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Pre-melting and the adsorption of formic acid at the air-ice interface at 253 K as seen by NEXAFS and XPS

Artikel i vetenskaplig tidskrift
Författare Astrid Waldner
Luca Artiglia
Xiangrui Kong
Fabrizio Orlando
Thomas Huthwelker
Markus Ammann
Thorsten Bartels-Rausch
Publicerad i Physical Chemistry Chemical Physics
Volym 20
Sidor 24408-24417
ISSN 14639076
Publiceringsår 2018
Publicerad vid
Sidor 24408-24417
Språk en
Länkar https://doi.org/10.1039/C8CP03621G
Ämneskategorier Yt- och kolloidkemi, Spektroskopi

Sammanfattning

© the Owner Societies. Interactions between trace gases and ice are important in environmental chemistry and for Earth's climate. In particular, the adsorption of trace gases to ice surfaces at temperatures approaching the melting point has raised interest in the past, because of the prevailing pre-melting. Here, we present Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy data at ambient partial pressure of water to better define the onset temperature of pre-melting at the interfacial region of ice. Further, this study directly compares the interaction between an organic acid common in the atmosphere, formic acid, and that of an aliphatic carbon with ice at 253 K. It makes use of X-ray Photoelectron Spectroscopy (XPS) with its inherent narrow probing depth covering both the surface and near-surface bulk region when detecting electrons. We use the tender X-ray range for excitation to locate the organic species within the interfacial region with an extended probing depth compared to published XPS work. Electron kinetic energy dependent C1s photoemission data indicate that, at low coverage of a few 1014 molecules cm−2, the presence of formic acid is restricted to the upper ice layers of the interfacial region. Increasing the dosage, formic acid penetrates 6-7 nm into the air-ice interface. The presence of the more hydrophobic aliphatic carbon is restricted to the upper ice monolayers. This direct comparison of an organic acid with an aliphatic compound confirms the emerging picture where solutes enter the interfacial region of ice at a depth related to their specific tendency to form solvation shells.

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