Collisional energy loss in cluster surface impact: Experimental, model, and simulation studies of some relevant factors

Impact of composite soft-rigid elastic projectiles: A numerical study

We study by numerical simulation the impact of a one-dimensional composite projectile, composed of two superposed homogeneous parts, on an infinitely rigid and massive wall. The coefficient of restitution and the contact time are systematically measured as functions of the contrasts of mass and stiffness between the two parts. For purely elastic parts, these quantities show complex trends associated with different dynamics of the deformation waves propagating inside the projectile. A significant portion of the initial kinetic energy can be trapped in the deformation modes: the coefficient of restitution is lowest, about 0.2, when there is a strong stiffness contrast between the two parts and the stiff and soft parts are at the leading and trailing edges of the projectile respectively. In this case, we highlight the presence of multiple bounces, whose number increases as the proportion of the soft part increases. Finally, viscoelastic parts can be implemented in the same numerical framework to successfully recover the results obtained in real composite projectile impact experiments [D'Angelo et al., Phys. Rev. E 103, 053005 (2021)2470-004510.1103/PhysRevE.103.053005].

Impact dynamics of composite elastorigid projectiles onto solid surfaces

We investigate the impact of composite objects. They consist of a soft layer on top of a rigid part with a hemispherical impacting end. The coefficient of restitution (e) of such objects is studied systematically as a function of the mass ratio and of the nature of the materials. For rather elastic materials, the coefficient of restitution is a nonmonotonic function of the mass ratio and exhibits important variations. The dynamics of the impact can be characterized by several bounces depending on the ratios between the four timescales at play. These include the duration of contact of the rigid part with the substrate and the time for the elastic waves to travel back and forth in the soft layer. In that sense, describing these projectiles requires one to take into account both the Hertzian theory of contact and the elastic waves described by Saint-Venant's approach.

Radical solutions: Principles and application of electron-based dissociation in mass spectrometry-based analysis of protein structure

In recent years, electron capture (ECD) and electron transfer dissociation (ETD) have emerged as two of the most useful methods in mass spectrometry-based protein analysis, evidenced by a considerable and growing body of literature. In large part, the interest in these methods is due to their ability to induce backbone fragmentation with very little disruption of noncovalent interactions which allows inference of information regarding higher order structure from the observed fragmentation behavior. Here, we review the evolution of electron-based dissociation methods, and pay particular attention to their application in "native" mass spectrometry, their mechanism, determinants of fragmentation behavior, and recent developments in available instrumentation. Although we focus on the two most widely used methods-ECD and ETD-we also discuss the use of other ion/electron, ion/ion, and ion/neutral fragmentation methods, useful for interrogation of a range of classes of biomolecules in positive- and negative-ion mode, and speculate about how this exciting field might evolve in the coming years.

An energy-filtering device coupled to a quadrupole mass spectrometer for soft-landing molecular ions on surfaces with controlled energy

We have developed an energy-filtering device coupled to a quadrupole mass spectrometer to deposit ionized molecules on surfaces with controlled energy in ultra high vacuum environment. Extensive numerical simulations as well as direct measurements show that the ion beam flying out of a quadrupole exhibits a high-energy tail decreasing slowly up to several hundred eV. This energy distribution renders impossible any direct soft-landing deposition of molecular ions. To remove this high-energy tail by energy filtering, a 127° electrostatic sector and a specific triplet lenses were designed and added after the last quadrupole of a triple quadrupole mass spectrometer. The results obtained with this energy-filtering device show clearly the elimination of the high-energy tail. The ion beam that impinges on the sample surface satisfies now the soft-landing criterion for molecular ions, opening new research opportunities in the numerous scientific domains involving charges adsorbed on insulating surfaces.

Role of cohesive energy in droplet fragmentation

Using molecular-dynamics simulation, we investigate the fragmentation behavior of droplets after collision with a wall. We demonstrate that the ratio of the impact to the cohesive energy E(coh) of the droplet is the key quantity characterizing the droplet fragmentation process. To show this both van der Waals-bonded Ar and N(2) droplets and polar H(2)O droplets are studied. If the impact energy per molecule E<(0.35-0.4)E(coh), the droplet is reflected without fragmenting. Beyond that impact energy fragmentation of the droplet abruptly starts. At E=E(coh), the fragmentation process already results in a fine dispersal of the droplet into daughter droplets; the maximum fragment contains only less than 4% of the initial droplet mass and around one-third of the droplet has been shattered into isolated molecules. The disintegration process continuously increases with collision energy. These findings are relevant for the process of droplet fragmentation as used in the method of impact desolvation of electrosprayed microdroplets mass spectrometry.

