Periodic Reporting for period 1 - PSI-COM (Photo Stability of Ice-bound Complex Organic Molecules)
Reporting period: 2024-02-01 to 2026-01-31
The chemical pathways which ice-bound molecules follow when subjected to energetic processing is a crucial part of accurately modelling the distribution of COMs -- many of which are considered the ``building blocks of life" -- in developing solar systems. This represents one of the key ways in which laboratory experiments assist in the understanding of astronomical processes; by recreating the conditions of space, and controlling factors such as incident flux, ice composition, and temperature, instrumentalists can obtain quantitative as well as qualitative insight into the chemistry occurring in star- and planet-forming regions. Such studies have shown that many of the molecular precursors to life are formed when icy mantles on dust grains are subjected to high-energy irradiation. Paradoxically, many complex organic molecules are unstable with respect to such irradiation, hinting at a balance between constructive and destructive forces in the molecular inventory of interstellar space.
This project was aimed at probing this balance by performing experiments where astronomically-relevant COMs were embedded in icy environments and irradiated with vacuum ultraviolet light. The extent to which the COM degraded was correlated with the composition of the surrounding ice matrix, and some mechanistic insight was gained by studying the root of these effects. The quantitative results for this work can be used in astronomical simulations, better accounting for the prevalence of ethanol in astronomical observations.
This project was aimed at probing this balance by performing experiments where astronomically-relevant COMs were embedded in icy environments and irradiated with vacuum ultraviolet light. The extent to which the COM degraded was correlated with the composition of the surrounding ice matrix, and some mechanistic insight was gained by studying the root of these effects. The quantitative results for this work can be used in astronomical simulations, better accounting for the prevalence of ethanol in astronomical observations.
The main activities were a series of experiments using a custom ultra-high vacuum (UHV) setup situated in the Leiden Observatory. Gas mixtures prepared with the desired composition are deposited onto a cryogenically-cooled mirror held in a UHV chamber with pressures below 5e-10 mbar, recreating the low temperature and pressure of interstellar environments. A microwave hydrogen discharge lamp was used to irradiate these ices, mimicking the secondary photons emitted when cosmic rays impinge upon molecular hydrogen in dense molecular clouds. The chemical evolution of the ice was monitored using reflection-absorption infrared spectroscopy, allowing the researcher to identify fragmentation pathways. The COM under investigation in these experiments was ethanol, which is known to form through high-energy irradiation of methanol, one of the most abundant COMs in space. As these molecules often form in CO ices, a series of experiments were carried out where the decomposition of ethanol was measured for ethanol:CO ices of different mixing ratios.
The results of this work show that when modelling the photodecomposition of COMs in interstellar environments -- a key step included in many astrochemical models of star-forming regions -- it is important to consider the environment of the COM itself. In other words, if a model includes ice-bound ethanol, then the gas-phase photodestruction cross section should not be used as a rate parameter for this decomposition channel.