2.5. Femtosecond Visible Transient Absorption Spectroscopy

A custom-built transient absorption (TA) spectrometer was used to measure all TA spectra. The initial stage of the TA light source is a Ti:sapphire-based fs laser oscillator (Coherent, Santa Clara, CA, USA), which produces an 80 MHz train of pulses centered at ~800 nm with a pulse energy of ~4 nJ/pulse. The output of the oscillator is used to seed a Ti:sapphire-based regenerative amplifier (Coherent), which produces a 1 kHz train of ~40 fs pulses centered at 800 nm that have been amplified to ~3 mJ/pulse. A portion of the regenerative amplifier output drives an optical parametric amplifier (Coherent), which was used to generate the 480 nm pump beam. A small fraction of the regenerative amplifier output was also used to generate the probe beam by focusing onto a 2 mm thick sapphire window (Newlight Photonics, Toronto, ON, Canada), producing a white light continuum spanning ~440 to 750 nm (Section S1). The pump and probe beams were spatially overlapped at the position of the sample. The relative time delay between the pump and probe at the sample position was controlled by varying the pump beam path length using a mechanical delay stage (Aerotech, Pittsburg, PA, USA). The TA spectra were collected by directing the probe beam into a spectrograph, which included a monochromator and a sCMOS array detector (Andor, Belfast, Northern Ireland). All measurements were carried out with the relative orientation of the linearly polarized pump and probe beams set to the magic angle of 55°. The beam power was measured in the presence and absence of a 50 μm diameter high-energy pinhole (Newport, Santa Clara, CA, USA) situated at the focal plane of the overlapped beams to estimate diameters of ca. 240 and 160 μm for the pump and probe beams, respectively. Pump pulse energies of ~0.5 µJ/pulse were used, which corresponded to a pump fluence of ~1.2 mJ/cm². All solutions were contained in a 2 mm quartz cuvette (Starna Cells) and stirred with a magnetic stirrer bar (Starna Cells) using a magnetic stirring apparatus (Ultrafast Systems, Sarasota, FL, USA) over the course of the measurement. The pump pulse duration was ~190 fs for all measurements, as was determined by performing an autocorrelation of the pump with a 2 mm quartz cuvette filled with distilled water and situated at the sample position (Section S1).

Measurements extending into the several nanosecond timescale were obtained on a TA spectrometer that has been described in detail previously [64,65].