Plume velocities were measured in air by an optical transmission technique as a function of laser fluence using a KrF laser for explosive boiling and lift-off of a thin variable thickness 2-propanol layer on a laser-heated Si substrate of interest for laser-assisted particle removal. The plume velocities were found to diminish with an increase in the thickness of the 2-propanol layer. This dependence was explained on the basis of the the momentum conservation rule, assuming explosive expansion of the superheated alcohol layer under spinodal conditions.