A method of processing one or more surfaces is provided, comprising: providing a switchable ion gun which is switchable between a cluster mode setting for producing an ion beam substantially comprising ionised gas clusters for irradiating a surface and an atomic mode setting for producing an ion beam substantially comprising ionised gas atoms for irradiating a surface; and selectively operating the ion gun in the cluster mode by mass selecting ionised gas clusters using a variable mass selector thereby irradiating a surface substantially with ionised gas clusters or the atomic mode by mass selecting ionised gas atoms using a variable mass selector thereby irradiating a surface substantially with ionised gas atoms. Also provided is a switchable ion gun comprising: a gas expansion nozzle for producing gas clusters; an ionisation chamber for ionising the gas clusters and gas atoms; and a variable (preferably a magnetic sector) mass selector for mass selecting the ionised gas clusters and ionised gas atoms to produce an ion beam variable between substantially comprising ionised gas clusters and substantially comprising ionised gas atoms. Preferably, the gun comprises an electrically floating flight tube for adjusting the energy of the ions whilst within the mass selector.