A sensitive variation of FM spectroscopy was used to detect photochemical holes burned in free-base phthalocyanine- doped poly(ethylene) in times as small as 100 nsec. These writing times are more than 3 orders of magnitude smaller than the cycle time of the previously observed photochemical bottleneck. The changes in absorption resulting from hole burning were calibrated against a known interferometer resonance. A simplified model of the hole-burning process fits the experimental data over 7 orders of magnitude in exposure time and predicts the parameters necessary to obtain a given absorption change. These results provide important evidence that the burning bottleneck is due to population buildup in the triplet state. © 1984, Optical Society of America.