Liquid-Beam Source

The instrumentation's liquid-beam source is designed to produce micron-sized beams (~10 μm to 100 μm) of homogeneous liquids and heterogeneous solutions in a high-vacuum environment.

Figure 1. Liquid beam source chamber. The jet travels vertically through the extraction region of a TOF mass spectrometer into a cryopump. Full Image (jpg 120 kb)

Figure 2. Liquid beam at 2 to 3 mm from the nozzle. The measured diameter of the beam is 7 μm. The nozzle had a 10-μm aperture. Full Image (jpg 31kb)

Figure 3. Once the jet is formed, it is stable only for a short distance before it becomes unstable and breaks up into a cloud of droplets. A cryopump at 77 K immediately condenses the droplets to maintain low pressure in the chamber.

A liquid jet is formed by forcing liquid through a small pinhole into a large evacuated chamber. The jet is mounted on a precision xyz manipulator, and points directly into a liquid nitrogen-cooled cryopump—located a few centimeters away—that condenses most of the beam. A second cryopump, located above the jet, and a 6-inch turbomolecular pump are used to pump out the chamber. Chamber pressure during operation typically ranges from mid-10-5 to mid-10-4 Torr for a water jet. The small size of the beam combined with low-background pressure is designed to permit free (or nearly free) evaporation of the liquid, enabling direct studies of the liquid itself. Evaporative cooling creates a large temperature gradient that allows the liquid temperature to be varied simply by translating along the beam. A differentially pumped time-of-flight (TOF) reflectron mass spectrometer protrudes into the beam chamber for direct photodesorption studies of the jet. Experiments that involve pulsed ultraviolet and vacuum ultraviolet irradiation of the liquid-vacuum interface are also available.

The jet source and TOF spectrometer are shown in Figure 1. Figure 2 depicts a photograph of an actual water jet taken several millimeters downstream of a 10-μm-diameter pinhole. Here, the jet diameter measured 7 microns. The jet breaks up into droplets a short distance downstream of the aperture. As the liquid evaporates, the diameter shrinks while the surface temperature rapidly drops. The jet itself rapidly becomes a super-cooled liquid, and extremely cold liquid temperatures have been measured on similar jets.

Related Publications

1. Crystalline Ice Growth on Pt(111) and Pd(111): Nonwetting Growth on a Hydrophobic Water Monolayer.