Quantitative Decoupling of Excited-State Absorption Cross Section and Population via Pump-Probe Spectroscopy with a Strong Probe

Photoinduced absorption signals measured by transient absorption spectroscopy are typically proportional to the product of absorption cross section (sigma) and excited-state density (N). We show that this approximation does not hold at high probe-pulse intensities, and introduce the use of probe-intensity-dependent spectroscopy to decouple the two parameters. The singlet excited-state (S-1 -> S-2) absorption cross section of the conjugated polymer F8BT is measured to be 1.6 x 10(-16) cm(2) +/- 40% at 800 nm and 3.7 x 10(-16) cm(2) +/- 30% at 900 nm, with no variation over the time window surveyed. The robustness of these parameters is established by observing that only N scales with excitation fluence and time delay, and conversely only s is dependent on probe wavelength. The technique may be useful for quantifying salient parameters in many systems, such as branching yields in systems exhibiting singlet fission or triplet production, or cross sections required for photophysical models.