Igor V. Veryovkin

Analyzing individual presolar grains with CHARISMA

Isotopic analysis of heavy elements in individual stardust grains is important in testing and constraining theories of stellar nucleosynthesis. These analyses are challenging in that the grains are very small, the largest being perhaps a few microns in diameter, and contain only trace concentrations of heavy elements, generally on the order of ppm. In addition, isotopic analysis requires the suppression of isobaric interferences. We describe a unique instrument, based on resonant ionization mass spectrometry, that has successfully characterized such grains for the past several years, and report on some recent upgrades that significantly enhance the instrumental capabilities. The fundamental principles and operational details are discussed, along with illustrative results and plans for future modifications.

MODEL FOR LARGE CLUSTER EMISSION IN ION SPUTTERING OF METALS APPLIED TO ATOMIC AND POLYATOMIC ION BOMBARDMENTS

Abstract We propose a model for the emission of large clusters (N⩾5) under ion bombardment of metals. In the frame of this model, the relative cluster yields have been calculated as a function of the number of atoms in the cluster for the sputtering of Al, Ag, Ta and Nb targets by atomic and polyatomic ions with energies of several keV/atom. The calculated data have shown a very good agreement with experimental results obtained by us and other researchers. On the basis of this agreement we have concluded that larger clusters are emitted as whole agglomerates during the early stages of the collision cascade evolution.

Estimation of useful yield in surface analysis using single photon ionisation

Abstract Secondary ion mass spectrometry (SIMS), laser sputter neutral mass spectrometry (SNMS) and laser desorption photoionisation (LDPI) have been used to investigate the desorption of molecules from self-assembled monolayers of phenylsulphides. LDPI, using an F2 excimer laser to single photon ionise gave the lowest fragmentation. A useful yield greater than 0.5% was found for analysis of diphenyldisulphide self-assembled monolayers. It is shown that using a free electron laser to postionise will lead, in the future, to analysis of many atoms and molecules with useful yields approaching 30%.

Fragmentation lifetimes and the internal energy of sputtered clusters

Abstract We have compared the distribution of internal energies and fragmentation rate constants determined experimentally for sputtered Fe n + cluster ions with theoretical Molecular Dynamics computer simulations. It is found that the experimental data and the simulation are complementary with respect to the fragmentation time scale involved. While the experiment is sensitive to fragmentation times of nanoseconds and above, the simulation can provide information about the time interval ranging from femtoseconds to about one nanosecond. From the experimental data, it is found that the distribution of fragmentation rates as a function of time after the emission of the clusters follows a power law rather than an exponential decay, thus indicating a broad distribution of fragmentation rate constants. From the simulation, we conclude that this dependence continues down into the sub-nanosecond time range with, however, increasing exponent as the times get shorter. Around fragmentation times at or below 10 −12 s, the rate distribution levels off due to a maximum possible rate constant of the order of the vibrational frequency of the cluster. The fragmentation rate constants are connected with the internal energy distribution of the sputtered clusters by means of statistical RRK theory. While the average internal energy determined by experiment and simulation agree quite well, significant differences are found in the width of the respective distributions, the origin of which is attributed to the different times scales explored by both techniques.

Non-additive sputtering of niobium and tantalum as neutral and charged clusters

Abstract An analysis of available literature data on both positive ion emission from Nb and Ta bombarded with 6 keV/atom Aum− atomic and molecular ions (m=1, 2, 3) and positive ionization probabilities of Nbn and Tan neutral clusters sputtered from the same metals by 5 keV Ar+ ions has been conducted. Dependencies of cluster yields Yn,m (regardless of charge state) on the number of atoms n in a sputtered particles were found to follow a power law as Yn,m∼n−σm where σm decreases with an increase of m. A non-linear enhancement of yields for large Nbn+ and Tan+ cluster ions (n>4) appeared to be a result of a non-additive process of sputtering rather than of a non-additive process of their ionization. A manifestation of the non-additive sputtering in kinetic energy distribution of secondary ions was found to be different for atomic and cluster ions.

