There is another problem which may occur while performing CPD measurements with light. If the gold tip is not clean, it can contribute to the result - the CPD may be changed due to the illumination of the tip.
It is worth checking, whether work function of the tip is changing upon irradiation. It can be done by performing CPD measurements with chopped light irradiation for some reference sample: gold, aluminum or other clean not passivated metal sample (or even carbon tape attached to a metal support), for which CPD should not change upon illumination. (Be carful with the gold reference. I believe that if the tip was contaminated for example by an adsorption of some volatile thiols, the same could happen with the gold reference)
I attach my results. It is a CPD measurement of the gold sample (gold film freshly sputtered on the aluminum plate). The sample was irradiated with monochromatic light (370 nm, power: 10%) for 10 min. Similar results were obtained for aluminum and carbon tape samples (which should not show any light-induced response). In my case, the CPD changes resulting from the irradiation of the gold tip are not big, however, they do contribute to the CPD changes recorded for every sample.
Cleaning of the gold tip by rinsing it with an alcohol or acetonitrile does not help.