Interpretations of High-Order Transient Absorption Spectroscopies

Transient absorption (TA) spectroscopy is an invaluable tool for determining the energetics and dynamics of excited states. When pump intensities are sufficiently high, TA spectra include both the generally desired third-order response and responses that are higher in order in the field amplitudes. Recent work demonstrated that pump-intensity-dependent TA measurements allow separating the orders of response, but the information content in those higher orders has not been described. We give a general framework for understanding high-order TA spectra. We extend to higher order the fundamental processes of standard TA: ground-state bleach (GSB), stimulated emission (SE), and excited-state absorption (ESA). Each order introduces two new processes: SE and ESA from previously inaccessible highly excited states and negations of lower-order processes. We show the new spectral and dynamical information at each order and show how the relative signs of the signals in different orders can be used to identify which processes dominate.