David G. Schultz

Classical dynamics simulations of SiMe3+ ion–surface scattering

This paper presents classical dynamics simulations of Si(CD3)3+scattering from a hexanethiolate self-assembled monolayer on Au(111) and from a clean Au(111) surface. Simulations are performed with a united atom model using purely repulsive scattering potentials. These simulations predict the partitioning of the incident ion kinetic energy into the scattered ion kinetic energy and the internal modes of both the surface and the ion. For the organic surface, the simulations predict energy transfer to surface, ion internal, and ion kinetic energies of 0.78, 0.11, and 0.12 of the collision energy. The corresponding transfer efficiencies of 0.12, 0.21, and 0.65 were calculated for the Au(111) surface. These computational results compare well with the experimental results on the same systems which are reported in the preceding paper. The simulations predict near specular scattering for both surfaces. They also demonstrate that the ion penetrates only the topmost two to three layers of Me atoms of the organic sur...

Shattering of SiMe3+ during surface-induced dissociation

We provide experimental evidence that upon hyperthermal impact of Si(CD3)3+ ions with an organic surface, a portion of the ions undergo dissociation while still in contact with the surface. We use a tandem configuration of quadrupole mass spectrometers along with an energy analyzer to measure the kinetic energy distributions of the fragments that form as a result of the surface scattering of 25 eV Si(CD3)3+. These distributions are different for scattering from a clean Au(111) surface versus scattering from an organic surface composed of a self-assembled monolayer of hexanethiolate on Au(111). Parent and fragment ions recoil from the clean Au(111) surface with the same velocity, as is expected for fragmentation away from the surface. However, the same scattering products recoil from the organic surface with different velocities but similar energies, suggesting that the fragmentation dynamics are modified by surface interactions. We perform molecular dynamics simulations which predict residence times of ∼2...

Energy transfer and surface-induced dissociation for SiMe3+ scattering off clean and adsorbate covered metals

We scatter 10–70 eV SiMe3+ from clean Au(111), a hexanethiolate self-assembled monolayer on Au(111) (C6), and a NiO(111) layer grown on top of Ni(111). We examine both the scattered ion fragmentation patterns and the kinetic energy distribution spectra (KEDS) as a function of the incident ion energy E. Surface infrared and KEDS data indicate that we have prepared a saturated monolayer of hexanethiolate (C6) on Au(111) where the C6 carbon backbone is predominantly upright on the surface. C6 monolayers with a mixture of prone and upright C6 can also be prepared, but only the upright C6 monolayers are used for ion scattering experiments. The fragment ion distributions and the KEDS are then used to determine the channeling of the incident SiMe3+ion energy into the scattered ion internal energy Eint, and the scattered ion kinetic energy Escat. Overall, we find the order of Eint/E for SiMe3+ to be Au(111)≫NiO(111)>C6. From the Escat values, we find that MiSe3+ scattering off C6 is highly inelastic while scatter...

X-ray studies of the interface between two polar liquids: neat and with electrolytes

We demonstrate the use of X-ray reflectivity to probe the electron density profile normal to the interface between two polar liquids. Measurements of the interfacial width at the neat nitrobenzene/water and the neat water/2-heptanone interfaces are presented. These widths are consistent with predictions from capillary wave theory that describe thermal interfacial fluctuations determined by the tension and bending rigidity of the interface. Variation of the temperature of the water/nitrobenzene interface from 25 degrees C to 55 degrees C indicates that the role of the bending rigidity decreases with increasing temperature. X-ray reflectivity measurements of the electrified interface between an aqueous solution of BaCl2 and a nitrobenzene solution of TBATPB demonstrate the sensitivity of these measurements to the electrolyte distribution at the interface. A preliminary analysis of these data illustrates the inadequacy of the simplest, classical Gouy-Chapman theory of the electrolyte distribution.

Energy partitioning in the surface‐induced dissociation of linear and cyclic protonated peptides at an organic surface

Full understanding of the surface-induced dissociation (SID) of biological ions requires the determination of the energy channeling into the surface and the scattered ion kinetic and internal energies. Parent and fragment ion kinetic energy distributions were measured for five peptide ions scattered off a hexanethiolate monolayer on Au(111). Singly protonated ions of triglycine, tetraglycine, cyclo(Pro-Gly), cyclo(His-Phe) and tentoxin were formed by electrospray ionization and scattered at 15–55 eV collision energies off the organic surface. The scattered parent ion kinetic energies were 24% of the incident ion energy for the linear peptides, 21% for the cyclic dipeptides and 17% for the four-peptide ring. These results suggest that ion size and/or structure influences the scattered kinetic energy. Using these values and assuming an average internal excitation efficiency of 17%, it is estimated that the final internal energy given to the surface is 59–66% of the initial collision energy. This energy transfer to the surface is very close to that previously estimated for a host of smaller polyatomic ions scattered from similar organic targets. However, comparison with small ion SID shows that the peptides leave the surface with a wider distribution of kinetic energies. Finally, the measured kinetic energy distributions show that the fragment ions for a given peptide leave the surface with a common velocity, suggesting that dissociation occurs away from the surface. All fragments were found to result from non-reactive, inelastic scattering off the organic surface.

