An experimental and computational study of the effect of aqueous solution on the multiphoton ionisation photoelectron spectrum of phenol.

Alice Henley,Helen H Fielding,Jamie W Riley,Bingxing Wang

doi:10.1039/c9fd00079h

Abstract

We revisit the photoelectron spectroscopy of aqueous phenol in an effort to improve our understanding of the impact of inhomogeneous broadening and inelastic scattering on solution-phase photoelectron spectra. Following resonance-enhanced multiphoton ionisation via the 11ππ* and 11πσ* states of phenol, we observe 11ππ*-D0/D1 ionisation and competing direct S0-D0/D1 ionisation. Following resonance-enhanced multiphoton ionisation via the 21ππ* state, we observe the signature of solvated electrons. By comparing the photoelectron spectra of aqueous phenol with those of gas-phase phenol, we find that inelastic scattering results in peak shifts with similar values to those that have been observed in photoelectron spectra of solvated electrons, highlighting the need for a robust way of deconvoluting the effect of inelastic scattering from liquid-phase photoelectron spectra. We also present a computational strategy for calculating vertical ionisation energies using a quantum-mechanics/effective fragmentation potential (QM/EFP) approach, in which we find that optimising the configurations obtained from molecular dynamics simulations and using the [phenol·(H2O)5]QM[(H2O)n≥250]EFP (B3LYP/aug-cc-pvdz) method gives good agreement with experiment.

Highlights

Ionisation is the most fundamental photophysical process accompanying the interaction of ionising radiation with biologically important molecules and plays a central role in radiation chemistry and biology
We present a computational strategy for calculating vertical ionisation energies using a quantum-mechanics/effective fragmentation potential (QM/effective fragment potential (EFP)) approach, in which we find that optimising the configurations obtained from molecular dynamics simulations and using the [phenol$(H2O)5]quantum mechanical (QM)[(H2O)n$250]EFP (B3LYP/aug-cc-pvdz) method gives good agreement with experiment
The residuals of the ts at the low eBE edges of the 265.5–249.7 nm spectra are plotted as insets in Fig. 3 and can be attributed to non-resonant S0–D0 multiphoton ionisation (MPI) that competes with resonance-enhanced MPI

Summary

Introduction

Ionisation is the most fundamental photophysical process accompanying the interaction of ionising radiation with biologically important molecules and plays a central role in radiation chemistry and biology. The most direct way to probe ionisation experimentally is to use photoelectron spectroscopy (PES), which measures the electron kinetic energies (eKEs) of electrons emitted following ionisation. The eKE distribution encodes the role of each vibrational mode of the radical in its subsequent structural relaxation and, in the case of solution-phase photoelectron spectra, contains information about solvent relaxation. The interpretation of solutionphase photoelectron spectra is complicated by the inhomogeneous environment of the solution causing spectral broadening and inelastic scattering of photoelectrons in the solution before emission causing the measured electron kinetic energies to be lower than their true values.[1,2] Disentangling the various contributions to solution-phase photoelectron spectra requires gas-phase PES as an essential reference and synergistic contributions from experiment and theory. We revisit the PES of aqueous phenol in an effort to improve our understanding of the impact of inhomogeneous broadening and inelastic scattering on the photoelectron spectra

Methods

Results

Conclusion