Sputtering of Oxygen Ice by Low Energy Ions

Naturally occurring ices lie on both interstellar dust grains and on celestial objects, such as those in the outer solar system. These ices are continu- ously subjected to irradiation by ions from the solar wind and/or cosmic rays, which modify their surfaces. As a result, new molecular species may form which can be sputtered off into space or planetary atmospheres. We determined the experimental values of sputtering yields for irradiation of oxygen ice at 10 K by singly (He+, C+, N+, O+ and Ar+) and doubly (C2+, N2+ and O2+) charged ions with 4 keV kinetic energy. In these laboratory experiments, oxygen ice was deposited and irradiated by ions in an ultra high vacuum chamber at low temperature to simulate the environment of space. The number of molecules removed by sputtering was observed by measurement of the ice thickness using laser interferometry. Preliminary mass spectra were taken of sputtered species and of molecules formed in the ice by temperature programmed desorption (TPD). We find that the experimental sputtering yields increase approximately linearly with the projectile ion mass (or momentum squared) for all ions studied. No difference was found between the sputtering yield for singly and doubly charged ions of the same atom within the experimental uncertainty, as expected for a process dominated by momentum transfer. The experimental sputter yields are in good agreement with values calculated using a theoretical model except in the case of oxygen ions. Preliminary studies have shown molecular oxygen as the dominant species sputtered and TPD measurements indicate ozone formation.Comment: to be published in Surface Science (2015

Vacuum-UV negative photoion spectroscopy of CF3Cl, CF3Br and CF3I

Using synchrotron radiation negative ions have been detected by mass spectrometry following vacuum-UV photoexcitation of trifluorochloromethane (CF$_3$Cl), trifluorobromomethane (CF$_3$Br) and trifluoroiodomethane (CF$_3$I). The anions F$^-$, X$^-$, F$_2^-$, FX$^-$, CF$^-$, CF$_2^-$ and CF$_3^-$ were observed from all three molecules, where X = Cl, Br or I, and their ion yields recorded in the range 8-35 eV. With the exception of Br$^-$ and I$^-$, the anions observed show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation. Dissociative electron attachment, following photoionization of CF$_3$Br and CF$_3$I as the source of low-energy electrons, is shown to dominate the observed Br$^-$ and I$^-$ signals, respectively. Cross sections for ion-pair formation are put on to an absolute scale by calibrating the signal strengths with those of F$^-$ from both SF$_6$ and CF$_4$. These anion cross sections are normalized to vacuum-UV absorption cross sections, where available, and the resulting quantum yields are reported. Anion appearance energies are used to calculate upper limits to 298 K bond dissociation energies for $D^0$(CF$_3$-X) which are consistent with literature values. We report new data for $D^0$(CF$_2$I$^-$-F) ≤ 2.7 ± 0.2 eV and $\Delta_fH^0_{298}$ (CF$_2$I$^+$) ≤ (598 ± 22) kJ mol$^{-1}$. No ion-pair formation is observed below the ionization energy of the parent molecule for CF$_3$Cl and CF$_3$Br, and only weak signals (in both I$^-$ and F$^-$) are detected for CF$_3$I. These observations suggest neutral photodissociation is the dominant exit channel to Rydberg state photoexcitation at these lower energies

A laboratory study of water ice erosion by low-energy ions

Water ice covers the surface of various objects in the outer Solar system. Within the heliopause, surface ice is constantly bombarded and sputtered by energetic particles from the solar wind and magnetospheres. We report a laboratory investigation of the sputtering yield of water ice when irradiated at 10 K by 4 keV singly (13C+, N+, O+, Ar+) and doubly charged ions (13C2+, N2+, O2+). The experimental values for the sputtering yields are in good agreement with the prediction of a theoretical model. There is no significant difference in the yield for singly and doubly charged ions. Using these yields, we estimate the rate of water ice erosion in the outer Solar system objects due to solar wind sputtering. Temperature-programmed desorption of the ice after irradiation with 13C+ and 13C2+ demonstrated the formation of 13CO and 13CO2, with 13CO being the dominant formed species.Comment: 10 pages, 7 figures, 3 tables, accepted for publication at MNRA

Implantation of multiply charged carbon ions in water ice

Context. Several objects in the Solar System like Europa, Ganymede and Callisto have frozen surface (main component: H2O). The associated thickness is bigger than the penetration depth of the relevant projectile ions. Additionally, other species such as H2O2, SO2 and CO2 have been detected on these surface. The formation mechanisms of these molecules are still under discussion. Aims. We present new experimental results on the implantation of 13Cq+ (q = 2, 3) ions at an energy of 30 keV in water ice at low temperatures (15 and 80 K). Experiments with multiply charged ions at energies of tens of keV are particularly relevant to simulating the complexity of the irradiation environment to which the surfaces of the icy moons in the outer solar system are exposed. Methods. The experiments were performed at the low energy ion beam facility ARIBE of GANIL in Caen (France). 30 keV 13Cq+ (q = 2, 3) ions have been used to bombard solid H,2O surface which were frozen at 15K and 80K. Fourier Transform Infrared Spectrometer (FTIR) was used to analyze the sample in the 5000 - 600 cm-1 (2-16.7 μm) region with a spectral resolution of 1 cm-1. Results. The results of our experiments indicate that implantation produces 13CO2 with yields in the range of 0.32-0.57 molecules ion-1. This yield seems to be independent of the temperature of the ices in the range studied. We have estimated the time scale necessary to accumulate by implantation of magnetospheric carbon ions the observed quantity of carbon dioxide on the surface of Europa, a Jovian moon. This time scale is of the order of 1.0-1.3x104 yrs which is higher than that evaluated for carbon dioxide production by other relevant processes. Conclusions.We conclude that although a relevant quantity of CO2 can be formed by carbon ion implantation, this is not the dominant formation mechanism at Europa

ASACUSA Status Report

ASACUSA progress during 2009 and plans for 201

Ionization and fragmentation of biologically relevant molecules by low energy ions and UV photons

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Desorption of small ionic fragments from oligonucleotides induced by low energy carbon ions

The degradation of oligonucleotide films containing differing base sequences induced by 4 keV C+ ions has been studied experimentally. The oligonucleotides were deposited onto a gold coated stainless steel substrate and the anions and cations released from the films were analyzed by a quadrupole mass spectrometer. The total ion desorption yield was recorded as a function of time using a constant C+ ion flux of 6 × 1014 ions cm-2 s-1. At low fluences the formation of small ionic fragments was observed, whilst for fluences greater than 1.2 × 1018 ions cm-2 molecules were sputtered from the substrate. In addition to studies of the influence of a particular base to the total cation desorption yield, the effect of base substitution with bromine was measured for negative ion desorption. These results showed a strong degradation of oligonucleotide films during ion bombardment