Surface energy transfer by low energy polyatomic ion collisions

Abstract Experiments and classical dynamics simulations are performed to determine the energy transferred when C4H4S+, Si(CD3)3+, and Fe(C5H5)2+ collide with clean Au(111), hexanethiolate/Au(111) and various organic multilayer surfaces. For these various ion-surface pairs, the energy transferred to the surface Esurf can be described by a linear function of the incident energy E over the range E = 10–70 eV. For the hexanethiolate and organic multilayer surfaces, the slope of this line varies from 0.65 to 0.94 and indicates an efficient transfer of energy to the surface. For the clean Au(111) surface, Esurf is much smaller with slopes of 0.15 and 0.43. Classical dynamics simulations quantitatively duplicate the Esurf behavior for Si(CD3)3+ scattering off hexanethiolate and find that Esurf varies roughly with Ecos0.8(θi), where θi is the incident ion angle.

Relative dissociation energies of protonated peptides by electrospray ionization/surface-induced dissociation.

Relative dissociation energies (RDEs) are obtained for the major fragment ions produced by electrospray ionization/surface-induced dissociation of singly protonated triglycine, tetraglycine, leucine enkephalin, and leucine enkephalin arginine. A previously described data analysis method (Lim, H.; et al. J. Phys. Chem. B 1998, 102, 4753) is employed to analyze the energy-resolved mass spectra by subtracting out the distribution of energy transferred to the surface, integrating over the distribution of the incident ion energy, and taking into account the precursor ion initial internal energy and kinetic energy distributions. These variables are optimized by anchoring the RDE for the lowest energy fragment of a given precursor ion to its literature values and then using these optimized parameters to obtain the other RDEs. The RDEs of the four major fragments of triglycine vary from 2.4 eV for the b(2) fragment ion to 6.0 eV for the a(2) ion. The RDEs of the four major fragments of tetraglycine vary from 3.2 eV for the y(2) ion to 5.7 eV for the a(2) ion. The leucine enkephalin RDEs range from 1.1 eV for the b(4) ion to 2.1 eV for the b(2) ion. The leucine enkephalin arginine RDEs all lay between 2.5 and 3.5 eV. The overall trend of fragmentation order for all peptides is (y(n), b(n)) < a(n) and is consistent with the results from other experiments. The peptide RDEs presented here are only as accurate as the literature values to which they are anchored. Determination of absolute dissociation energies from SID data will require further refinement of the data analysis method.

Energetics, timescales, and chemistry of low energy molecular ion-organic surface collisions

Abstract Several basic topics are presented here that are related to the modification of organic surfaces by 10–100 eV molecular ions. First, experimental results are presented for energy transfer for Si(CD3)3+ and m/z 190–713 peptide ions scattered off hexanethiolate self-assembled monolayers adsorbed on Au(111). Next, experimental and computer simulation results are presented on the dissociation timescales of these same ion-surface pairs. Finally, the surface chemical modification of polystyrene polymer surfaces by CF3+ and C3F5+ ions is analyzed by X-ray photoelectron spectroscopy. These results combine to demonstrate that low energy molecular ion–organic surface modification is highly surface selective, transfers a large fraction of the incident ion energy to the uppermost layer of the surface, can present unique reactive species to the surface, and can transfer to the surface a portion of the ions chemical functionality.

Surface modification by molecular ions

There are several advantages in using molecular ions for surface modification. The modification can be confined to the uppermost layer of the surface, the molecular character of the ion can be imparted to the surface, and sputter yields are often higher. These effects are demonstrated by the use of mass selected ion beams incident on well characterized surfaces. Energy transfer is examined by detecting the masses and energies of ions scattered off surfaces and performing molecular dynamics simulations. Surface modification is followed by chemical analysis with x-ray photoelectron spectroscopy and surface mass spectrometry. TRIDYN monte carlo simulations are used to support some of the modification experiments. Energy transfer is examined for Si(CD3)3+ scattered off clean and hexanethiolate covered Au(111). Adsorbate desorption cross sections and substrate damage depths for NH3/CO/Ni(111) are compared for 10–1000 eV isobaric atomic and polyatomic ions, Xe+ and SF5+. The surface chemical modification of pol...