Abstract
CO2-rich planetary atmospheres are continuously exposed to ionising radiation driving major photochemical processes. In the Martian atmosphere, CO2 clusters are predicted to exist at high altitudes motivating a deeper understanding of their photochemistry. In this joint experimental-theoretical study, we investigate the photoreactions of CO2 clusters (≤2 nm) induced by soft X-ray ionisation. We observe dramatically enhanced production of {{{{{{{{rm{O}}}}}}}}}_{2}^{+} from photoionized CO2 clusters compared to the case of the isolated molecule and identify two relevant reactions. Using quantum chemistry calculations and multi-coincidence mass spectrometry, we pinpoint the origin of this enhancement: A size-dependent structural transition of the clusters from a covalently bonded arrangement to a weakly bonded polyhedral geometry that activates an exothermic reaction producing {{{{{{{{rm{O}}}}}}}}}_{2}^{+}. Our results unambiguously demonstrate that the photochemistry of small clusters/particles will likely have a strong influence on the ion balance in atmospheres.
Highlights
CO2-rich planetary atmospheres are continuously exposed to ionising radiation driving major photochemical processes
The most prominent experimentally observed fragmentation channels correspond to coincident detection of ðCO2Þþm=ðCO2Þþn (m, n = 1, 2, 3, . . . ), i.e. related to the dissociation of clusters into smaller units, in agreement with previous studies[21,31]
To investigate the competition between the multiplefragmentation channels, we look at the ratio of measured events in regions A and B in the Dalitz plots that were generated for different mean cluster sizes
Summary
CO2-rich planetary atmospheres are continuously exposed to ionising radiation driving major photochemical processes. In the Martian atmosphere, CO2 clusters are predicted to exist at high altitudes motivating a deeper understanding of their photochemistry. In this joint experimental-theoretical study, we investigate the photoreactions of CO2 clusters (≤2 nm) induced by soft X-ray ionisation. In CO2-rich atmospheres like that of Mars, the presence of CO2 particles at different altitudes between 60 and 100 km has been confirmed in the form of CO2 ice aerosols[6] and CO2 ice clouds[7,8,9,10]. Whilst classical nucleation theories rule out homogeneous nucleation of CO2 molecules into larger particles in the atmosphere of Mars[11], recent quantum chemical calculations predict the existence of pure CO2 clusters at high altitudes[12]. Recently revealed the fundamental role that aerosols and water clusters play in the photochemistry that takes place at altitudes 100 km in the Martian atmosphere is well understood[15,16]
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