Electronic Sputtering Produced by Fission Fragments on Condensed CO and CO 2

• Published in: Journal of the American Society for Mass Spectrometry 2006-08, Vol.17 (8), p.1120-1128
• Main Authors: Ponciano, C.R., Martinez, R., Farenzena, L.S., Iza, P., da Silveira, E.F., Homem, M.G.P., Naves de Brito, A., Wien, K.
• Format: Article
• Language: English
• ISSN: 1044-0305; 1879-1123
• DOI: 10.1016/j.jasms.2006.04.018

Condensed CO and CO 2 are bombarded by ∼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 δ-electrons with CO and CO 2, and (2) pronounced series of cluster ions. The basic ice cluster structures (CO) n and (CO 2) n are present in the emitted cluster series such as (CO) nCO +, (CO 2) nCO 2 +, or (CO 2) nCO 3 −. In the case of CO ice, however, the intense production of the series C n +, C n −, and (CO) mC n + shows that C n 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 CO 2 ice. Ion cluster abundance is observed to decrease exponentially with cluster mass. The decay constant is k n ≅ 0.13, about the same for series based on (CO) n and (CO 2) n, but a factor 3.3 higher for the C n series. The C n clusters are formed by gas-phase condensation, but the (CO) n and (CO 2) 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 CO 2, when fast sublimation occurs and ice thickness vanishes. Close to sublimation temperature, the decay constant of the (CO) 2C n + series increases due to a decreasing formation probability of large C n clusters.