +/- 0.0009 eV) and the appearance potentials of the H loss product (13.065 +/- 0.003 eV) and the F loss product (14.30 +/- 0.06 eV) of the cation are reported. Ab initio harmonic calculations were performed at the MP2 level with quadruple-zeta basis sets in an attempt to assign the newly observed vibrational structure which, in its previously published low resolution form, led to numerous speculations regarding its true origin. The adiabatic ionisation potential and the fragmentation appearance potentials for the three lowest dissociation channels are also predicted in the complete basis set limit of CCSD(T) theory.

The uptake of sodium and the fragmentation before and after "soft" photoionization with ultraviolet light are investigated for small acetic acid clusters. The acetic acid clusters are generated in a supersonic expansion and ionized with ultraviolet light after doping with sodium in a pick-up chamber. The composition of the bare acetic acid clusters in the molecular beam is determined independently from complementary photoionization experiments using extreme ultraviolet light. The experimental results are analyzed with the help of density functional calculations for energetics and statistical adiabatic channel calculations for fragmentation kinetics. The study demonstrates that the detected ions originate from fragmentation in the neutral as well as in the ionic state, and in particular that the fragmentation pathway strongly depends on the cluster size.

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Pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy and supersonic cooling are used to investigate the CH(3) torsional dynamics of the acetic acid cation and to determine an accurate value for the first adiabatic ionization potential of acetic acid (IP=85 912+/-5 cm(-1)), which has been the subject of debates for more than 40 yr. A doubling of the torsional barrier upon ionization is due to a significant shortening of the C-C bond and reduces the tunneling efficiency by an order of magnitude.