Data acquisition: After passing through the sample, the probe pulse was spectrally dispersed using a Czerny-Turner spectrograph (Andor Technology, Shamrock 303) and detected with a multichannel detector (Andor Technology, Newton DU920P-BEX2-DD). Transient spectra were recorded up to a pump-probe delay of 30 ps using an optical delay stage. A semi-logarithmic delay scheme was utilized, providing a dense linear sampling during the ultrafast period, and logarithmic sampling afterwards. The maximum detector read-out speed was about 200 Hz, limiting the number of integrated pump-probe spectra per delay time to a few hundred. Further information on the data treatment is available in the Electronic Supplementary Information (ESI).