Vibronic dynamics resolved by global and target analysis of ultrafast transient absorption spectra

We present a methodology that provides a complete parametric description of the time evolution of the electronically and vibrationally excited states as detected by ultrafast transient absorption (TA). Differently from previous approaches, which started fitting the data after AP;100 fs, no data are left out in our methodology, and the coherent artifact and the instrument response function are fully taken into account. In case studies, the method is applied to solvents, the dye Nile blue, and all-trans beta-carotene in cyclohexane solution. The estimated Damped Oscillation Associated Spectra (DOAS) and phases express the most important vibrational frequencies present in the molecular system. By global fit alone of the experimental data, it is difficult to interpret in detail the underlying dynamics. Since it is unfeasible to directly fit the data by a theoretical simulation, our enhanced DOAS methodology thus provides a useful middle ground where the theoretical description and the fit of the experimental data can meet. beta-carotene in cyclohexane was complementarily studied with femtosecond stimulated Raman spectroscopy (FSRS). The fs-ps dynamics of beta-carotene in cyclohexane in TA and FSRS experiments can be described by a sequential scheme S2 -> hot S1 -> S1 ' -> S1 -> S0 with lifetimes of 167 fs (fixed), 0.35, 1.1, and 9.6 ps. The correspondence of DOAS decaying concomitantly with hot S1 and the Species Associated Difference Spectra of hot S1 in TA and FSRS suggest that we observe here features of the vibrational relaxation and nuclear reorganization responsible for the hot S1 to S1 transition. (c) 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Automated summary

A new methodology is presented to fully describe the time evolution of electronically and vibrationally excited states detected by ultrafast transient absorption in various systems. By global fit of experimental data, important vibrational frequencies in molecular systems are revealed. The enhanced method provides a balanced approach between theoretical description and fit of experimental data, aiding in the interpretation of molecular dynamics.