UV irradiation of ethanol-containing interstellar ice analogs: Photostability in CH3CH2OH:CO mixtures

TL;DR

This study probes how the photostability of solid ethanol is controlled by its ice environment, focusing on CH$_3$CH$_2$OH:CO mixtures under UV irradiation at 16 K. By combining RAIRS, QMS, and TPD with a layered radiative-transfer model, it quantifies how effective photon exposure evolves with ice composition and thickness, and how this shapes ethanol destruction and photoproduct formation. The key finding is that CO stabilizes ethanol, reducing the photodestruction cross section to $\sim1.6\times10^{-17}$ cm$^2$ per photon in highly dilute mixtures, with a cascade of photoproducts including acetaldehyde, formaldehyde, and radical species such as HCO$^\bullet$ and HOCO$^\bullet$; acetoin and possibly diacetyl are discussed as larger COM candidates. These results illuminate the balance of constructive and destructive processes governing molecular complexity in interstellar ices and imply that ethanol can persist longer in CO-rich, polar environments, impacting astrochemical models and JWST-era observations.

Abstract

Ethanol (CH3CH2OH) has been detected in interstellar ices within regions associated with the early stages of star and planet formation. Its solid-phase pathways can lead to diverse conditions that can significantly influence its photostability and -chemistry. Laboratory studies have explored the effects of energetic processing on pure ethanol ices, there is a gap in understanding how ethanol behaves in astrophysically relevant mixed ices. This proof-of-principle study aims to quantify how the ice composition influences the photostability of ethanol mixed with CO, from both physical and chemical perspectives. It also seeks to highlight the importance of balancing constructive and destructive processes. Mixtures with ethanol to CO ratios ranging from 1:0 to 1:11 are exposed to UV irradiation from a microwave discharge H lamp under UHV conditions, at 16 K. The evolution of the solid phase is tracked using reflection-absorption infrared spectroscopy, and changes in the gas phase are monitored with a quadrupole mass spectrometer. Temperature-programmed desorption experiments aid in the identification of infrared spectral features. A radiative-transfer model has been developed to account for the influence of ice composition on the effective photon flux. The model reveals that, during later stages of irradiation, photoproducts play a significant role in the absorbing of incident photons, highlighting the complex cascade of processes initiated by single-photon absorption in ethanol-containing ices. By evaluating photodestruction cross sections as a function of the initial ice composition, we found that CO exerts a stabilizing effect on ethanol. For highly dilute ethanol:CO mixtures, representative of astronomical ices, the photodestruction cross section of ethanol is estimated to ~1.6E-17 cm2/photon after correcting for the effective absorbed UV fluence of the studied interstellar ice analogs.

Figures (15)

• Figure 1: (top) Wavelength distribution of the flux of the MDHL used for irradiation. (bottom) VUV absorption cross sections for CO CruzDiaz2014a as well as ethanol and CH$_3$CHO Hrodmarsson2023 in the lamp region. 1 Mb = 10$^{-18}$ cm$^{2}$. Note that for the two COMs, these correspond to the gas-phase cross sections, and so they should not be quantitatively compared to CO.
• Figure 2: An illustration of the layered model of Section \ref{['section:layermodel']}, used to determine effective photon exposure in mixed ices, is shown in panel (a). In this case, the ice consists of two components, CO and ethanol. The effective flux is considered to be the average of the flux incident on each layer. Panels (b-c) show the result of applying this model to our samples by comparing the ratio of the effective values to the zeroth-order values for the flux $\varphi_0^{eff}$ and fluence $\phi_{avg}$.
• Figure 3: (a) RAIR spectra from the initial (black) and final (red) pure ethanol experiments, with the data representing the sum of all pure experiments (A, F, G, N). Peak assignments are taken from Boudin1998. (b) Adjusted difference spectra for the sample represented in panel (a) taken at several times during the irradiation.
• Figure 4: Partial reaction diagram reflecting processes in the pure ethanol photolysis experiments. Purple arrows indicate processes induced by photon absorption. Highlighted species have been identified in the RAIR spectra; those with boxes around them are considered to be "stable,", i.e., do not react further unless exposed to another reactive species. Acetoin is highlighted in gray due to the tentative nature of this assignment.
• Figure 5: Data is provided for a pure ethanol sample (experiment A) as well as an ethanol:CO mixture (experiment D) to illustrate the importance of acetaldehyde as a driver of chemistry in our photolysis experiments. (a) The fractional composition of the sample, over the course of irradiation, as determined from the column densities. Only species with appreciable contributions are shown for clarity. (b) The partitioning of the photon absorption events among the various ice components, as defined in Eq. \ref{['eq:NstarFrac']}. (c) The probability that an arbitrary molecule in the sample will have the indicated molecule as one of its nearest-neighbors; see Appendix \ref{['section:app2']}.