# Laboratory Investigation of Simultaneous Ultraviolet Photoprocessing and Temperature-Programmed Desorption of Interstellar Ice Analogs

## TL;DR

This study explores how UV light and heat affect ice mixtures in space-like conditions, revealing that simultaneous exposure changes chemical outcomes compared to sequential exposure.

## Contribution

The paper introduces new insights into how simultaneous UV irradiation and heating influence ice chemistry, challenging assumptions in interstellar ice analog experiments.

## Key findings

- Simultaneous UV irradiation and heating of ice mixtures leads to different chemical outcomes compared to sequential exposure.
- Enhanced mobility of larger radicals during simultaneous exposure increases complex molecule formation.
- Sequential irradiation and heating increases production of CO, H2CO, and CH4 but suppresses more complex species.

## Abstract

Ice mantles on dust grains in the interstellar medium
and protoplanetary
disks are sites that allow for complex chemistry to occur. The formation
of interstellar complex organic molecules (iCOMs) in these astronomical
environments relies on energetic processes such as photochemistry
driven by ultraviolet (UV) photons and thermal processes. Simultaneous
versus subsequent UV photoprocessing and temperature-programmed desorption
(TPD) on pure methanol and methanol and water mixtures (14–21%)
under laboratory conditions were studied to mimic conditions in dense
clouds and disks. In experiments where the ice was irradiated and
heated simultaneously, results suggest that comparing between different
experiments with fluence but not flux or time held constant may be
unreliable for mixed ices for some chemical species. This finding
indicates that experiments might not be comparable to interstellar
conditions if ice mixtures are involved. For experiments where the
ice was irradiated and then warmed in sequential steps, the methanol
and water ice mixtures show an enhancement of CO, H2CO,
and CH4 production as compared to experiments with simultaneous
irradiation and heating. Additionally, the production of more complex
species (i.e., (CH2OH)2, HOCH2CHO,
and CH3OCH3) is suppressed. This effect is best
explained by the enhanced mobility of larger radicals with simultaneous
heating and irradiation, yielding more complex molecules. Additional
studies with a well characterized UV lamp are needed to explore this
trend with other ice mixtures, photon fluxes, and fluences. Nonetheless,
these results call into question a common assumption made in the study
of ice analogs and may impact the interpretation of experimental and
observational results.