Liquid Jet Photoelectron Spectroscopy
Our work focuses on the electronic structure and on (soft) X-ray induced electron dynamics of liquid water and solutions, using photo-/Auger-electron spectroscopy from a liquid microjet. Systems of interest extend from small ionic solutes in water and at the solution interface, to biological relevant molecules, such as amino acids and DNA components, molecular surfactants, to acids and bases in aqueous solution. Valence and core-level binding (chemical shifts) energies in the aqueous milieu are reflect solute local structure details as well as the resulting charge transfers which depend on counter ions, concentration, and pH. But also the propensity of solute species to exist at the solution surface is an important property, and is of particular interest for atmospheric chemistry. Experimentally, interfacial density profiles into solution can be probed by exploiting the variable electron inelastic mean free path. For elucidating ultrafast dynamical processes in aqueous solution, we apply core-level resonant electron spectroscopies, as in resonance Auger or resonance photoemission spectroscopy. The very short core-hole lifetimes – of a few femtoseconds – are exploited to determine ultrafast energy and electron transfers in solution, including charge-transfer-to-solvent processes or X-ray initiated photochemistry.
Electrons emitted from highly volatile solution experience multiple elastic and inelastic collisions with gas-phase (water)molecules, and the latter must be avoided for detection of electron kinetic energies. The seemingly contradictory concept of achieving undisturbed electron travel in a region of high vapor pressure has been realized by the development of the vacuum liquid microjet technique. If the diameter of the liquid jet (usually formed in a glass capillary) is on the order of ten microns, gas-phase (water)vapor density decreases quickly in radial directions such that the electron transfer-length increases to the 1 mm range. That explains why in a typical liquid-jet experiment the electron energy-analyzer is approximately 0.5 mm away from the jet. After injection into vacuum the jet is laminar on a length of several millimeters. Jet velocity is typically 100 ms-1 and temperature is < 10°C. Such fast flow eliminates sample aging effects and prevents the free-water-surface from early freezing. Efficient evaporative cooling is counteracted by the continuous rapid replacement of liquid, and warrants local thermodynamic equilibrium because liquid-liquid molecule collisions occur on a much slower time scale.