Dependence of size distribution for liquid cluster ions on retarding voltage

We have succeeded in producing liquid clusters such as ethanol and water clusters by an adiabatic expansion phenomenon, and the cluster size distribution was analyzed by using the time-of-flight method. For ethanol cluster ions, the size was distributed between a few hundreds and a few thousands, and the peak size was approximately 1200 molecules per cluster. For water cluster ions, the size distribution shifted to higher values, and the peak size was approximately 2500 molecules per cluster. Furthermore, the cluster size distribution was measured by changing the retarding voltage, and it was found that the minimum size increased with increase of the retarding voltage. In addition, the minimum size was larger than the critical size estimated by the theoretical calculation.