Fluid Flow and Effusive Desorption: Dominant Mechanisms of Energy Dissipation after Energetic Cluster Bombardment of Molecular Solids

Effect of the Impact Angle on the Kinetic Energy and Angular Distributions of β-Carotene Sputtered by 15 keV Ar2000 Projectiles

Molecular dynamics (MD) computer simulations are used to model ejection of particles from β-carotene samples bombarded by 15 keV Ar₂₀₀₀. The effect of the incidence angle on the angular and kinetic energy distributions is investigated. It has been found that both of these distributions are sensitive to the variation of the incidence angle, particularly near the normal incidence. For impacts along the surface normal, material ejection is azimuthally symmetric, and a significant emission occurs along the surface normal. The kinetic energy distribution of intact molecules has a maximum near 1 eV and terminates below approximately 2 eV. An increase of the incidence angle breaks the azimuthal symmetry. Most of the intact molecules become ejected in the forward direction. The maximum in the polar angle distribution shifts toward large off-normal angles. In addition, the most probable kinetic energy of ejected molecules is significantly increased. The mechanisms of molecular emission responsible for the observed changes are delineated. The implications of the observed ejection characteristics for the utilization of large gas cluster projectiles in secondary neutral mass spectrometry are discussed.

Hypervelocity cluster ion impacts on free standing graphene: Experiment, theory, and applications

We present results from experiments and molecular dynamics (MD) simulations obtained with C₆₀ and Au₄₀₀ impacting on free-standing graphene, graphene oxide (GO), and graphene-supported molecular layers. The experiments were run on custom-built ToF reflectron mass spectrometers with C₆₀ and Au-LMIS sources with acceleration potentials generating 50 keV C₆₀ ²⁺ and 440-540 keV Au₄₀₀ ⁴⁺. Bombardment-detection was in the same mode as MD simulation, i.e., a sequence of individual projectile impacts with separate collection/identification of the ejecta from each impact in either the forward (transmission) or backward (reflection) direction. For C₆₀ impacts on single layer graphene, the secondary ion (SI) yields for C₂ and C₄ emitted in transmission are ∼0.1 (10%). Similar yields were observed for analyte-specific ions from submonolayer deposits of phenylalanine. MD simulations show that graphene acts as a trampoline, i.e., they can be ejected without destruction. Another topic investigated dealt with the chemical composition of free-standing GO. The elemental composition was found to be approximately COH₂. We have also studied the impact of Au₄₀₀ clusters on graphene. Again SI yields were high (e.g., 1.25 C^-/impact). 90-100 Au atoms evaporate off the exiting projectile which experiences an energy loss of ∼72 keV. The latter is a summation of energy spent on rupturing the graphene, ejecting carbon atoms and clusters and a dipole projectile/hole interaction. The charge distribution of the exiting projectiles is ∼50% neutrals and ∼25% either negatively or positively charged. We infer that free-standing graphene enables detection of attomole to zeptomole deposits of analyte via cluster-SI mass spectrometry.

Gas Cluster Ion Beams for Secondary Ion Mass Spectrometry

Gas cluster ion beams (GCIBs) provide new opportunities for bioimaging and molecular depth profiling with secondary ion mass spectrometry (SIMS). These beams, consisting of clusters containing thousands of particles, initiate desorption of target molecules with high yield and minimal fragmentation. This review emphasizes the unique opportunities for implementing these sources, especially for bioimaging applications. Theoretical aspects of the cluster ion/solid interaction are developed to maximize conditions for successful mass spectrometry. In addition, the history of how GCIBs have become practical laboratory tools is reviewed. Special emphasis is placed on the versatility of these sources, as size, kinetic energy, and chemical composition can be varied easily to maximize lateral resolution, hopefully to less than 1 micron, and to maximize ionization efficiency. Recent examples of bioimaging applications are also presented.

