Liquid microjets can be formed by a cylindrical glass capillary (ø=10-50 μm) positioned in vacuum. Supported by a high backing pressure the liquid jet shoots into vacuum at a typical speed of 100 m/s. The jet is intersected by the light source (typically synchrotron radiation) just after ejection into vacuum (i.e. when the jet is still laminar); the generated photoelectrons travel through a skimmed orifice leading to the detector region. The short distance between jet and skimmer (≈0.5 mm) and the strong differential pumping in the detector region enables detection of undisturbed electrons emitted from the jet. Our new setup is a highly flexible version of the traditional liquid jet photoelectron spectroscopy setup. The geometric flexibility around two lab frame axis enables us to perform two variations of angular dependent measurements, where we either study conventional orbital anisotropies or the newly recognized dichroic asymmetries. (Photo of the new setup is coming up!)
Liquid jet photoelectron spectroscopy is an excellent tool to probe electronic structures of liquid samples by direct photo-induced electron emission, but the subsequent (ultrafast) electron dynamics induced by ionization also give rise to secondary electrons via electronic decay and auto-ionization. Molecular-level information on electronic structure properties and intermolecular interactions can thus be obtained from the energetic and angular distributions of the detected electrons arising from direct as well as indirect processes.
Binding energies of valence as well as core electrons are sensitive to the local environment surrounding the molecule. Chemical shifts of the orbital energies either with respect to gas phase values or as a function of a solution parameters (pH, solute concentration, temperature, etc.) can thus reveal important information on aspects such as solvation and solvation dependent conformational changes. Solvent effects comprises non-specific electrostatic interactions, specific interaction such as hydrogen bonding, and cooperative effects, and these effects can all influence both conformational and electronic structures and thus have high impact on chemical reactivity.
A notable feature of photoelectron spectroscopy is the possibility to sample electronic structure information from different depths into the solution, and thereby to investigate changes in solute densities across the solution interface. This is for example important for atmospherically relevant chemistry (eg. aerosol chemistry), as surface active solutes can alter surface and interfacial chemistry drastically.
Upon core ionization of a molecule highly excited cations are formed; in the liquid phase both local and non-local (ultrafast) decay processes contribute in a competition that depend on solvation configuration and the nature of the intermolecular interactions that form the immediate solvation shell.
Local Auger decay often dominates over non-local processes; in a conventional Auger-process the core-hole is refilled by a local electron, the relaxation energy is then used to eject another valence electron from the same molecule thus forming a dication. The alternative non-local processes comprise intermolecular Columbic decay (ICD), proton-transfer-mediated charge separation (PTM-CS), and electron-transfer-mediated-decay (ETMD) and all produce ultrafast charge delocalization.