Fig. 4: Setup for double excitation and working principle for data analysis.

a, Schematic representation of the commercial transient absorption spectroscopy setup, into which an external continuous-wave (cw) laser has been integrated as an additional pump. The result is a cw-pump–pump–probe setup, where one pump (447 nm) is continuously irradiating the solution and the other pump (460 nm) is pulsed. b, Photograph of the actual setup from a with the directions of the individual beams shown by coloured lines. c, Arrangement of the three different beams at the cuvette, to highlight the fact that the cw pump beam is approximately seven times larger than the pulsed pump beam. d, Measurement and processing of transient absorption spectra in classical pump–probe experiments. The unpumped spectrum (i) is subtracted from the spectrum recorded after a pump pulse (ii) providing the transient absorption (difference) spectrum (iii). The grey background in d and e highlights that the two respective spectra ((ii) in d; (i) in e) are identical. The blue background in e highlights the cw-pump–pump–probe experiment as a whole. e, In the cw-pump–pump–probe experiment, the cw pump laser continuously forms some photoproducts (red and blue in i), which become part of the background signal. Therefore, the actual observable of the cw-pump–pump–probe experiment is not the sum of all photoproducts, but only of those photoproducts formed by the pulsed pump laser (orange and green in iii).