Dry etch damage is a potential worry when etching III-V semiconductors. Even though very low levels of damage are possible in modern dry etch processes, it is nevertheless of interest to be able to detect any residual damage and understand its origins. We have used measurements of photoluminescence intensity from GaAs and InGaAs quantum well heterostructures to probe the damage in a non-invasive way. At low damage levels, two factors influencing possible damage penetration have been found: The ratio of atomic to molecular ions in the discharge and the effect of ambient light. Samples were etched using SiCl4 reactive ion etching and also a more complex multi-component discharge. Comparative studies were made on samples bombarded by the separate constituent ions of a SiCl4 discharge using a mass-resolving ion implanter fitted with a deceleration lens. The experiments show that molecular ions contribute less to deep damage than do atomic ions. This is relevant, for example, if using high pump powers to sustain the discharge, such as in ICP or ECR. Further studies of laser illumination on the sample during etching show that a form of radiation enhanced diffusion can modify the damage distribution. This is relevant, for example, if laser interferometry is used for end-point detection. Thus for low damage in low damage etch processes, etch parameters that minimize atomic ion content in the discharge should be used and ambient illumination should be avoided if possible.