Laboratory Evidence that Methanol-Rich Ice Mantles Lower Methyl Formate Binding Energies
Abstract
Complex organic molecules are widely detected in star-forming regions and are important precursors to prebiotic species. Because these molecules are thought to form primarily within icy grain mantles, their desorption is assumed to be regulated by the sublimation of the surrounding H2O ice matrix. However, observations increasingly reveal gas-phase abundances of methyl formate (MF) at temperatures well below the sublimation temperature of water ice (T ~ 100K), challenging this view. To investigate how ice composition and phase influence MF desorption, we present laboratory temperature-programmed desorption measurements of MF from four astrophysically relevant substrates: amorphous and crystalline H2O and CH3OH. Desorption kinetics were analyzed using pre-exponential factors derived from transition-state theory with leading-edge analysis and non-negative least-squares inversion to recover binding-energy distributions. We find that MF binds more weakly to CH3OH ice than to H2O-rich substrates, with representative binding energies of ~6247K for amorphous H2O, ~6213K for crystalline H2O, and ~5469K for amorphous CH3OH. MF deposited on crystalline CH3OH exhibits coverage-independent leading edges consistent with multilayer or island desorption, yielding an effective binding energy of 5333K. Enhanced surface mobility and reduced trapping efficiency on methanol ice, combined with weaker MF binding, provide a natural explanation for the early appearance of gas-phase MF observed in star-forming environments, with characteristic desorption temperatures of ~85K on CH3OH and ~95-100K on H2O. Together, these effects support a two-step thermal desorption pathway in which MF associated with CH3OH-rich ice can be released prior to bulk H2O sublimation.
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BibTeX
@misc{gross2026laboratory,
title = {{Laboratory Evidence that Methanol-Rich Ice Mantles Lower Methyl Formate Binding Energies}},
author = {Rachel E. Gross and Jeroen Terwisscha van Scheltinga and L. Ilsedore Cleeves and Catherine A. Dukes and Robin T. Garrod and Alexia Simon and Shane Sawyer and Adam Woodson},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.27031},
eprint = {2609.27031},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.27031},
}
RIS
TY - UNPB
TI - Laboratory Evidence that Methanol-Rich Ice Mantles Lower Methyl Formate Binding Energies
AU - Rachel E. Gross
AU - Jeroen Terwisscha van Scheltinga
AU - L. Ilsedore Cleeves
AU - Catherine A. Dukes
AU - Robin T. Garrod
AU - Alexia Simon
AU - Shane Sawyer
AU - Adam Woodson
PY - 2026
DA - 2026/09/22
PB - arXiv
DO - 10.48550/arxiv.2609.27031
UR - https://arxiv.org/abs/2609.27031
AB - Complex organic molecules are widely detected in star-forming regions and are important precursors to prebiotic species. Because these molecules are thought to form primarily within icy grain mantles, their desorption is assumed to be regulated by the sublimation of the surrounding H_2O ice matrix. However, observations increasingly reveal gas-phase abundances of methyl formate (MF) at temperatures well below the sublimation temperature of water ice (T ~ 100K), challenging this view. To investigate how ice composition and phase influence MF desorption, we present laboratory temperature-programmed desorption measurements of MF from four astrophysically relevant substrates: amorphous and crystalline H_2O and CH_3OH. Desorption kinetics were analyzed using pre-exponential factors derived from transition-state theory with leading-edge analysis and non-negative least-squares inversion to recover binding-energy distributions. We find that MF binds more weakly to CH_3OH ice than to H_2O-rich substrates, with representative binding energies of ~6247K for amorphous H_2O, ~6213K for crystalline H_2O, and ~5469K for amorphous CH_3OH. MF deposited on crystalline CH_3OH exhibits coverage-independent leading edges consistent with multilayer or island desorption, yielding an effective binding energy of 5333K. Enhanced surface mobility and reduced trapping efficiency on methanol ice, combined with weaker MF binding, provide a natural explanation for the early appearance of gas-phase MF observed in star-forming environments, with characteristic desorption temperatures of ~85K on CH_3OH and ~95-100K on H_2O. Together, these effects support a two-step thermal desorption pathway in which MF associated with CH_3OH-rich ice can be released prior to bulk H_2O sublimation.
ER -