Abstract
The heterogeneous radiolysis of organic molecules in clays is a matter of considerable interest in astrochemistry and environmental sciences. However, little is known about the effects of highly ionizing soft X-rays. By combining monochromatized synchrotron source irradiation with in situ Near Ambient Pressure X-ray Photoelectron Spectroscopy (in the mbar range), and using the synoptic view encompassing both the gas and condensed phases, we found the water and pyridine pressure conditions under which pyridine is decomposed in the presence of synthetic Sr2+-hydroxyhectorite. The formation of a pyridine/water/Sr2+ complex, detected from the Sr 3d and N 1s core-level binding energies, likely presents a favorable situation for the radiolytic breaking of the O-H bond of water molecules adsorbed in the clay and the subsequent decomposition of the molecule. However, decomposition stops when the pyridine pressure exceeds a critical value. This observation can be related to a change in the nature of the active radical species with the pyridine loading. This highlights the fact that the destruction of the molecule is not entirely determined by the properties of the host material, but also by the inserted organic species. The physical and chemical causes of the present observations are discussed.
Highlights
In UHV conditions, 1.5 1012 photons × s−1, and 3 1012 photons × s−1 and 2 1012 photons × s−1 reached the sample at 450, 750 and 1050 eV respectively
Along its way across the gas phase, the photon flux loss due to absorption was almost negligible in the present pressure conditions
Under a pressure of 0.5 mbar of water and 0.1 mbar of pyridine, one estimates that 97%, 98% and 99% of the photons reach the sample surface at photon energies of 450, 750 and 1050 eV, respectively[76]
Summary
In UHV conditions, 1.5 1012 photons × s−1, and 3 1012 photons × s−1 and 2 1012 photons × s−1 reached the sample at 450, 750 and 1050 eV respectively. Along its way across the gas phase (about 5 cm), the photon flux loss due to absorption was almost negligible in the present pressure conditions. Typical soft X-ray photon fluxes in space considered in ref.[37] are orders of magnitude smaller than the present fluxes. They can reach 1010 photons × cm−2 × s−1 in the X-ray dominated photodissociation regions of molecular clouds, but are much smaller in protoplanetary disks, 103 photons × cm−2 × s−
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