A method and device for adjusting a beam property, such as a beam size, a beam shape or a beam divergence angle, in a gas cluster beam prior to ionization of the gas cluster beam is described. A gas cluster ion beam (GCIB) source is provided, comprising a nozzle assembly having a gas source, a stagnation chamber and a nozzle that is configured to introduce under high pressure one or more gases through the nozzle to a vacuum vessel in order to produce a gas cluster beam. Additionally, the GCIB source comprises a gas skimmer positioned downstream from the nozzle assembly that is configured to reduce the number of energetic, smaller particles in the gas cluster beam. Furthermore, the GCIB source comprises a beam adjustment device positioned downstream from the gas skimmer that is configured to adjust at least one beam property of the gas cluster beam, and an ionizer positioned downstream from the beam adjustment device that is configured to ionize the gas cluster beam to produce a GCIB.