Liquid Jet Experiment
Liquid-Jet Photoelectron Spectroscopy with Synchrotron Radiation
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. This is about the distance at which electrons pass a pressure barrier into a low-pressure detection chamber.
The jet, the X-ray beam (with focal size matching the diameter of the jet), and imaging focus of a hemispherical electron energy analyzer (EA) are spatially overlapped in the main interaction chamber. The jet is allowed to travel several centimeters until hitting the surface of a cryo trap. Or it shoots after few mm of travel into a small container (liquid-jet catcher; see photograph) with a pinhole opening, in which case solutions can be recovered. Cryo traps combined with mechanical pumping maintain a low 10-4 mbar pressure under operation conditions.
After injection into vacuum the jet is laminar on a length of several millimeters. Jet velocity is typically <100 ms-1 and temperature is <10oC. 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.
Impressions of the Experiment
From small to large – When in operation, jet nozzle, jet performance, and distance to the EA-skimmer are monitored by a camera, mounted at a telescope (x100 magnification). The 100-200μm orifice of the skimmer causes the pressure barrier between main chamber and EA. With the jet (and optionally the catcher directly facing the jet) mounted onto a high precision x-,y,-,z-manipulator it can be accurately positioned. EA detection axis is perpendicular to the light beam axis and jet flow direction. EA and main chamber can be rotated around the light beam axis, which allows variation of the electron detection angle with respect to the polarization axis of the light. Main Chamber and EA are enclosed within a 3-axes Helmholtz cage.