Search

Water ice exists on many objects in space. The most abundant icy species, among them water, are present in the icy satellites of the outer Solar System giant planets. The nuclei of comets, which are mainly composed of water ice, give another example of its abundance. In the interstellar medium (ISM), ice mantles, formed by molecular species sticking on dust grains, consist mainly of water ice. All these objects are exposed to ionizing radiation like ions, UV photons, and electrons. Sputtering of atoms, molecules, ions, and radicals from icy surfaces may populate and maintain exospheres of icy objects in the Solar System. In other respects, ionized hydrides such as OH+, H2O+, and H3O+have been detected in the gas phase in star-forming regions. Interactions with cosmic rays could be an additional explanation to the current models for the formation of those species. In fact, laboratory simulations showed that the main components of the sputtered ionic species from water ice are oxygen hydrides. In this work, water ice targets were irradiated at several temperatures (10–200 K) by 90 keV O6+ions, yielding an electronic stopping power of about 12 eV/Å, when the nuclear stopping power is comparable to the electronic stopping power. Sputtering of secondary ions after bombardment of the ice target was analyzed by time-of-flight mass spectrometry (TOF-SIMS). Besides hydrogen ions (H+, H2+, H3+), also O+, O2+, OH+, (H2O)+, and clusters of (H2O)nH+with n= 1–8 are emitted. Our results show a progressive yield decrease with increasing temperature of all of the detected species. This is related to the structure of the ice: the ionic sputtering yield for crystalline ice is much lower than for an amorphous ice. For instance, amorphous ice at 10 K exhibits a yield of the order of 2 × 10–6secondary (H2O)nH+hydride ions/projectile (with n= 1–8). As the temperature is increasing toward the phase transition to crystalline ice, the yields decrease by about one order of magnitude.

Focused ion beams are becoming important tools in nanofabrication. The underlying physical processes in the substrate were already explored for several projectile ions. However, studies of ion interaction with precursor molecules for beam-assisted deposition are almost nonexistent. Here, we explore the interaction of various projectile ions with iron pentacarbonyl. We report fragmentation patterns of isolated gas-phase iron pentacarbonyl after interaction with 4He+at a collision energy of 16 keV, 4He2+at 16 keV, 20Ne+at 6 keV, 20Ne4+at 40 keV, 40Ar+at 3 keV, 40Ar3+at 21 keV, 84Kr3+at 12 keV, and 84Kr17+at 255 keV. These projectiles cover interaction regimes ranging from collisions dominated by nuclear stopping through collisions dominated by electronic stopping to soft resonant electron-capture interactions. We report a surprising efficiency of Ne+in the Fe(CO)5decomposition. The interaction with multiply charged ions results in a higher content of parent ions and slow metastable fragmentation due to the electron-capture process. The release of CO groups during the decomposition process seems to take off a significant amount of energy. The fragmentation mechanism may be described as Fe being trapped within a CO cluster.

Chemical reactivity is observed following electron irradiation of a binary mixture of ammonia (NH3) and acetic acid (CH3COOD) at 25 K, without any subsequent thermal activation, as evidenced by vibrational high resolution electron energy loss spectroscopy (HREELS). Analysis of the HREEL spectra and comparison with infrared and Raman data of different molecules are compatible with glycine formation in its zwitterionic form. The onset for electron induced reaction is found to be at about ∼13 eV. The mechanisms may involve NH radicals interaction with CH3COOD molecules. Then glycine formation does not imply any displacement of reactants, so that it involves only NH3 and CH3COOD neighboring molecules.

Absolute total cross section (TCS) for electron-trimethylborane-d9(B(CD3)3, TMB-d9) collisions has been measured at energies ranging from 0.4 to 370 eV using the linear electron-transmission technique. The most visible feature of the TCS energy function is a pronounced broad enhancement peaked between 5 and 10 eV. Some weak structures are also perceptible in the TCS curve. At intermediate energies the experimental results are compared with the cross section obtained as a sum of the integral elastic and ionization cross sections calculated in this work for the B(CH3)3molecule. Similarities observed in cross sections for planar, boron-containing BX3(X = F, Cl, CD3) molecules are also pointed out and discussed.

Absolute total cross section (TCS) for electron scattering from sulfuryl chloride (SO2Cl2) molecules was measured in a linear transmission experiment at energies ranging from 0.5 to 150 eV. The most distinct features found in the TCS are the deep minimum located near 1.8 eV and the broad enhancement peaked around 9.5 eV with a shoulder spanned between 3 and 5 eV. At intermediate energies, the present experimental TCS results agree well with our total cross-section estimations, based on calculations of elastic and ionization cross sections. In addition, the calculated total cross section for SO2F2is reported. The results for e?-SO2Cl2collisions are compared, for the energy dependence and magnitude, with experimental and computed cross sections for other molecules (SO2F2, SO2ClF) comprising a sulfone group.

