Simulations of light induced processes in water based on ab initio path integrals molecular dynamics. II. Photoionization

Abstract

We have applied ab initio based reflection principle to simulate photoelectron spectra of small water clusters, ranging from monomer to octamer. The role of quantum and thermal effects on the structure of the water photoelectron spectra is discussed within the ab initio path integral molecular dynamics (PIMD) framework. We have used the PIMD method with up to 40 beads to sample the ground state quantum distribution at temperature T = 180 K. We have thoroughly tested the performance of various density functionals (B3LYP, BHandHLYP, M06HF, BNL, LC-ωPBE, and CAM-B3LYP) for the ionization process description. The benchmarking based on a comparison of simulated photoelectron spectra to experimental data and high level equation-of-motion ionization potential coupled clusters with singles and doubles calculations has singled out the BHandHLYP and LC-ωPBE functionals as the most reliable ones for simulations of light induced processes in water. The good performance of the density functional theory functionals to model the water photoelectron spectra also reflects their ability to reliably describe open shell excited states. The width of the photoelectron spectrum converges quickly with the cluster size as it is controlled by specific interactions of local character. The peak position is, on the other hand, defined by long-range non-specific solvent effects; it therefore only slowly converges to the corresponding bulk value. We are able to reproduce the experimental valence photoelectron spectrum of liquid water within the combined model of the water octamer embedded in a polarizable dielectric continuum. We demonstrate that including the long-range polarization and the state-specific treatment of the solvent response are needed for a reliable liquid water ionization description.

Enabling Forbidden Processes: Quantum and Solvation Enhancement of Nitrate Anion UV Absorption

“…The precise localization of the maximum is, however, difficult because it intersects with the water absorbing near 200 nm. 58 On the other hand, we can see clearly that (unlike for the weak absorption band) the onset of the absorption band is independent of the number of water molecules.…”

Structural Progression in Clusters of Ionized Water, (H2O)n=1–5+

“…This value is notably lower than the IP of the monomer, a trend which has been shown to continue toward bulk water. 113 Due to the large structural rearrangement upon ionizationand the concomitantly abysmal Franck−Condon overlapcomparison to experimental values 114 is somewhat dubious. Proton transfer to form the ion−radical pair serves as the driving force for approximately 1 eV of structural relaxation upon ionization.…”

Reaction selectivity in an ionized water dimer: nonadiabatic ab initio dynamics simulations

“…Earlier we have shown that the PIMD based scheme describes well both the photon absorption and photoionization of systems in both harmonic and anharmonic modes. 31,51,58 Initial geometries were sampled from the PIMD simulation using 30 beads. We used the massive Nosé-Hoover chains thermostatting scheme 59 to maintain the temperature at 180 K. The simulation details are explained in ref.…”

“…Later, ionization of water clusters and liquid water drew much attention of both experimentalists 12,[20][21][22][23][24][25][26][27][28] and theoreticians. 20,[29][30][31] The structure of the photoelectron spectrum remains the same in liquid water but the whole spectrum shifts by more than 1 eV (ref. 5) to lower energies due to the long range polarization and the width of the spectrum increases due to the inhomogeneous broadening.…”

“…In our recent paper, we showed that quantum effects significantly influence the absorption and photoelectron spectra of small water clusters. 31,51 Therefore, we use here the path integral molecular dynamics (PIMD) sampling scheme for the simulation of initial conditions of the system. We further perform a detailed mapping of the potential energy surfaces (PES) in the relevant coordinates for different electronic states of the ionized water dimer in the valence electron manifold, and we rationalize the observed dynamical patterns.…”