Adaptive Control of Ion Yield in Femtosecond Laser Post-ionization for Secondary Ion Mass Spectrometry

Secondary ion mass spectrometry is an excellent technique of analytical chemistry, where primary ions sputter a solid sample generating the secondary ions which are determined. Although the ion yield is inherently low, it can be enhanced by using a post-ionization of sputtered neutral species. Our novel approach integrates this technique with a near infrared femtosecond laser post-ionization based on an adaptive control through a laser pulse shaper. The shaping of the laser pulse provides adaptive control to select a mass peak of interest and to enhance this peak intensity. Versatility is confirmed by optimizing the ion yield for different molecules (tryptophan, anthracene, polyethylene, and oxalic acid) with focus on parent ion enhancement, fragmentation process, sublimation effect, and excited secondary species. This proof-of-concept experiment provides not only a nonspecific increase of the overall ion yield, but also the selection of specific secondary species and the adaptive enhancement of their intensities on the order of 100, potentially simplifying data interpretation. Such tailored spectra might advance the (secondary ion) mass spectrometry to new capabilities.

Metal-assisted polyatomic SIMS and laser desorption/ionization for enhanced small molecule imaging of bacterial biofilms

Mass spectrometry imaging (MSI) has become an important analytical tool for many sectors of science and medicine. As the application of MSI expands into new areas of inquiry, existing methodologies must be adapted and improved to meet emerging challenges. Particularly salient is the need for small molecule imaging methods that are compatible with complex multicomponent systems, a challenge that is amplified by the effects of analyte migration and matrix interference. With a focus on microbial biofilms from the opportunistic pathogen Pseudomonas aeruginosa, the relative advantages of two established microprobe-based MSI techniques-polyatomic secondary ion mass spectrometry (SIMS) and laser desorption/ionization-are compared, with emphasis on exploring the effect of surface metallization on small molecule imaging. A combination of qualitative image comparison and multivariate statistical analysis demonstrates that sputtering microbial biofilms with a 2.5 nm layer of gold selectively enhances C60-SIMS ionization for several molecular classes including rhamnolipids and 2-alkyl-quinolones. Metallization also leads to the reduction of in-source fragmentation and subsequent ionization of media-specific background polymers, which improves spectral purity and image quality. These findings show that the influence of metallization upon ionization is strongly dependent on both the surface architecture and the analyte class, and further demonstrate that metal-assisted C60-SIMS is a viable method for small molecule imaging of intact molecular ions in complex biological systems.

Characteristics of positive and negative secondary ions emitted from [Formula: see text] and [Formula: see text] impacts

The current limitation for SIMS analyses is insufficient secondary ion yields, due in part to the inefficiency of traditional primary ions. Massive gold clusters are shown to be a route to significant gains in secondary ion yields relative to other commonly used projectiles. At an impact energy of 520 keV, [Formula: see text] is capable of generating an average of greater than ten secondary ions per projectile, with some impact events generating >100 secondary ions. The capability of this projectile for signal enhancement is further displayed through the observation of up to seven deprotonated molecular ions from a single impact on a neat target of the model pentapeptide leu-enkephalin. Positive and negative spectra of leu-enkephalin reveal two distinct emission regimes responsible for the emission of either intact molecular ions with low internal energies or small fragment species. The internal energy distribution for this projectile is measured using a series of benzylpyridinium salts and compared with the small polyatomic projectile [Formula: see text] at 110 keV as well as distributions previously reported for electrospray ionization and fast atom bombardment. These results show that [Formula: see text] offers high secondary ion yields not only for small fragment ions, e.g. CN-, typically observed in SIMS analyses, but also for characteristic molecular ions. For the leu-enkephalin example, the yields for each of these species are greater than unity.

Bi-Directional Ion Emission from Massive Gold Cluster Impacts on Nanometric Carbon Foils

Carbon cluster emission from thin carbon foils (5-40 nm) impacted by individual Au(n) (+q) cluster projectiles (95-125 qkeV, n/q = 3-200) reveals features regarding the energy deposition, projectile range, and projectile fate in matter as a function of the projectile characteristics. For the first time, the secondary ion emission from thin foils has been monitored simultaneously in both forward and backward emission directions. The projectile range and depth of emission were examined as a function of projectile size, energy, and target thickness. A key finding is that the massive cluster impact develops very differently from that of a small polyatomic projectile. The range of the 125 qkeV Au(100q) (+q) (q ≈ 4) projectile is estimated to be 20 nm (well beyond the range of an equal velocity Au(+)) and projectile disintegration occurs at the exit of even a 5 nm thick foil.