Ultrafast proton transfer in a photoionized glycine by a mixed quantum–classical and quantum dynamics

Abstract

We have theoretically investigated the ultrafast intramolecular hydrogen transfer in the glycine molecule after ionization, as observed by Castrovilli et al., J. Phys. Chem. Lett. 9, 6012–6016 (2018), following excitation of the molecule with an XUV attosecond pulse train of 1.5 fs duration. In this experiment, the interaction of the glycine molecule with the XUV pulse creates a superposition of electronic states, whose dynamics is coupled to the nuclear one. We employed the static exchange restricted active space density functional theory correlated approach, as implemented in the Tiresia code [Decleva et al., Molecules, 27(6), 2026 (2022)], to evaluate ionization probabilities. Coherence effects were studied through quantum dynamics simulations using the multi-layer multi-configuration time-dependent Hartree method on a vibronic coupling Hamiltonian model. Our findings indicate that, for the pulses used in the Castrovilli et al. experiment, electronic coherence dissipates very rapidly, in less than 3 fs. Consequently, we performed simulations starting from single electronic state. In addition, we described the long-term coupled electron–nuclear dynamics using the trajectory surface hopping method. Our results reveal that hydrogen transfer predominantly occurs when the active state reaches the cationic ground state. Charge analysis confirms that this process corresponds to a proton transfer.