A liquid cluster ion source, which offers many possibilities for new and advanced applications in the surface processing of materials, was developed, and the cluster size distribution was measured by using time-of-flight method. When the vapor pressure of liquid materials such as ethanol and water was larger than 1 atm, the liquid clusters could be produced by an adiabatic expansion phenomenon. The peak size of the cluster ions increased with the increase of the vapor pressures, and decreased with the increase of the electron voltage and the electron current for ionization. In another case of producing liquid cluster ions at lower vapor pressures, He gas was used to mix up with vapors of liquid materials. Even if a vapor pressure was less than 0.1 atm, the liquid cluster ions with smaller size could be produced at the He gas pressures larger than 0.5 atm. The vapors of the liquid materials were cooled down by the collision with the He gas, which resulted in the effective formation of the liquid clusters even at lower vapor pressures.
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