Mechanically induced generation of highly reactive excited-state oxygen molecules in cluster scattering

Molecular electronic excitation in (O(2))(n) clusters induced by mechanical collisions via the "chemistry with a hammer" is investigated by a combination of molecular dynamics simulations and quantum chemistry calculations. Complete active space self-consistent field augmented with triple-zeta polarizable basis set quantum chemistry calculations of a compressed (O(2))(2) cluster model in various configurations reveal the emergence of possible pathways for the generation of electronically excited singlet O(2) molecules upon cluster compression and vibrational excitation, due to electronic curve-crossing and spin-orbit coupling. Extrapolation of the model (O(2))(2) results to larger clusters suggests a dramatic increase in the population of electronically excited O(2) products, and may account for the recently observed cluster-catalyzed oxidation of silicon surfaces, via singlet oxygen generation induced by cluster impact, followed by surface reaction of highly reactive singlet O(2) molecules. Extensive molecular dynamics simulations of (O(2))(n) clusters colliding onto a hot surface indeed reveal that cluster compression is sufficient under typical experimental conditions for nonadiabatic transitions to occur. This work highlights the importance of nonadiabatic effects in the "chemistry with a hammer."

Kinetics of surface-induced dissociation of N(CH3)4(+) and N(CD3)4(+) using silicon nanoparticle assisted laser desorption/ionization and laser desorption/ionization

The implementation of surface-induced dissociation (SID) to study the fast dissociation kinetics (sub-microsecond dissociation) of peptides in a MALDI TOF instrument has been reported previously. Silicon nanoparticle assisted laser desorption/ionization (SPALDI) now allows the study of small molecule dissociation kinetics for ions formed with low initial source internal energy and without MALDI matrix interference. The dissociation kinetics of N(CH(3))(4)(+) and N(CD(3))(4)(+) were chosen for investigation because the dissociation mechanisms of N(CH(3))(4)(+) have been studied extensively, providing well-characterized systems to investigate by collision with a surface. With changes in laboratory collision energy, changes in fragmentation timescale and dominant fragment ions were observed, verifying that these ions dissociate via unimolecular decay. At lower collision energies, methyl radical (CH(3)) loss with a sub-microsecond dissociation rate is dominant, but consecutive H loss after CH(3) loss becomes dominant at higher collision energies. These observations are consistent with the known dissociation pathways. The dissociation rate of CH(3) loss from N(CH(3))(4)(+) formed by SPALDI and dissociated by an SID lab collision energy of 15 eV corresponds to log k = 8.1, a value achieved by laser desorption ionization (LDI) and SID at 5 eV. The results obtained with SPALDI SID and LDI SID confirm that (1) the dissociation follows unimolecular decay as predicted by RRKM calculations; (2) the SPALDI process deposits less initial energy than LDI, which has advantages for kinetics studies; and (3) fluorinated self-assembled monolayers convert about 18% of laboratory collision energy into internal energy. SID TOF experiments combined with SPALDI and peak shape analysis enable the measurement of dissociation rates for fast dissociation of small molecules.

Noncovalent protein tetramers and pentamers with "n" charges yield monomers with n/4 and n/5 charges

In recent years mass spectrometry based techniques have emerged as structural biology tools for the characterization of macromolecular, noncovalent assemblies. Many of these efforts involve preservation of intact protein complexes within the mass spectrometer, providing molecular weight measurements that allow the determination of subunit stoichiometry and real-time monitoring of protein interactions. Attempts have been made to further elucidate subunit architecture through the dissociation of subunits from the intact complex by colliding it into inert gas atoms such as argon or xenon. Unfortunately, the amount of structural information that can be derived from such strategies is limited by the nearly ubiquitous ejection of a single, unfolded subunit. Here, we present results from the gas-phase dissociation of protein-protein complexes upon collision into a surface. Dissociation of a series of tetrameric and pentameric proteins demonstrate that alternative subunit fragments, not observed through multiple collisions with gas atoms, can be generated through surface collision. Evidence is presented for the retention of individual subunit structure, and in some cases, retention of noncovalent interactions between subunits and ligands. We attribute these differences to the rapid large energy input of ion-surface collisions, which leads to the dissociation of subunits prior to the unfolding of individual monomers.

