R. Pedrys

Reactive sputtering of simple condensed gases by keV ions II: Mass spectra

Abstract Condensed gas layers of H2O, NH3 and CO at 15–20 K have been bombarded by 6 keV H+2 and 3 keV He+ and Ar+ ions. Mass spectra of the neutral species sputtered from these layers have been measured. There is a substantial yield of products which originally were not in the target material, and which have thus been formed in chemical reactions induced by the ion bombardment. The relative yields of some of the products have been found to increase with decreasing incident ion mass. This is mainly attributed to the larger amount of energy deposited by electronic stopping in such situations. From CO a nonvolatile residue is left after ion irradiation. From a layer of H2O frozen on top of the CO-residue H2CO was detected.

Sputtering of condensed noble gases by keV heavy ions

Abstract Bombardment of condensed Kr and Xe by 2–8 keV noble gas ions results in very high sputtering yields. A considerable fraction (10–30%) of the sputtered particles consists of Van der Waals clusters, with Kr2, Kr3, Xe2, XeKr, XeKr2 and ArKr having been observed. The kinetic energy distributions of the sputtered monomer-species are in agreement with a collision cascade mechanism. However, at the low energy side an excess yield is observed. This is explained by a model which takes into account the large sputtering yields and the damage of the surface during the sputtering process. The energy distributions of the dimers and trimers are satisfactorily explained by a statistical model. It is concluded that the dimer and trimer species are sputtered during an early stage of the cascade.

Reactive sputtering of simple condensed gases by kev ions III: Kinetic energy distributions

Abstract Condensed gas layers of H2O, NH3 and CO have been bombarded by 3–6 keV H+, H+2, He+ and Ar+ ions. Kinetic energy distributions of the sputtered neutral particles have been measured using a time-of-flight technique. The mechanism of ejection of the sputtered products in nearly all cases appears to involve momentum transfer in the final steps, as is inferred from the collision cascade-like energy spectra. The effective surface binding energies observed are much lower than the sublimation energies. Nevertheless most sputtered species have energies far in excess of those expected for evaporation at the ambient temperature. Only H2 from NH3 shows evaporation as the main desorption mechanism under irradiation by light ions.

Sputtering of Van der Waals molecules

Abstract The impact of keV heavy ions on frozen gases has been observed, for the first time, to result in the sputtering of Van der Waals molecules. The masses of these molecules were determined by mass spectrometry. Very high yields of molecules like Kr 2 , KrXe, Xe 2 , Kr 2 Xe were observed. From the energy distributions of the sputtered molecules it is inferred that they are formed by the so-called statistical mechanisms, due to the simultaneous ejection of two or three atoms.

Emission of large molecules from methane by ion bombardment

Abstract Condensed layers ot methane at 20 K have been bombarded by 6–8 keV Ar+, He+ and H2+ ions. Mass spectra and Kinetic energy distributions of neutral species sputtered from these layers have been measured. We have found sputtered species with masses up to 72 amu and thus with at least 5 carbon atoms. In addition to this an involatile residue was formed. Analysis by pyrolysis mass spectrometry showed this residue to contain species with masses up to at least 170 amu which therefore contain at least 12 carbon atoms. The kinetic energy distributions of sputtered methane molecules lie between those of a Maxwell-Boltzmann distribution and a collision cascade. Higher values are reached for Ar+ than for the light ions. From these observations we conclude: for both light and heavy ions radicals are formed, which combine to new molecules. These exothermic reactions produce heat which causes desorption. The high energy tail for bombardment with argon ions shows that part of the sputtering is caused by momentum transfer.

Sputtering of solid nitrogen and oxygen by keV hydrogen ions

Abstract Electronic sputtering of solid nitrogen and oxygen by keV hydrogen ions has been studied at two low-temperature setups. The yield of the sputtered particles has been determined in the energy regime 4–10 keV for H+, H 2+ and H3+ ions. The yield for oxygen is more than a factor of two larger than that for nitrogen. The energy distributions of the sputtered N2 and O2 molecules were measured for hydrogen ions in this energy regime as well. The yields from both solids turn out to depend on the sum of the stopping power of all atoms in the ion. The yield increases as a quadratic function of the stopping power for oxygen, but slightly slower for nitrogen. The energy distributions do not exhibit strong features, but are similar to those published earlier for electron sputtering.

Erosion of rare gas solids by electron bombardment

Abstract Ar and Xe solids have been bombarded by 0.5 keV electrons. Mass spectrometry and time of flight measurements were made of the ejected neutral particles. The former measurements indicate that only atoms are emitted from the surface in contrast to sputtering by ions in which also clusters are found. The time of flight distribution of Ar atoms shows features distinctly different from those for Xe. The results are explained by creation of excitations, their transport towards the surface and subsequent decay of molecular excitons to a repulsive state.

Erosion of frozen SF6 by electron bombardment

Abstract Mass spectra were taken of neutral particle fluxes sputtered from frozen SF 6 gas during bombardment by electrons with an energy of 750 eV. This spectrum is very different from that of gaseous SF 6 . In particular the small SF x fragments are much more abundant in the sputtering case. Also time of flight spectra have been taken. These show different TOF distributions for different particles. Conversion to energy spectra show these also to be different for the various sputtered species. F 2 molecules have energies up to a few tenths of an eV, the other particles up to a few eV. From these observations we infer that 1) the conversion of electronic into kinetic energy is caused mainly by dissociative excitation and/or ionization, 2) the various species are sputtered as such, 3) F 2 molecules are obtained from recombination of F atoms and subsequent desorption from the surface, 4) the results cannot be explained by assuming degradation of the initial electronic energy into a thermal spike.

Energy distributions from electron-sputtered solid nitrogen

Abstract Solid N2 has been bombarded by 0.5 keV electrons. Mass spectrometry and time of flight measurements were performed for the sputtered neutral particles. The dominant component is molecular N2, but also N-atoms and a small number of N3- and N4-molecules were observed. The energy distribution of N2 and N indicates that the sputtering is caused by non-radiative transitions that release up to more than 6 eV. The important non-radiative transitions in solid N2 are discussed.

Inelastic Effects in Sputtering of Frozen Gases by keV Ions

Recent experiments show that MeV ions passing through frozen gases cause an erosion that is at least one order of magnitude higher than predicted by classical theory. When frozen molecules of different atomic species are bombarded, mobile fragments and even new molecules as compared to the starting material have been observed within the target. Since a cascade of collisions resulting from single elastic collisions of the incident particles with target atoms account for only a very small fraction of the energy loss of MeV ions, the inelastic collisions are assumed to be exclusively responsible for the observed features [1,2].