The Characterization Of Secondary Electron Emitters For Use In Large Area Photo‐Detectors

The Large‐Area Picosecond Photo‐Detector Project is focused on the development of large‐area systems to measure the time‐of‐arrival of relativistic particles with, ultimately, 1 pico‐second resolution, and for signals typical of Positron‐Emission Tomography (PET), a resolution of about 30 pico‐seconds. Our contribution to this project is to help with identification and efficient fabrication of novel electron emitting materials with properties optimized for use in such detectors. We have assembled several techniques into a single ultra‐high vacuum apparatus in order to enable characterization of both photocathode and secondary electron emission (SEE) materials. This apparatus will examine how photocathode quantum efficiency and SEE material electron yield correlate to surface chemical composition, state, and band structure. The techniques employed in this undertaking are X‐ray photoelectron spectroscopy (XPS) for surface chemical composition, ultraviolet photoelectron spectroscopy (UPS) for the determinati...

New Cs sputter ion source with polyatomic ion beams for secondary ion mass spectrometry applications

A simple design for a cesium sputter ion source compatible with vacuum and ion-optical systems as well as with electronics of the commercially available Cameca IMS-4f instrument is reported. This ion source has been tested with the cluster primary ions of Si(n)(-) and Cu(n)(-). Our experiments with surface characterization and depth profiling conducted to date demonstrate improvements of the analytical capabilities of the secondary ion mass spectrometry instrument due to the nonadditive enhancement of secondary ion emission and shorter ion ranges of polyatomic projectiles compared to atomic ones with the same impact energy.

High lateral resolution vs molecular preservation in near-IR fs-laser desorption postionization mass spectrometry

Ultrashort pulse length lasers operating in the near-infrared region show promise for submicrometer lateral resolution by laser desorption-based mass spectrometry (MS) imaging. However, these experiments must balance lateral resolution and molecular fragmentation since abundant atomic ions are observed at the high laser irradiances that can be generated by tightly focused ultrashort pulse laser beams. It is shown here that combining ultrashort pulse laser desorption with laser postionization (fs-LDPI) allows for a considerable increase of molecular ion signal while operating with lower laser irradiances, yielding the added benefit of reduced molecular fragmentation. This Letter presents several experimental results in support of the fs-LDPI approach for MS imaging. First, the lateral resolution for MS imaging of molecular species desorbed by ∼75 fs, 800 nm laser pulses was determined to be <2 μm for a simulated organic electronic device under vacuum. Next, the dependence of precursor ion survival on both desorption laser fluence and delay between desorption and photoionization laser pulses was observed for a small molecule desorbed from an organic multilayer that was originally devised as a model of a bacterial biofilm. When considered in light of recent results in the literature (Milasinovic et al. J. Phys. Chem. C 2014, DOI: 10.1021/jp504062u), these experiments demonstrate the potential for submicrometer spatial resolution MS imaging by fs-LDPI.

Single photon ionisation of self assembled monolayers

Self assembled monolayers formed from benzenethiol, diphenylsulphide and diphenyldisulphide have been analysed using secondary ion mass spectrometry (SIMS), sputter neutral mass spectrometry (SNMS) and laser desorption photoionisation mass spectrometry (LDPI). The peak corresponding to the parent ion was much stronger in LDPI than with SIMS or SNMS analysis and fragmentation was lower. A useful yield of order 0.5% was obtained for LDPI from diphenyldisulphide.

Cn− and CsmCn− clusters sputtered from fullerite compared with those from graphite

Abstract The mass spectra and unimolecular fragmentation of the C n − and Cs m C n − clusters sputtered from fullerite and graphite by Cs + ions have been investigated. The obtained results have shown that a drastic difference between the sputtering of the fullerene substance and graphite was not observed provided that the region of collision cascade is limited by the carbon volume only. The possible mechanisms of cluster emission have been discussed.