Absolute total cross section (TCS) for electron scattering from sulfuryl chloride fluoride (SO2ClF) molecules was measured in a linear transmission experiment at energies ranging from 0.6 to 370 eV. The experimental TCS energy function has a deep minimum (30 × 10?20m2) at 3 eV which separates two distinct TCS enhancements. Below 3 eV the TCS curve increases steeply towards lower energies with a small resonant maximum visible near 2.2 eV. Above 3 eV the TCS is dominated by second pronounced enhancement with the maximum (45 × 10?20m2) peaked near 9.5 eV. The experimental TCS findings are-at intermediate energies-satisfactorily reproduced with our total cross section estimation based on calculations of integral elastic (in independent atom approximation) and ionization (binary-encounter-Bethe approach) cross sections. The TCS measured for SO2ClF is also compared with experimental TCSs for molecules containing sulfur as a central atom and it is found that some features observed for SO2ClF are shared by others of the compared molecules.

Absolute total cross section (TCS) for electron scattering from sulfur tetrafluoride (SF4) molecules was measured in a linear transmission experiment at energies ranging from low (0.1 eV) to intermediate (370 eV). Below 1.6 eV the TCS function increases steeply towards lower energies and shows small resonant-like maximum near 0.4 eV. Two other distinct enhancements are located at higher energies: a narrow one is peaked at 12 eV, and a very broad hump is centred near 40 eV. Our experimental TCS results are compared with the previous TCS measurements at low impact energies and at intermediate energies-with our total cross section estimations based on calculations of elastic and ionization cross sections. The TCS for SF4is also compared with SFn(n= 1, 2, 6) total cross sections.

Laboratory data of CH4ice radiolysis promoted by fast heavy ions are obtained by infrared spectroscopy (FTIR). CH4molecules are condensed on a CsI substrate at 15 K, and the ice layer is bombarded by 220 MeV 16O7+ion beam. The ice thickness is thin enough to be traversed by projectiles at constant velocity close to the equilibrium charge state. The induced CH4dissociation gives rise to the formation of molecular species CH3, C2H2, C2H4, C2H6, and C3H8. Their formation and dissociation cross sections are determined. C2H6represent the most abundant daughter molecules. The carbon budget analysis of CH4and its radiolysis products shows that the column density of carbon atoms contained in the methane destroyed during ion irradiation is 30 − 50% greater than the sum for the column densities of the newly formed species. As an astrophysical implication, the current results allow estimation of chemical reaction rates in ices covering interstellar grains.

An experimental study of the interaction of heavy, highly charged, and energetic ions (52 MeV 58Ni13+) with pure H2O, pure CO2and mixed H2O:CO2astrophysical ice analogs is presented. This analysis aims to simulate the chemical and the physicochemical interactions induced by heavy cosmic rays inside dense and cold astrophysical environments, such as molecular clouds or protostellar clouds. The measurements were performed at the heavy ion accelerator GANIL (Grand Accélérateur National d’Ions Lourds in Caen, France). The gas samples were deposited onto a CsI substrate at 13 K. In-situ analysis was performed by a Fourier transform infrared (FTIR) spectrometer at different fluences. Radiolysis yields of the produced species were quantified. The dissociation cross sections of pure H2O and CO2ices are 1.1 and 1.9  × 10-13cm2, respectively. For mixed H2O:CO2(10:1), the dissociation cross sections of both species are about 1  ×  10-13cm2. The measured sputtering yield of pure CO2ice is 2.2  ×  104molec ion-1. After a fluence of 2−3  ×  1012ions cm-2, the CO2/CO ratio becomes roughly constant (~0.1), independent of the initial CO2/H2O ratio. A similar behavior is observed for the H2O2/H2O ratio, which stabilizes at 0.01, independent of the initial H2O column density or relative abundance.

Context. Within dense interstellar clouds, from their periphery to regions deep inside, ice mantles on dust grains are exposed to cosmic-ray irradiation. Various swift ions contribute from protons to iron in the keV to TeV energy range. Observations show that in some lines of sight condensed CO molecules are an important component of the ice.Aims. We irradiate CO ices with Ni ions of relatively high energy (50 and 537 MeV) to simulate the effects produced by fast heavy cosmic-ray ions in interstellar grain mantles.Methods. CO gas is condensed on a CsI substrate at 13 K and irradiated by 50 MeV 58Ni13+and 537 MeV 64Ni24+ions up to a final fluence of ≈1 × 1013cm-2, at a flux of 1 × 109cm-2s-1. The sputtering yields, the destruction rate of CO, and the rate of formation of new molecular species are measured in situ by Fourier transform infrared spectroscopy (FTIR).Results. The measured CO destruction cross-sections and sputtering yields induced by Ni ions are, respectively, (i) for 50 MeV, $\sigma_{\rm d}$= 1.0 × 10-13cm2and Y= 7 × 104molecules/impact; (ii) for 537 MeV, $\sigma_{\rm d}$= 3.0 × 10-14cm2and Y= 5.85 × 104molecules/impact. Based on the present and previous results, the desorption rates induced by H, Ni, and Fe ions are estimated for a wide range of energies. The contribution of the heavy ions is found to dominate over that of protons in the interstellar medium.

