Femtosecond Transient Absorption (fTA) Spectroscopy Information

fTA spectroscopy is a differential absorption (ΔAbs) measurement, where the broad, probe continuum is used to probe changes in the absorption spectra of the sample with and without excitation as a function of t after photoexcitation. For a single measurement at a given t, two transmission spectra of the probe laser are collected. One spectrum is the probe spectrum acquired with the excitation beam blocked. A second transmission probe spectrum is recorded with the excitation laser pulse traversing the sample at t before the probe pulse. The femtosecond TA experiments were performed using the output of a commercial Ti:sapphire amplifier laser system (800 nm, ∼2.3 W at 1 kHz, and ∼120 fs pulse width) and a commercial TA spectrometer. A fraction of the laser output was focused into a sapphire crystal to generate a white light continuum (450–760 nm) for use as the probe beam. The remainder of the laser output pumped a commercial optical parameter amplifier to provide excitation pulses at either 500 nm (2.48 eV) or 450 nm (2.76 eV) with a bandwidth of ∼12 nm. A motorized delay stage was used to control the delay time between the pump and probe pulses, t, with t < 1.5 ns. The data were collected for 2 s at each t. The unpolarized excitation and probe beams were gently focused on the sample, which was positioned orthogonally to the probe beam path. The excitation fluence was kept to ∼50 μJ cm^–2 pulse^–1 to minimize multicarrier interactions and heating effects. A temporal resolution of ∼200 fs was achieved in these fTA measurements.