The consumption of C 18 O2 molecules and the formation of the possible oxygen exchange products C 16 O 18 O and C 16 O2 are presented in Figure . Also here, the general trends are comparable to those in Figures and. Most obvious difference to the previous experiments is the much later appearance of the maximum in the C 16 O 18 O signal, which was reached only after about 300 pulses, i.e., significantly later than in Figure and much later than in Figure . Also, the subsequent decay was significantly slower than in the previous cases. The formation of C 16 O2 (Figure ) occurred rather similar as described for Figure , with an initial maximum followed by an about exponential decay. But in this case the decay was even slower than in Figure , with the loss of 50% of the maximum intensity being reached only after about 150 pulses.