To understand negative ion formation under Cs+ bombardment, we have studied the steady-state Cs depth distribution in Si and Ge with a focus on high-resolution analysis conditions i.e. Cs+ energies between 500 eV and 5 keV and impact angles from 0 degrees to 60 degrees. The in situ internal Cs depth profiles have been measured with a low energy O-2(+) beam (0.5-1 keV) similar to the work of Yoshikawa [S. Yoshikawa, H. Morita, et al., Appl. Surf. Sci. 203-204 (2003) 252-255]. To minimize the distortions on Cs profiles, a non-oxidizing condition (at impact angle 60 degrees) was chosen instead of an oxidizing condition. The internal Cs profiles in Si have a flat saturation region near the surface, with an intensity independent from the bombardment conditions. Only the profile width scales with impact angles and energies. Whereas in the Ge-case, a Gaussian profile is always obtained. Based on the internal Cs profiles, a possible near surface altered layer formation in Si and Ge under Cs bombardment is suggested. (c) 2006 Elsevier B.V. All rights reserved.