B. J. Hinch

Sulfur adatom diffusion on the Cu(111) surface

Abstract This paper presents the first low-energy, quasi-elastic He-atom scattering study of adsorbate atom diffusion. At 820 K sulfur adatoms segregated onto the close-packed (111) surface of a copper crystal at a rate ∼ 0.01 ML/h. These adatoms produce significant diffuse intensity of scattered He and, as evident from the quasielastic energy broadening, are highly mobile. The diffusion coefficient (D = 2.9 × 10−5 cm2s−1) exhibits no surface coverage dependence, indicating relatively weak sulfur-sulfur interactions on the Cu(111) substrate.

Cu(001) to HD energy transfer and translational to rotational energy conversion on surface scattering

Intense peaks are observed in angular intensity distributions for HD scattering from the Cu(001) surface. These can be ascribed to coherent diffraction with translational energy transfers of ΔE=0, −11.0, or 33.11 meV to molecular rotational energy. Time-of-flight spectra, at other scattering angles, display inelastic peaks that are assigned to phonon creation or annihilation processes with either the HD rotationally elastic or inelastic transitions. The HD phonon dispersion curves suggest a strong HD coupling both with surface Rayleigh modes and with bulk phonon modes of the metal surface. Comparisons of both elastic and inelastic scattering intensities are also made with those reported for the Ni(001) surface. To explain an apparent anomalous diffraction peak intensity ratio for Ni, a preferential coupling for the HD J=0 rotational state into a predissociation channel above the Ni surface is proposed.

An electrostatic autoresonant ion trap mass spectrometer

A new method for ion extraction from an anharmonic electrostatic trap is introduced. Anharmonicity is a common feature of electrostatic traps which can be used for small scale spatial confinement of ions, and this feature is also necessary for autoresonant ion extraction. With the aid of ion trajectory simulations, novel autoresonant trap mass spectrometers (ART-MSs) have been designed based on these very simple principles. A mass resolution approximately 60 is demonstrated for the prototypes discussed here. We report also on the pressure dependencies, and the (mV) rf field strength dependencies of the ART-MS sensitivity. Importantly the new MS designs do not require heavy magnets, tight manufacturing tolerances, introduction of buffer gases, high power rf sources, nor complicated electronics. The designs described here are very inexpensive to implement relative to other instruments, and can be easily miniaturized. Possible applications are discussed.

APPLICATION OF A NOVEL CONTACTLESS CONDUCTIVITY SENSOR IN CHEMICAL VAPOR DEPOSITION OF ALUMINUM FILMS

A novel contactless method for conductivity sensing is introduced that utilizes a driving coil and two tunable and near resonant coils. The design uses only inexpensive electronic components and a variable frequency rf generator. An algebraic expression for the response has been derived and simulations indicate a linear response to surface conductivity changes over at least four orders of magnitude. The sensitivity is shown to depend on the conductivity of the substrate, with a limit to conductivity changes as low as 10−4 Ω−1 for insulating substrates. An ultrahigh vacuum compatible version of this probe has been used to monitor in situ aluminum thin film growth by chemical vapor deposition on a native oxide covered, highly doped, Si(111) wafer. On this semiconducting substrate (3 Ω−1) a sensitivity to sheet conductivity changes as low as ∼2×10−2 Ω−1 has been demonstrated. The Al films show a discrete jump in differential sheet conductivity associated with Al cluster coalescence during growth.

Atomic scale friction of molecular adsorbates during diffusion

Experimental observations suggest that molecular adsorbates exhibit a larger friction coefficient than atomic species of comparable mass, yet the origin of this increased friction is not well understood. We present a study of the microscopic origins of friction experienced by molecular adsorbates during surface diffusion. Helium spin-echo measurements of a range of five-membered aromatic molecules, cyclopentadienyl, pyrrole, and thiophene, on a copper(111) surface are compared with molecular dynamics simulations of the respective systems. The adsorbates have different chemical interactions with the surface and differ in bonding geometry, yet the measurements show that the friction is greater than 2 ps(-1) for all these molecules. We demonstrate that the internal and external degrees of freedom of these adsorbate species are a key factor in the underlying microscopic processes and identify the rotation modes as the ones contributing most to the total measured friction coefficient.