Deep inside dense molecular clouds and protostellar disks, interstellar ices are protected from stellar energetic UV photons. However, X-rays and energetic cosmic rays can penetrate inside these regions triggering chemical reactions, molecular dissociation, and evaporation processes. We present experimental studies of the interaction of heavy, highly charged, and energetic ions (46 MeV 58Ni13+) with ammonia-containing ices H2O:NH3(1:0.5) and H2O:NH3:CO (1:0.6:0.4) in an attempt to simulate the physical chemistry induced by heavy-ion cosmic rays inside dense astrophysical environments. The measurements were performed inside a high vacuum chamber coupled to the IRRSUD (IR radiation SUD) beamline at the heavy-ion accelerator GANIL (Grand Accelerateur National d'Ions Lourds) in Caen, France. The gas samples were deposited onto a polished CsI substrate previously cooled to 13 K. In-situ analysis was performed by a Fourier transform infrared spectrometer (FTIR) at different fluences. The average values of the dissociation cross-section of water, ammonia, and carbon monoxide due to heavy-ion cosmic ray analogs are ~$2 \times 10^{-13}$, $1.4 \times 10^{-13}$, and $1.9 \times 10^{-13}$cm2, respectively. In the presence of a typical heavy cosmic ray field, the estimated half life of the studied species is 2–$3 \times 10^6$years. The ice compaction (micropore collapse) produced by heavy cosmic rays seems to be at least 3 orders of magnitude higher than that produced by (0.8 MeV) protons. The infrared spectra of the irradiated ice samples exhibit lines of several new species including HNCO, N2O, OCN-, and NH$_4^+$. In the case of the irradiated H2O:NH3:CO ice, the infrared spectrum at room temperature contains five bands that are tentatively assigned to vibration modes of the zwitterionic glycine (NH$_3^+$CH2COO-).

We have studied collisions between 22.5 keV He2+ ions and mixed clusters [(C60)m(C24H12)n] of m C60 and n coronene molecules where m and n range up to about ten. Surprisingly, the cluster fragmentation behavior in distant collisions is dramatically different for pure coronene clusters (m=0) and clusters containing a single C60 molecule (m=1). In the latter case, the clusters may be ionized without also being fragmented on the experimental time scale of tens of microseconds. This does not occur for pure coronene clusters, but is a main characteristic of pure fullerene clusters. For ion trajectories penetrating the mixed cluster, we observe covalent bond formations between C59 or C58 and C60, but not between coronene fragments and C60, or between C60 fragments and coronene. These results are explained by means of classical molecular dynamics simulations of collisions inside the fragmenting mixed clusters. [ABSTRACT FROM AUTHOR]

Dumbbell-shaped fullerene dimers C+118 and C+119 have recently been observed in mass spectra resulting from collisions between clusters of C60 molecules and keV He2+ or Ar2+ ions [H. Zettergren et al., Phys. Rev. Lett. 110, 185501 (2013) and F. Seitz et al., J. Chem. Phys. 139, 034309 (2013)]. To unveil the formation mechanisms of these fullerene dimers, systematic molecular dynamics (MD) simulations based on the self-consistent charge density functional tight-binding method have been performed for Cn+ + C60 (n=58,59,60) collisions following prompt atom knockouts by the fast ions. The statistics from the MD simulations indicate a much higher reactivity of C59+ and C58+ fragments compared to that of C60+. It is found that the covalently bonded dumbbell-shaped fullerene dimers C118+ and C119+ can be formed at very low-collision energies within 1 ps and are stable enough to survive on the microsecond time scale of the experiment. The thermodynamic and kinetic stabilities, as well as the bonding features, have been investigated for the most stable dumbbell dimers C118+, C119+, and C120+. [ABSTRACT FROM AUTHOR]

We report on an experimental study of the ionization and fragmentation of clusters of κ polycyclic aromatic hydrocarbon (PAH) molecules using anthracene, C14H10, or coronene, C24H12. These PAH clusters are moderately charged and strongly heated in small impact parameter collisions with 22.5-keV He2+ ions, after which they mostly decay in long monomer evaporation sequences with singly charged and comparatively cold monomers as dominating end products. We describe a simple cluster evaporation model and estimate the number of PAH molecules in the clusters that have to be hit by He2+ projectiles for such complete cluster evaporations to occur. Highly charged and initially cold clusters are efficiently formed in collisions with 360-keV Xe20+ ions, leading to cluster Coulomb explosions and several hot charged fragments, which again predominantly yield singly charged, but much hotter, monomer ions than the He2+ collisions. We present a simple formula, based on density-functional-theory calculations, for the ionization energy sequences as functions of coronene cluster size, rationalized in terms of the classic electrostatic expression for the ionization of a charged conducting object. Our analysis indicates that multiple electron removal by highly charged ions from a cluster of PAH molecules rapidly may become more important than single ionization as the cluster size k increases and that this is the main reason for the unexpectedly strong heating in these types of collisions. [ABSTRACT FROM AUTHOR]