Avoidance of the photochemical production of oxygen atoms in one-dimensional, two-photon laser-induced fluorescence imaging.

We demonstrate that one-dimensional, two-photon laser-induced fluorescence imaging of oxygen atoms in flames can be achieved with reasonable signal intensities and spatial resolution but without the interference caused by the photolysis of O(2), by the use of a beam telescope instead of a focusing lens. The increase in probed volume (and concomitantly the number of excitable atoms) caused by the larger beam diameter in the telescope experiments offsets, to a certain extent, the loss in signal that is due to the reduced laser fluence. Further, the results show that the "correct" dependence of the fluorescence intensity on laser fluence (in this case quadratic) does not guarantee the absence of photochemistry.