Observation of multiple missing dimer structures on the Si(001)-(2 × n) surface

Helium atom diffraction intensities from ordered missing dimer structures on Si(001) surfaces indicate that, in stable (2 × n) n = 8, 11 configurations, the unit cell contains a pair of missing dimers rather than the single missing dimer predicted by theory.

Jumping, rotating, and flapping: the atomic-scale motion of thiophene on Cu(111)

Self-assembled monolayers of sulfur-containing heterocycles and linear oligomers containing thiophene groups have been widely employed in organic electronic applications. Here, we investigate the dynamics of isolated thiophene molecules on Cu(111) by combining helium spin-echo (HeSE) spectroscopy with density functional theory calculations. We show that the thiophene/Cu(111) system displays a rich array of aperiodic dynamical phenomena that include jump diffusion between adjacent atop sites over a 59-62 meV barrier and activated rotation around a sulfur-copper anchor, two processes that have been observed previously for related systems. In addition, we present experimental evidence for a new, weakly activated process, the flapping of the molecular ring. Repulsive inter-adsorbate interactions and an exceptionally high friction coefficient of 5 ± 2 ps(-1) are also observed. These experiments demonstrate the versatility of the HeSE technique, and the quantitative information extracted in a detailed analysis provides an ideal benchmark for state-of-the-art theoretical techniques including nonlocal adsorbate-substrate interactions.

Quantum Influences in the Diffusive Motion of Pyrrole on Cu(111)

Pyrrole diffuses in channels on Cu(111), hopping between adjacent bridge sites over a barrier above hollow sites. Strong lateral interactions alter the lineshapes in helium-3 spin-echo measurements from a predicted double exponential toward an apparent single exponential decay. Molecular dynamics simulations reproduce the centre-of-mass motion of pyrrole and reveal a friction coefficient of 2.0 \(\pm \) 0.4 \(\mathrm{ps}^{-1}\). Density functional theory calculations reveal that a large contribution to the experimentally determined activation barrier of 53 \(\pm \) 4 meV arises from the quantum character of internal vibrational modes.

Adsorption of PF3 on Cu(001): Ordered overlayer structures and frustrated translational modes

The adsorption of PF3 on Cu(001) was studied by means of high resolution helium atom scattering (HAS) and SPALEED. PF3 adsorbs at surface temperatures below 210 K and forms a lattice gas at exposures less than 0.8 L. Saturation is reached for exposures greater than 3 L. At these coverages PF3 forms an ordered c(4×2) layer for surface temperatures above 145 K. For temperatures below 145 K an incommensurate, hexagonal PF3 layer was observed that necessarily excludes uniquely on-top site PF3 molecule to surface coordination. A new vibrational mode in the thermal energy regime corresponding to the frustrated translation parallel to the surface was identified; the energies of excitation were 3.3 meV and 3.5 meV, respectively, for the isolated molecules and the c(4×2) structure. No dispersion of the frustrated translation in the ordered c(4×2) phase was found. The results are discussed in terms of adsorbate–adsorbate and adsorbate–substrate interactions.

Implementation of New TPD Analysis Techniques in the Evaluation of Second Order Desorption Kinetics of Cyanogen from Cu(001)

The interactions of cyanide species with a copper (001) surface were studied with temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Adsorbed cyanide species (CN(a)) undergo recombinative desorption evolving molecular cyanogen (C(2)N(2)). As the adsorbed CN species charge upon adsorption, mutually repulsive dipolar interactions lead to a marked desorption energy reduction with increasing CN(a) coverages. Two new TPD analysis approaches were developed, which used only accurately discernible observables and which do not assume constant desorption energies, E(d), and pre-exponential values, ν. These two approaches demonstrated a linear variation of E(d) with instantaneous coverage. The first approach involved an analysis of the variations of desorption peak asymmetry with initial CN coverages. The second quantitative approach utilized only temperatures and intensities of TPD peaks, together with deduced surface coverages at the peak maxima, also as a function of initial surface coverages. Parameters derived from the latter approach were utilized as initial inputs for a comprehensive curve fit analysis technique. Excellent fits for all experimental desorption curves were produced in simulations. The curve fit analysis confirms that the activation energy of desorption of 170-180 kJ/mol at low coverage decreases by up to 14-15 kJ/mol at CN saturation.