Bright Imperfections?

The interaction of light with ice is at the heart of many atmospheric and planetary surface processes. Yet, there is still much about it that we do not understand. A collaboration between ICTP’s Condensed Matter and Statistical Physics section and the University of Chicago has recently used accurate first-principle simulations to study what happens when ice crystals interact with UV light. They have found that defects in the crystal affect ice’s properties and determine its capacity to fluoresce. The study, which was published in the journal Proceedings of the National Academy of Sciences (PNAS), is the first that investigates the theory of how defects control the interplay between light and ice.

Understanding how ice interacts with light is as fundamental as it has been challenging. From a theoretical point of view, that requires studying the microscopic processes taking place in the crystal when an ice sample is excited with electromagnetic radiation. Eager to find out, ICTP’s Ali Hassanali and his group teamed up with Giulia Galli’s group at the University of Chicago. Galli, who has served on ICTP’s Scientific Council since 2022, is a leading expert in electronic structure calculations, and her group has developed techniques to accurately simulate the electronic excited states of materials.

As part of their ERC-funded project “HyBOP” (Hydrogen Bond Networks as Optical Probes), Hassanali’s group was particularly interested in investigating ice’s capacity to absorb and emit light, specifically a property called fluorescence. While looking for experimental results on the topic, their attention was grabbed by a paper from the late 1980s by a research group at the University of Western Australia. “They observed fluorescence in their experiments on ice, but the microscopic origins of the phenomenon and the chemical reactions triggered by light in their sample were not known,” explains Marta Monti, a postdoc in Hassanali’s research group and the study’s first author.

To understand what could cause fluorescence in ice, Monti spent a few weeks in Galli’s group at the University of Chicago. “Their expertise is to accurately study the chemistry and the physics of light-ice interaction. By using their techniques, we were able to consider a large-enough ice sample and treat all of it as a quantum mechanical system. Only in this way could our simulations give us a realistic picture of what happens inside the ice crystal when it is excited with light,” Monti says.

In Chicago, Monti worked closely with Yu Jin, a postdoc at the Flatiron Institute, who at the time was doing his PhD at the University of Chicago. He is the one who implemented a key feature in the code used by the group to study the optical properties of solid-state systems, time-dependent density functional theory – exactly what Monti needed to look at the light-ice interaction in detail. “These calculations are very computationally expensive, but our code is designed to be very efficient,” says Yu Jin.

Monti’s simulations of pure ice did not reproduce the experimental observations until an extra ingredient was added. “It was only when we introduced defects into our ice samples that we observed features in the absorption and fluorescence emission spectra that were consistent with experiments,” Monti explains. Despite not having been considered as a possible cause of fluorescence before, it came as no surprise that defects play an important role. “The electrical and optical properties of many materials are known to be strongly affected by defects and there is no reason to think ice would be any different,” Galli adds.

The study opens up new possible research directions. “We would like to continue our collaboration with Giulia’s group to make our simulations more accurate and efficient, for example by combining them with machine learning methods that could speed up the calculations,” explains Hassanali, adding, “We also hope that soon someone will take up the challenge to perform new experiments that will test our theoretical predictions.”

Despite the longstanding relationship that links ICTP with Galli’s group, this is the first time that Galli’s and Hassanali’s groups collaborated on a research question. “I remember first meeting Giulia at my first talk as a graduate student at a Gordon Research Conference many years ago. She asked me a difficult question and I thought that it must have been interesting to work with her. This paper feels like coming full circle,” Hassanali commented.

Full article: Defects at play: Shaping the photophysics and photochemistry of ice