Lars Breuer

On the SIMS Ionization Probability of Organic Molecules

AbstractThe prospect of improved secondary ion yields for secondary ion mass spectrometry (SIMS) experiments drives innovation of new primary ion sources, instrumentation, and post-ionization techniques. The largest factor affecting secondary ion efficiency is believed to be the poor ionization probability (α+) of sputtered material, a value rarely measured directly, but estimated to be in some cases as low as 10−5. Our lab has developed a method for the direct determination of α+ in a SIMS experiment using laser post-ionization (LPI) to detect neutral molecular species in the sputtered plume for an organic compound. Here, we apply this method to coronene (C24H12), a polyaromatic hydrocarbon that exhibits strong molecular signal during gas-phase photoionization. A two-dimensional spatial distribution of sputtered neutral molecules is measured and presented. It is shown that the ionization probability of molecular coronene desorbed from a clean film under bombardment with 40 keV C60 cluster projectiles is of the order of 10−3, with some remaining uncertainty arising from laser-induced fragmentation and possible differences in the emission velocity distributions of neutral and ionized molecules. In general, this work establishes a method to estimate the ionization efficiency of molecular species sputtered during a single bombardment event. Graphical Abstract

Formation of Neutral InmCn Clusters under C60 Ion Bombardment of Indium

The formation of neutral gas phase indium carbide clusters under C60(+) ion bombardment of solid indium was investigated using laser based postionization prior to mass spectrometric detection. Two different postionization methods were used and shown to provide saturated photoionization efficiency, thereby delivering nearly the same information about the composition of the sputtered material. The resulting size distributions of neutral In(m)C(n) clusters are compared with those of the corresponding cationic secondary cluster ions and discussed in terms of calculated cluster properties. Investigating the dependence on C60(+) ion fluence, we demonstrate that clusters containing only one carbon atom are formed in single impact events, whereas the formation of more carbon-rich clusters results from carbon accumulation at the bombarded surface.

A new setup for the investigation of swift heavy ion induced particle emission and surface modifications.

The irradiation with fast ions with kinetic energies of >10 MeV leads to the deposition of a high amount of energy along their trajectory (up to several ten keV/nm). The energy is mainly transferred to the electronic subsystem and induces different secondary processes of excitations, which result in significant material modifications. A new setup to study these ion induced effects on surfaces will be described in this paper. The setup combines a variable irradiation chamber with different techniques of surface characterizations like scanning probe microscopy, time-of-flight secondary ion, and neutral mass spectrometry, as well as low energy electron diffraction under ultra high vacuum conditions, and is mounted at a beamline of the universal linear accelerator (UNILAC) of the GSI facility in Darmstadt, Germany. Here, samples can be irradiated with high-energy ions with a total kinetic energy up to several GeVs under different angles of incidence. Our setup enables the preparation and in situ analysis of different types of sample systems ranging from metals to insulators. Time-of-flight secondary ion mass spectrometry enables us to study the chemical composition of the surface, while scanning probe microscopy allows a detailed view into the local electrical and morphological conditions of the sample surface down to atomic scales. With the new setup, particle emission during irradiation as well as persistent modifications of the surface after irradiation can thus be studied. We present first data obtained with the new setup, including a novel measuring protocol for time-of-flight mass spectrometry with the GSI UNILAC accelerator.

Secondary ion and neutral mass spectrometry with swift heavy ions: Organic molecules

The authors report on experiments regarding the electronic and nuclear sputtering of organic films. The newly built swift heavy ion induced particle emission and surface modifications setup [Meinerzhagen et al., Rev. Sci. Instrum. 87, 013903 (2016)] at the M1 Branch at the universal linear accelerator (UNILAC) beam line at GSI in Darmstadt, Germany, has been used for research on organic molecules in the electronic sputtering regime. This setup has the unique capability not only to investigate electronically sputtered ions by projectiles with kinetic energies up to several giga-electron-volt but also to detect their neutral counterparts as well by laser postionization. For this purpose, the experiment is equipped with a laser system delivering 157 nm pulses with photon energies of 7.9 eV to be utilized in single photon ionization. In addition to the investigation of sputtered ions and neutrals in the electronic sputtering regime, a comparison of typical fragments between fundamentally different sputtering ...

Secondary ion formation on indium under nuclear and electronic sputtering conditions

The electronic sputtering of indium under swift heavy ion bombardment is investigated using time of flight secondary ion mass spectrometry in combination with 157 nm laser postionization. Secondary ion and neutral mass spectra generated under the impact of 4.8 MeV/u 48Ca10+ ions are analyzed in order to determine the ionization probability of the emitted indium atoms, and the results are compared to those measured under nuclear sputtering conditions via bombardment by 5 keV Ar+ primary ions. The influence of surface contamination on the ionization probability is studied by comparing (1) a pristine surface covered by a native oxide layer, (2) a kilo-electron-volt sputter-cleaned surface, and (3) a controlled oxygen coverage established by dosing the precleaned surface with O2. It is found that the native oxide layer increases the ionization probability for both kilo-electron-volt and mega-electron-volt primary ions. In contrast, oxygen deposited on a sputter-cleaned surface results in the well-known matrix effect for kilo-electron-volt ions, but has no influence on the ionization probability for the mega-electron-volt ions. In the case of a thoroughly sputter-cleaned surface a four- to sevenfold higher ionization probability for indium atoms is found for 4.8 MeV/u 48Ca10+ as compared to 5 keV Ar+ bombardment.The electronic sputtering of indium under swift heavy ion bombardment is investigated using time of flight secondary ion mass spectrometry in combination with 157 nm laser postionization. Secondary ion and neutral mass spectra generated under the impact of 4.8 MeV/u 48Ca10+ ions are analyzed in order to determine the ionization probability of the emitted indium atoms, and the results are compared to those measured under nuclear sputtering conditions via bombardment by 5 keV Ar+ primary ions. The influence of surface contamination on the ionization probability is studied by comparing (1) a pristine surface covered by a native oxide layer, (2) a kilo-electron-volt sputter-cleaned surface, and (3) a controlled oxygen coverage established by dosing the precleaned surface with O2. It is found that the native oxide layer increases the ionization probability for both kilo-electron-volt and mega-electron-volt primary ions. In contrast, oxygen deposited on a sputter-cleaned surface results in the well-known matrix...