Dissociative scattering of hyperthermal energy CF3+ ions from modified surfaces

Dissociative scattering of CF3+ ions in collision with a self-assembled monolayer surface of fluorinated alkyl thiol on a gold 111 crystal has been studied at low ion kinetic energies (from 29 to 159 eV) using a custom built tandem mass spectrometer with a rotatable second stage energy analyzer and mass spectrometer detectors. Energy and intensity distributions of the scattered fragment ions were measured as a function of the fragment ion mass and scattering angle. Inelastically scattered CF3+ ions were not observed even at the lowest energy studied here. All fragment ions, CF2+, CF+, F+, and C+, were observed at all energies studied with the relative intensity of the highest energy pathway, C+, increasing and that of the lowest energy pathway, CF2+, decreasing with collision energy. Also, the dissociation efficiency of CF3+ decreased significantly as the collision energy was increased to 159 eV. Energy distributions of all fragment ions from the alkyl thiol surface showed two distinct components, one corresponding to the loss of nearly all of the kinetic energy and scattered over a broad angular range while the other corresponding to smaller kinetic energy losses and scattered closer to the surface parallel. The latter process is due to delayed dissociation of collisionally excited ions after they have passed the collision region as excited parent ions. A similar study performed at 74 eV using a LiF coated surface on a titanium substrate resulted only in one process for all fragment ions; corresponding to the delayed dissociation process. The intensity maxima for these fragmentation processes were shifted farther away from the surface parallel compared to the thiol surface. A new mechanism is proposed for the delayed dissociation process as proceeding via projectile ions' neutralization to long-lived highly excited Rydberg state(s), reionization by the potential field between the collision region and entrance to the energy analyzer, and subsequent dissociation several microseconds after collisional excitation. A kinematic analysis of experimental data plotted as velocity Newton diagrams demonstrates that the delayed dissociation process results from the collisions of the ion with the bulk surface; i.e., the self-assembled monolayer surface acts as a bulk surface. A similar analysis for the highly inelastic collision processes shows that these are due to stronger collisions with a fraction of the thiol molecular chain, varying in length (mass) with the ion energy.

A comparative study of cluster-surface collisions: Molecular-dynamics simulations of (H2O)1000 and (SO2)1000

A classical molecular-dynamics study of (H2O)1000 and (SO2)1000 clusters impacting with velocities between 6 x 10(2) and 8 x 10(3) ms at normal incidence on a repulsive target is presented. Using the ratio of total kinetic energy to total binding energy of the cluster as a scaling parameter, a general description of the fragmentation dynamics as well as the final fragment size distributions is achieved for the different systems. With increasing ratio, the angular distribution of the emitted monomers rapidly shifts from isotropic to anisotropic. At the highest investigated velocities, a tendency to recover more isotropic distributions is observed. Comparable transient compression of the impacting cluster is reached, on the other hand, for the same, unscaled collision velocities in both systems. For both H2O and SO2 the obtained internal temperatures of the cluster fragments are found to be independent of impact energy and close to the boiling temperature of the respective systems.

Symmetrical gas-phase dissociation of noncovalent protein complexes via surface collisions

Previous gas-phase dissociation experiments of protein-protein complexes have resulted in product ion distributions that are asymmetric by charge and mass, providing limited insight into the chemical nature of subunit organization and interaction. In these experiments, a symmetric charge distribution results from an "energy sudden" collision of protein-protein complexes with a surface, indicating that it may be possible to probe the suboligomeric structure of noncovalent complexes in the gas phase. It is proposed that energy sudden surface activation of cytochrome C homodimers results in dissociation without significant unfolding of one of the monomeric subunits. Previously proposed mechanisms for the dissociation of protein-protein complexes are discussed in the context of these results. These experiments demonstrate the potential to preserve the structural details of subunit interaction within a protein-protein complex and help elucidate the asymmetric nature of macromolecular dissociation in the gas phase.

