Not limited by the emission of photons, transient absorption spectroscopy can be used to study the excited-state properties and dynamics in a time-resolved manner from 300 fs up to 450 µs. Solid and liquid samples can be photoexcited monochromatically between 240 nm and 2.6 µm wavelength and the sample response can be characterized in real time over a spectral range from 320 nm to 1.6 µm. This way, we gain valuable information about the origin and dynamics of photoexcited charge carriers in materials including intersystem crossing rates, photoluminescence lifetimes, quenching dynamics, hot carrier cooling and band structure changes. [1-4]

Our pump-probe setup is located in a temperature- and humidity-controlled room. It consists of an Astrella-F ultrafast Ti:sapphire amplifier from Coherent which produces 90 fs excitation pulses at a central wavelength of 800 nm and a repetition rate of 1 kHz. This fundamental beam is split into a pump (90 %) and a probe beam (10 %). The pump is directed to the optical parametric amplifier Apollo-T from Ultrafast Systems to alter the excitation wavelength from 240 nm to 2.6 µm. The delay between probe and pump pulse up to 8 ns is controlled via an automatic delay line and the broadband two-channel supercontinuum is generated by using nonlinear processes in a CaF2 (320–650 nm), sapphire (420-780 nm) or YAG (820-1600 nm) plate (Helios Fire, Ultrafast Systems). For the nanosecond setup, the white light probe is generated with a photonic crystal fiber (350-800 nm & 800-1600 nm) and the delay up to 450 µs is controlled electronically (EOS Fire, Ultrafast Systems).

Samples can be measured under inert conditions eighter in solution, as thin films or in a potassium bromide matrix. A 2D optical noise-corrected difference absorption spectrum as a function of wavelength and delay time can be calculated by subtracting the absorbance spectrum of the excited sample from the ground state absorption of the unpumped sample at different delay times. Analyzing the resulting data with global and target fitting tools gives information about the spectral and temporal evolution of the excited species and relaxation pathways.

[1] Pachel, F.; Frech, P.; Ströbele, M.; Enseling, D.; Romao, C. P.; Jüstel, T.; Scheele, M.; and Meyer, H.-J, Preparation, photoluminescence and excited state properties of the homoleptic cluster cation [(W₆I₈)(CH₃CN)₆]⁴⁺. Dalton Trans. 2023, 52, 3777 – 3785

[2] Kirsch, C.; Naujoks, T.; Haizmann, P.; Frech, P.; Peisert, H.; Chassé, T.; Brütting, W.*; and Scheele, M.* Zwitterionic Carbazole Ligands Enhance the Stability and Performance of Perovskite Nanocrystals in Light Emitting Diodes. ACS Appl. Mater. Interfaces 2023, 15, 27, 32744–32752

[3] Niebur, A.; Söll, A.; Haizmann, P.; Strolka, O.; Rudolph, D.; Tran, K.; Renz, F.; Frauendorf, A. P.; Hübner, J.; Peisert, H.; Scheele, M.; Lauth, J. Untangling the Intertwined: Metallic to Semiconducting Phase Transition in Colloidal MoS2 Nanoplatelets and Nanosheets. Nanoscale 2023, 15, 5679 - 5688

[4] Maulbetsch, T.; Frech, P.; Scheele, M.; Törnroos, K. W.; and Kunz, D. A saddle-shaped expanded porphyrinoid fitting C60. Chem. Eur. J. 2023, 29, 202302104.

[5] Bassler, M.*; Hiller, J.*; Wackenhut, F.; zur Oven-Krockhaus, S.; Frech, P.; Schmidt, F.; Kertzscher, C.; Rammler, T.; Ritz, R.; Braun, K.; Scheele, M.; Meixner, A. J.; Brecht, M. Fluorescence lifetime imaging unravels the pathway of glioma cell death upon hypericin-induced photodynamic therapy. RSC Chem. Biol. 2024, 5, 1219-1231.

[6] Strolka, O.; Rauthe, P.; Muschik, T.; Frech, P.; Niebur, A.; Unterreiner, A.-N.; Lauth, J. Continuous Tuning of Intersystem Crossing Times in Rose Bengal Water/Methanol Solutions. J. Phys. Chem. B 2024, 128, 12189–12196.

[7] Habibpourmoghadam, A.; Xie, W.; Bessel, P.; Niebur, A.; Antanovich, A.; Dorfs, D.; Lauth, J.; Lesina, A. C.; Numerical Modeling of Transient Absorption in Hybrid Dual-Plasmonic Au/CuS Nanocrystals. J. Phys. Chem. C 2024, 128, 21237–21244.

[8] Niebur, A.; Klein, E.; Lesyuk, R.; Klinke, C.; Lauth, J. Understanding the Optoelectronic Processes in Colloidal 2D Multi-Layered MAPbBr3 Perovskite Nanosheets: Funneling, Recombination and Self-Trapped Excitons. Arxiv 2024, published online. https://doi.org/10.48550/arXiv.2408.04571