Fig. 4: Femtosecond tracking of proton transfer dynamics.

a Calculated time-dependent yield of the PT channel. The yield is fitted with a growth function (pink dashed line), giving a PT time constant of τ = 52.8  ± 0.5 fs. b Schematic of the laser pump-probe experiment. Black, red, and purple arrows represent the 800 nm pump, 800 nm probe, and 400 nm probe, respectively. Black, blue, red and purple shadows represent wave packets on the PECs of C₄H₅N-H₂O, C₄H₅N²⁺-H₂O, C₄H₅N⁺-H₂O⁺, and C₄H₄N⁺-H₃O⁺. Red shadows at 3.1 eV and 3.9 eV delineated by black dotted lines represent the expected KER of the 800 nm pump - 800 nm probe and 800 nm pump - 400 nm probe schemes, respectively. c H₂O⁺ + C₄H₅N⁺ yields versus pump-probe time delay, which are smoothed with a moving average. The PT times τ₁ = 34  ± 5 fs and τ₂ = 31  ± 6 fs are determined by fitting with decay functions (pink/purple dashed lines). The orange line shows the instrument response function (IRF). d Time delay-dependent KER distribution. Dashed box highlights the 3.9 eV transient. e Time-dependent yield curve for the direct dissociation channel at KER  ~ 3.9 eV in the case of 800 nm pump - 400 nm probe. The PT time τ = 55  ± 24 fs is determined by fitting with decay functions. Error bars represent statistical s.d., and were calculated by the square root of the true coincidence counts. The PECs (blue, red, and purple solid curves) in (b) are calculated using constrained DFT with Configurational Interaction approach and the B3LYP/aug-cc-PVTZ method. Source data are provided as a Source Data file.