Early Microjet Experimentation with Liquid Water in Vacuum

Area of Science:

• Physical Chemistry
• Surface Science
• Spectroscopy

Background:

• High-vacuum techniques are essential for atomic-level surface and gas-phase studies but are incompatible with volatile liquids like water.
• Liquid microjets, developed over the past three decades, provide a solution by creating a stable liquid surface in a vacuum environment.
• The working principle relies on a high-velocity liquid stream (e.g., >50 m/s) to prevent freezing and minimize vapor collisions, analogous to molecular beam sources.

Purpose of the Study:

• To review the historical development and challenges of using liquid microjets for vacuum-based studies.
• To highlight key experimental results and unexpected phenomena observed with this technique.
• To showcase the application of liquid microjets in photoelectron spectroscopy for liquid interface analysis.

Main Methods:

• Utilizing liquid microjets with diameters smaller than the vapor mean free path (e.g., <10 μm for water) to maintain a non-collisional vapor environment.
• Employing molecular beam time-of-flight experiments to verify the free surface concept and measure vapor velocity distributions.
• Adapting photoelectron spectroscopy apparatus for vacuum microjets, including modifications for high vapor loads and utilization of synchrotron radiation for tunable X-ray sources.

Main Results:

• Experimental verification of Maxwellian velocity distributions for water vapor from microjets, confirming the free surface concept.
• Observation of unexpected non-equilibrium evaporation temperatures for acetic acid dimers, correlated with surface tension.
• Successful recording of photoelectron spectra from liquid water interfaces, including studies on alkali halide solutions and pH-dependent protonation of amino acids.

Conclusions:

• Liquid microjets represent a significant advancement, enabling direct investigation of liquid interfaces under high-vacuum conditions.
• The technique has overcome initial challenges and yielded novel insights into liquid evaporation and solution properties.
• Liquid microjets have opened new avenues for studying a wide range of chemically and biologically relevant liquids.