A velocity map imaging photoelectron spectrometer for the study of ultrafine aerosols with a table-top VUV laser and Na-doping for particle sizing applied to dimethyl ether condensation.

We present a new experimental configuration for the study of size-dependent, angle-resolved photoelectron and photoion spectra of weakly bound ultrafine aerosol particles targeted at particle sizes below ~20 nm. It combines single photon ionization by a tunable, table-top vacuum ultraviolet laser at energies up to 18 eV with velocity map imaging detection and independent size determination of the aerosol particles using the Na-doping method. As an example, the size-dependence of the valence photoelectron spectrum of dimethyl ether clusters and ultrafine aerosols is investigated. Up to a mean particle diameter of ~3-4 nm, the first ionization energy (value at band maximum) decreases systematically (up to ~1 eV) and the corresponding band broadens systematically (up to a factor of ~3) with increasing aggregate size. Plateau values for band positions and bandwidths are reached beyond a diameter of ~3-4 nm. Experimental evidence for the dominance of the fast intermolecular proton transfer over monomer fragmentation reactions upon ionization is presented via photoion imaging.