Fig. 1. Development of pump-probe technology (from microsecond^[6] and nanosecond^[7] time resolution to picosecond^[8] and femtosecond^[9] time resolution, from classical transient absorption spectroscopy^[10] to two-dimensional electronic spectroscopy^[11])

Fig. 2. Relationship between ΔA signal and dynamics of photo generated charge carriers in semiconductors^[49]. (a) Absorption intensity of probe pulse after it passes through sample; (b)‒(d) intensity of probe pulse after it passes through sample with pump pulse present; (e) corresponding electronic transition and relaxation pathways in semiconductor

Fig. 3. Typical fs-TAS spectra of corresponding electronic transitions and relaxation pathways in a semiconductor with Stokes shift^[49]. (a) Small Stokes shift; (b) large Stokes shift

Fig. 4. Typical femtosecond time-resolved absorption spectrum measurement and acquisition system^[49]

Fig. 5. Pulse sequence and corresponding time variables in two-dimensional spectra

Fig. 6. Two-dimensional spectral device with BOXCARS configuration based on beam splitter^[60]

Fig. 7. Retrieving procedure of the amplitude and phase of signal in the heterodyne detection experiment. (a) Interference signal in frequency domain; (b) time domain signal obtained by inverse Fourier transform; (c) filtering using window function; (d) Fourier transform obtains amplitude and phase signals of signal light

Fig. 8. Photoelectric conversion process in organic solar cells^[65]. (a) Schematic of excited state and related charge process of the solar cell; (b) exciton dissociation and charge separation process at D/A interface

Fig. 9. Spectroscopic investigations of triplet formation in model NFA blends^[78]. Transient absorption spectra of (a) PM6∶Y6 and (d) PTB7-Th∶IEICO-2F mixture in infrared region; normalized transient absorption kinetics of (b) PM6∶Y6 and (e) PTB7-Th∶IEICO-2F blend; transient electron paramagnetic resonance spectroscopy of (c) PM6∶Y6 and (f) PTB7-Th∶IEICO-2F blends after 532 nm excitation

Fig. 10. Exciton/carrier dynamics in bulk heterojunctions^[83]. (a) Extraction of transient absorption spectral components and related kinetics from transient absorption spectra of PTQ10∶IDIC blend films excited at 700 nm with a ratio of 1∶1.5; (b) model for charge transfer and separation in a 1∶1.5 blend membrane

Fig. 11. 2DES probes wave packet motion through an intermolecular conical intersection in a thin film of molecular aggregates^[93]. (a) Indicating coherence τ and waiting time T; (b)‒(d) for T<20 fs, 2DES plot displays a grid like peak pattern; (e)‒(g) for 20 fs<T<40 fs, the peak spacing along detection axis gradually decreases, pointing towards the anharmonicity of the S₂ state potential; (h) at T ≅ 45 fs, the grid pattern disappears and the 2DES spectrum becomes essentially featureless; waiting time dynamics of the period (k) and amplitude (l) of the main vibration modes