Zbigniew Postawa

Computational view of surface based organic mass spectrometry

Surface based mass spectrometric approaches fill an important niche in the mass analysis portfolio of tools. The particular niche depends on both the underlying physics and chemistry of molecule ejection as well as experimental characteristics. In this article, we use molecular dynamics computer simulations to elucidate the fundamental processes giving rise to ejection of organic molecules in atomic and cluster secondary ion mass spectrometry (SIMS), massive cluster impact (MCI) mass spectrometry, and matrix-assisted laser desorption ionization (MALDI) mass spectrometry. This review is aimed at graduate students and experimental researchers.

Internal energy of molecules ejected due to energetic C60 bombardment.

The early stages of C(60) bombardment of octane and octatetraene crystals are modeled using molecular dynamics simulations with incident energies of 5-20 keV. Using the AIREBO potential, which allows for chemical reactions in hydrocarbon molecules, we are able to investigate how the projectile energy is partitioned into changes in potential and kinetic energy as well as how much energy flows into reacted molecules and internal energy. Several animations have been included to illustrate the bombardment process. The results show that the material near the edge of the crater can be ejected with low internal energies and that ejected molecules maintain their internal energies in the plume, in contrast to a collisional cooling mechanism previously proposed. In addition, a single C(60) bombardment was able to create many free and reacted H atoms which may aid in the ionization of molecules upon subsequent bombardment events.

AFM/LFM surface studies of a ternary polymer blend cast on substrates covered by a self-assembled monolayer

Nanometer films composed of model ternary blend of deuterated polystyrene (dPS), poly(2-vinylpyridine) (PVP) and poly(methyl methacrylate) (PMMA) were studied after spin-coating from a common solvent. Surface undulations and the distribution of phase-separated domains at the surface and in the bulk are closely related as revealed by atomic (AFM) and lateral (LFM) force microscopy. For the first time the chemical sensitivity of LFM is demonstrated for a ternary polymer mixture. In this case PMMA intercalates between dPS and PVP leading to extended interfaces and surface patterns with two dominant length scales (∼1 μm and ∼100 nm). Both of these length scales as well as the film thickness increase linearly with total polymer concentration in the solvent. Phase separation on two length scales is concluded.

Understanding collision cascades in molecular solids

Abstract This paper describes simulations of the sputtering of a molecular solid that uses a reactive potential with both covalent bonding and van der Waals interactions. Recently, the adaptive intermolecular REBO (AIREBO) potential has been developed, which incorporates intermolecular interactions in a manner that maintains the reactivity of the original reactive empirical bond-order (REBO) potential. Preliminary simulations of the keV bombardment of a molecular solid have been performed using the AIREBO potential. Molecules that are initially struck by the bombarding particle break into fragments. The fragments initiate molecular collision cascades leading to the ejection of intact molecules and molecular fragments from the surface.

Electron-stimulated desorption from ionic crystal surfaces

Abstract Inelastic interactions of electrons with surfaces of ionic crystals result in emission of various particles such as ions, atoms and molecules. We will review such electron-stimulated desorption processes for the particular class of ionic crystals, namely for alkali halides. In this case, a dominant fraction of the emission is in the form of halogen and alkali atoms characterized by a thermal (Maxwellian) spectrum of translational energies. For several alkali halides (potassium and rubidium chlorides, bromides, and iodides), however, a significant part of the halogen atoms is ejected with nonthermal energies, i.e. energies of the order of 0.1 eV. The results of recent systematic studies of angular-resolved kinetic energy distributions of the emitted particles will be reported and current views on the electronic mechanisms of desorption will be described. In particular, it will be shown that the ESD mechanism can be understood in terms of the model involving a surface localisation of the so called “hot-holes” created by electron bombardment of alkali halides. A role of hot holes in ESD processes will further be discussed in relation to very recent experimental results obtained for the KBr crystals doped with In impurities which act as efficient hole traps.

