Simultaneously, the signal at 3676 cm -1 first shifts to 3665 cm -1 and finally transforms into a small, positive signal at 3676 cm -1 . While the two signals at 2188 and 1370 cm -1 disappear completely again, the broad signal at around 1600 cm -1 only decreases in intensity and remains even after saturation. Interestingly, the bands at 2188 and around 1600 cm -1 also reach their maximum after a different number of pulses, i.e. 8 and 14 pulses, respectively.