Transient absorption (TA) spectroscopy gives dynamical information about excited states and spectral information about their excitations. If the pump pulses are strong, then multiple excitations can be produced, and the signal has contributions from single excitations mixed with those from multiple excitations.
I will describe a new technique in which TA spectra are acquired at several pump intensities, enabling extraction of high signal-to-noise TA spectra and systematically separated high-order spectra [1,2]. I will show the spectral and dynamical information in high-order spectra. The higher orders contain information both about multiply excited states and singly excited states that are usually dark. I will give an intuitive taxonomy of the response pathways that characterize these high-order signals, extending the standard TA pathways -- stimulated emission, excited-state absorption, and ground-state bleach -- to higher orders. I will show examples from several molecular and solid-state systems.
[1] Malý, Lüttig, Rose, Turkin, Lambert, Krich, and Brixner, Nature 616 280 (2023)
[2] Lüttig, Rose, Malý, Turkin, Bühler, Lambert, Krich, and Brixner, J Chem Phys 158 234201 (2023)
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