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. 2020 Sep 11;11:4569. doi: 10.1038/s41467-020-18393-w

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© The Author(s) 2020

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 3 — a FTIR spectrum of DOPA melanin dissolved in phosphate-buffered D₂O, including vibrational assignments. b Ultrafast vibrational fingerprinting (UVF) spectroscopy vs. two-dimensional electronic-vibrational (2D EV) spectroscopy for a hypothetical sample containing multiple electronic chromophores that can be distinguished by the redox state of their oxidizable IR-active functional groups. With time-resolved infrared (TRIR) spectroscopy (right), a subensemble of eumelanin chromophores is photoselected using a particular visible wavelength and its time-resolved vibrational spectrum is measured. These spectra are essentially horizontal slices through the hypothetical 2D EV spectrum (left). The 2D signals are assumed to only contain negative absorptive (bleaching) contributions for simplicity. Note that the presence of positive signal contributions from new vibrational frequencies in excited electronic states can alter the positions of the negative bands.