A nearly monoenergetic beam of argon and chlorine ions with kinetic energy from 40 to 900 eV and intensity up to 30 mu A/cm(2) is used to study the etching of Si. The etch rate and ion current density at the surface are simultaneously measured to obtain the etch yield. The ratio of atomic chlorine (Cl+) to molecular chlorine (Cl-2(+)) ions striking the surface is measured by a quadrupole mass spectrometer. In the absence of any chlorine, the Ar ion sputtering of silicon at 120 eV is about 0.05 silicon atoms removed per ion and monotonically increases to 1 silicon atom per ion at 800 eV. The Ar+-induced etch yield increases by about an order of magnitude when the surface is simultaneously exposed to a neutral molecular chlorine flux sufficient to produce saturation coverages from the effusive doser. An etch yield of 0.13 silicon atoms removed per incident Ar ion is measured at an Ar+ energy of 50 eV, which also monotonically increases to 4 silicon atoms removed per incident argon ion at 700 eV. The etch yield further increases by approximately a factor of 20% as the surface is bombarded with energetic chlorine ions. The etch yield with energetic chlorine ions increases from 0.07 to similar to 5 silicon atoms per incident ion with increasing kinetic energy from 40 to 900 eV.