The experimental setup and data analysis used for the femtosecond broadband transient absorption spectroscopy (TAS) technique have been described in great deal elsewhere. Briefly, the TAS spectrometer (Helios, Ultrafast Systems) make use of a Ti:Sapphire oscillator (Vitesse, Coherent), which seeds a regenerative amplifier (Libra-HE, Coherent) producing 100 fs pulses, centered at 800 nm, and with a 1 kHz repetition rate. Generation of the 267 nm excitation pulse was done as described previously by pumping an optical rail kit (FKE series, EKSMA optics). A translating 2 mm CaF 2 window was used to generate the white light continuum in the spectral probe window from 320 to 700 nm.