In this work, the fragmentation, survival, and chemical reactions of formic acid (HCOOH) molecules condensed at 56 K are analyzed using plasma desorption mass spectrometry (PDMS) and photon-stimulated ion desorption (PSID) in an effort to simulate the effects of energetic charged particles (e.g., cosmic rays) and radiation fields on interstellar/cometary ices. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing soft X-ray photons (535.1 eV) and energetic ions (∼65 MeV) obtained as 252 Cf fission fragments. Mass spectra of positive and negative desorbed ions were obtained using a time-of-flight (TOF) spectrometer, providing information on the fragmentation pattern and abundance of the ionic species released from the icy surface. In both techniques, the major contribution to the released/desorbed ions were positively charged fragments. The production of several series of clusters, some of them with mass/charge ratios of up to 500 u/e, was observed in the PDMS spectra. Comparison between the employed techniques (photon and ion impacts) indicates that the interaction of energetic ions with formic acid ice produces a greater variety of ions than soft X-ray photon impact. This suggests that cosmic rays and high-energy solar wind particles, despite its reduced flux compared to other lower-energy particles, might play an important role in the synthesis of prebiotic molecules.
Condensed CO and CO2 are bombarded by approximately 65 MeV 252Cf fission fragments and the desorbed ions are analyzed by time-of-flight mass spectrometry as a function of target temperature, in the ranges 25-33 K and 75-91 K, respectively. Absolute desorption yields are measured up to complete ice sublimation. The mass spectra of both ice targets reveal the emission of: (1) low mass ions, produced by direct Coulomb interaction of the highly charged projectiles and delta-electrons with CO and CO2, and (2) pronounced series of cluster ions. The basic ice cluster structures (CO)n and (CO2)n are present in the emitted cluster series such as (CO)nCO+, (CO2)nCO2+, or (CO2)nCO3-. In the case of CO ice, however, the intense production of the series Cn+, Cn-, and (CO)mCn+ shows that Cn is the main cluster structure, consequence of a higher concentration of free carbon atoms in the nuclear track plasma of CO ice than in that of CO2 ice. Ion cluster abundance is observed to decrease exponentially with cluster mass. The decay constant is k(n) congruent with 0.13, about the same for series based on (CO)n and (CO2)n, but a factor 3.3 higher for the Cn series. The Cn clusters are formed by gas-phase condensation, but the (CO)n and (CO2)n clusters are produced by fracturing of the highly excited solid around the nuclear track. A dramatic reduction of the ion desorption yield is observed near T = 29 K for CO and near T = 85 K for CO2, when fast sublimation occurs and ice thickness vanishes. Close to sublimation temperature, the decay constant of the (CO)2Cn+ series increases due to a decreasing formation probability of large Cn clusters.
Two ices, O2 and a mixture of O2 and N2, are bombarded by 252Cf fission fragments (FF) (approximately 65 MeV at target surface); the emitted positive and negative secondary ions are analyzed by time-of-flight mass spectrometry (TOF-SIMS). These studies shall enlighten sputtering from planetary and interstellar ices. Three temperature regions in the 28-42-K range are analyzed: (1) before N2 sublimation, in which hybrid chemical species are formed, (2) before O2 sublimation, in which the TOF mass spectrum is dominated by low-mass (O2)p cluster ions and (3) after O2 sublimation, in which (N2)p or (O2)p cluster ions are practically inexistent. In the first region, four hybrid ion series are observed: NOn-1+, N2On-2(+/-), and N4On-4(-). In the second region, two positive and negative ion series are identified: (O2)pO(+/-) and (O2)pO2(+/-). Their yield distributions are fitted by the sum of two decreasing exponentials, whose decay constants are the same for all series. It is observed that the cluster ion desorption from solid oxygen is very similar to that of other frozen gases, but its yield distribution oscillates with a three- or six-atom periodicity, suggesting O3 or 3O2 units in the cluster structure, respectively.
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