Thickness effects of water overlayer on its explosive evaporation at heated metal surfaces

Abstract Molecular dynamics (MD) simulations have been employed to investigate the effect of the thickness of a water overlayer on the character of its ejection from a heated Au surface. The simulations are performed for five systems differing in the thickness of the water overlayer which was adsorbed on a metal substrate heated to 1000 K. For each system, an explosive evaporation occurs in the part of the water film adjacent to the metal surface and the upper part of the film is pushed off by the generated force. The average maximum temperature of the water film decreases as the film thickness increases. In contrast, the temperature achieved by the fast cooling due to the explosive evaporation shows an inverse trend. The significance of these model calculations to matrix-assisted laser desorption and ionization (MALDI) mass spectrometry is discussed.

Sputtering of atoms in fine structure states: A probe of excitation and de-excitation events

The electronic mechanisms leading to the formation of excited atoms from ion-bombarded metal surfaces have been examined in light of recent experimental observations. Specifically, populations and kinetic energy distributions are compared for metastable fine structure states of In, Rh, Ni, Co and Ag. The comparison shows that the populations of these depend strongly on the electronic configuration of the departing atom and its correspondence with the metallic band structure. Current hypotheses about fundamental processes are discussed. Missing parts of our understanding of these processes are enumerated, and a number of new experiments aimed toward filling in these gaps are proposed.# 1998 John Wiley & Sons, Ltd. Atomic and molecular desorption from ion bombarded surfaces is initiated not only by classical momentum transfer between colliding species but also by various processes. The electronic processes are particularly important in controlling the degree to which the desorbing species leave the surface in excited states or as positive or negative ions. An improved fundamental understanding of the basic mechanisms associated with these electronic events may indeed lead to more effective strategies for enhancing the ionization efficiency of desorbing species and to improve the prospects for mass spectral-based surface analyses. In general, excited atoms may be classified into two categories. Atoms in short-lived states, on one hand, are easy to detect by their radiative decay and, therefore, a wealth of experimental information on atoms sputtered in such states can be found in the literature. 1 The interpretation of these data, however, is extremely complicated due to the

Angle‐resolved velocity distributions of excited Rh atoms ejected from ion‐bombarded Rh{100}

The distributions of metastable excited state (4F7/2) and ground state (4F9/2) Rh atoms ejected from Ar+‐bombarded Rh{100} are experimentally determined as a function of ejection velocity and angle. Corresponding theoretical predictions are made by incorporating a nonradiative deexcitation model into molecular dynamics simulations of the bombardment process. There is good agreement between the experimental and theoretical distributions. The simulations show that a fraction of the ejected atoms are excited via collisions 1–20 A above the surface, and that these atoms make a significant contribution to the excited atom yield at low ejection velocities.

Energy‐ and angle‐resolved measurements of the Rh(4F9/2) and Rh(4F7/2) populations from ion bombarded Rh{100}

The first energy‐ and angle‐resolved measurements are presented for ground and excited state atoms ejected from a single crystal metal surface due to keV ion bombardment. These results show that at high velocities the ratio of the excited state to ground state intensity varies as exp(−A/av⊥) but at lower velocities the ratio is almost independent of velocity. We use a collisional excitation model to show conclusively that the details of the atomic motions are necessary to explain the experimental data.

Electronic and nuclear effects in ion-induced desorption from NaCl {100}

Multiphoton resonance ionization (MPRI) spectroscopy has been employed to investigate the ejection mechanisms of neutral and ionic particles from an ion‐bombarded NaCl{100} single crystal. The results are used to reveal the similarities and the differences between ion bombardment and electron irradiation of alkali halides. The mass spectra of neutral species and positive and negative ions have been measured. The yield of Na+ ions is found to be two orders of magnitude higher than in measurements with electron bombardment. It is suggested that the secondary ions are created by direct emission from the collision cascade. The ejection of neutral Na atoms is observed to be very sensitive to the temperature of the target, the angle of incidence, and the state of the surface as determined by the time‐of‐flight (TOF) measurements. In particular, it is found that most of the neutral Na atoms are emitted with thermal energies, which indicates that desorption via electronic transitions dominates over ejections from...