Measurement of velocities of clusters generated in supersonic flow

An apparatus is described for measuring the speeds of molecular clusters in a flight tube under conditions of gas density that would overwhelm the system in conventional measurements of gas velocities. Although the apparatus incorporates no new design principles it is effective in practice. It provides the information needed in timing the evolution of the extremely fast phase changes that occur in highly supercooled clusters. The state of clusters is monitored as a function of their distance and, hence, time-of-flight along their path. Illustrative examples of velocity measurements are given.