Mechanical Simulation of the Pressure and the Relaxation to Thermal Equilibrium of a Hot and Dense Rare Gas Cluster

A cold atomic cluster can be very rapidly heated and compressed by a hypersonic impact at a hard surface. The impact can be simulated by computing a classical trajectory for the motion of the atoms. By suddenly confining the hot and dense cluster within a rigid container, it is possible to monitor the time evolution of the force acting on the faces of the container. It is found that the pressure computed this way very rapidly decays to a time-independent value. After a somewhat longer time, this value reproduces the value for the pressure computed as the sum of the kinetic and internal pressures. This agreement is expected for a system in equilibrium. These observations support the conclusion that there is a fast relaxation to thermal equilibrium in these essentially hard-sphere systems. The deviation from equilibrium is primarily due to the propagation of shock waves within the cluster. The equilibrium pressure can reach up to the megabar range.

Soft deposition of organic macromolecules with fast atom bombardment mass spectrometry

In order to investigate the requirements for soft deposition of intact positively charged organic macromolecules, an homogenous series of modal compounds such as polyphenylene dendronized perylenes (PDPs), C(80)H(52), C(200)H(132) and C(320)H(212) and a series of derivatives involving perylene derivative, C(98)H(104)N(8)O(4), terrylene derivative, C(78)H(82)N(6)O(4) and quaterrylene derivative, C(140)H(138)N(10)O(8), were used for soft-landing experiments on a metallic or matrix coated surface using fast atom bombardment mass spectrometry. Soft-landing can be achieved at impact energies below 180 eV with no production of fragments. The deposition rate shows strong energy dependence with similar behavior of the different organic compounds. A single isotope of the molecule was selected and soft-landed at increased resolution.

Low-energy impact of X−(H2O)n (X=Cl,I) onto solid surface

We investigated dissociation of X-(H2O)n (X = Cl, I, n = 13-31) by the impact onto a (La0.7Ce0.3)B6(100) surface at a collision energy Ecol of 1-5 eV per water molecule in a tandem time-of-flight mass spectrometer equipped with a translation-energy analyzer. The mechanism of the dissociation was elucidated on the basis of the measurements of the mass spectrum and the translational energies of the product anions, X-(H2O)m (m = 0-4), scattered from the surface. It was concluded that (1) the parent cluster anion impacted on the surface undergoes dissociation on the surface under quasiequilibrium with its characteristic time varying with Ecol and n, and (2) the total collision energy introduced is partitioned preferentially to the translational motions of the products on the surface and to the rotational, the vibrational, and the lattice vibrational motions (surface) in this order. The quasiequilibrium model is applicable, even at the collision energy as low as 1 eV, because the translational modes are found to be statistically distributed while the other modes are not much populated by dynamical and energetics limitation.

Trapping, reflection, and fragmentation in a classical model of atom-lattice collisions

A classical one-dimensional model of the collision of an atom of mass M with a cold, semi-infinite harmonic lattice comprised of identical atoms of mass m is considered. In the model, the interactions between the incident atom (adatom) and the lattice are described in terms of a truncated parabolic potential by which the adatom is harmonically bound to the lattice at short distances but evolves freely when its distance is larger than a critical length R(c). The dynamics of the adatom colliding with an infinitely cold lattice is studied as a function of the initial velocity of the adatom. In order to determine whether the colliding atom is bound or reflected from the lattice in the asymptotic time limit, "secondary" collision events in which the incident atom leaves and reenters the interaction zone of the lattice are carefully considered. It is demonstrated that secondary collisions anticipated to be important for heavy adatoms (mu=m/M<1) also occur in the case of light adatoms (mu > or = 1). It is shown that the neglect of secondary collisions leads to an underestimation of the lower energy bound for adatom reflection of roughly 10% for mu close to 1. By generalizing the model to allow for the breaking of lattice bonds, the phenomenon of collision-induced lattice fragmentation is investigated.

Coefficient of restitution for one-dimensional harmonic solids

Using a numerical algorithm based on the time evolution of normal modes, we calculate the coefficient of restitution eta for various one-dimensional harmonic solids colliding with a hard wall. We find that, for a homogeneous chain, eta=1 in the thermodynamic limit. However, for a chain in which weaker springs are introduced in the colliding front half, eta remains significantly less than one even in the thermodynamic limit, and the "lost" energy goes mostly into low frequency normal modes. An understanding of these results is given in terms of how the energy is redistributed among the normal modes as the chain collides with the wall. We then contrast these results with those for collisions of one-dimensional harmonic solids with a soft wall. Using perturbation theory, we find that eta=1 for all harmonic chains in the extremely soft wall limit, but that inelasticity grows with increasing chain size in contrast to hard wall collisions.