Defects at Play: Shaping the Photophysics and Photochemistry of Ice

TL;DR

This work probes how UV light interacts with ice Ih and how lattice defects sculpt the photophysics and photochemistry. By applying time-dependent dielectric-dependent hybrid density functional theory to defect-free and defective ice Ih models, the authors map excited-state potential-energy surfaces and identify photoproducts such as H$_3$O$^+$, OH$^\cdot$, and e$^-$, elucidating how defects control absorption onsets and emission energies. They show that vacancies trap hydrated electrons, OH$^-$ defects alter charge-localization patterns, and Bjerrum defects enable low-energy, BD-localized emissions, potentially explaining long-exposure spectral features around 5.6 eV. The findings provide a microscopic framework connecting defect chemistry to ice photochemistry, with implications for atmospheric, environmental, and astrophysical processes and guidance for future pump-probe experiments and defect-aware modeling of ice photophysics.

Abstract

The mechanisms by which light interacts with ice and the impact of photo-induced reactions are central to our understanding of environmental, atmospheric and astrophysical processes. However, a microscopic description of the photoproducts originating from UV absorption and emission processes has remained elusive. Here we explore the photochemistry of ice using time-dependent hybrid density functional theory on various models of pristine and defective ice Ih. Our investigation of the excited state potential energy surface of the crystal shows that UV absorption can lead to the formation of hydronium ions, hydroxyl radicals and excess electrons. One of the dominant mechanisms of decay from the excited to the ground-state involves the recombination of the electron with the hydroxyl radical yielding hydronium-hydroxide ion-pairs. We find that the details of this charge recombination process sensitively depend on the presence of defects in the lattice, such as vacancies and pre-existing photoproducts. We also observe the formation of Bjerrum defects following UV absorption; we suggest that, together with hydroxide anions, they are likely responsible for prominent features experimentally detected in long UV exposure absorption spectra, remarkably red-shifted relative to short exposure spectra. Our results highlight the key role of defects in determining the onset of absorption and emission processes in ice.

Figures (23)

• Figure 1: Schematic representation of absorption and emission processes investigated in our work (a) for four different ice models: defect-free proton disordered ice (b); ice with vacancy defects (c), with ionic (OH$^-$) defects (d), and with Bjerrum defects (e). The ground and excited state potential energy surfaces are denoted by GS and ES, respectively. Hydrogen and oxygen atoms are represented by white and red spheres, respectively.
• Figure 2: Distributions of absorption (blue) and emission (red) onset energies for 100 configurations of defect-free, disordered ice Ih are shown in panel a. Solid lines represent Gaussian kernel density estimation (KDE) curves fitted to the corresponding normalized histograms (see Methods section). Panel b shows the unrelaxed differential density computed for one representative configuration optimized in the excited state. The electron depletion and electron accumulation regions are displayed in blue and yellow, respectively. Distributions of absorption (orange) and emission (green) onset energies for 100 configurations of vacancy-containing, disordered ice Ih are shown in panel c, along with their respective Gaussian KDE curves. The unrelaxed differential density for one representative configuration is reported in panel d, with the same color scheme adopted in (b).
• Figure 3: Distances between the oxygen belonging to an hydroxyl OH$^.$ and its nearest oxygen neighbor (NN) are shown as a function of the emission energy of disordered ice with a vacancy defect, for 100 configurations. NN oxygen atoms belonging to H$_2$O (group (i)) or H$_3$O$^+$ (group (ii)) are represented with red and cyan dots, respectively. An example of the region near the vacancy for a configuration in group (i) is shown in the inset.
• Figure 4: Distributions of absorption (cyan) and emission (magenta) onset energies for 40 ionic defect (OH$^-$) configurations of disordered ice Ih are shown in panel a. Solid lines represent Gaussian kernel density estimation (KDE) curves fitted to the corresponding normalized histograms (see Methods). Average (Avg.) values of the absorption (abs.) and emission (emis.) onset of the defect-free (DF) ice model are shown as red and blue dashed vertical lines, respectively. An example of unrelaxed differential density is displayed in panel b, showing electron depletion and electron accumulation in blue and yellow, respectively.
• Figure 5: Absorption onset energy values (a) calculated for intermediate configurations determined by nudged elastic band (NEB) calculations, from defect-free(DF)-like structures to Bjerrum defects (BDs) structures. An example of DF and BDs moieties are shown in the same panel. The corresponding emission onset energy values are shown in panel b. Blue and red dots represent configurations optimized in the excited state, where OH$^.$, H$_3$O$^+$, and e$^-$ (DF-like) and Bjerrum defects are found, respectively. An example of unrelaxed differential density for one of the BDs-like conformations is shown in the same panel. Electron depletion and electron accumulation are shown in blue and yellow, respectively.