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Molecular self-assembly is driven by intermolecular interactions between the functional groups on the component molecules. Small changes in molecular structure can make large differences in extended structure, and understanding this connection will lead to predictive power and control of the self-assembly process. Scanning tunneling microscopy is used to study self-assembly in two-dimensional clusters and monolayers, and the experimental approach is to study "families" of molecules where one or more functional groups is varied in a methodical way. Studied families include indole carboxylic acids, isatin derivatives (which have the indole backbone), quinaldic acid, thioethers, and fluorenone derivatives. In these systems, a variety of intermolecular interactions drive the assembly of the molecular monolayer, including hydrogen bonds, van der Waals forces, zwitterionic interactions, surface interactions, and halogen interactions.

Monolayers of methylisatin structural isomers are prepared on the Au(111) surface and imaged using scanning tunneling microscopy in ultrahigh vacuum. Monolayer preparation via solution deposition r...

Scanning tunneling microscopy (STM) was used to observe the chemical transformations of an octanethiolate monolayer on Au(111) exposed to gas-phase atomic hydrogen at 27 ○C. Reaction begins with th...

In this study, the pharmaceutical carbamazepine (CBZ) and the related molecule dibenzazepine (DBZ) are prepared on the Au(111) surface and visualized using scanning tunneling microscopy. Monolayers...

Scanning tunneling microscopy was used to study the self-assembly of three molecules on the Au(111) surface: 2-naphthoic acid, quinaldic acid, and 3-quinoline carboxylic acid. All three compounds c...

Monolayers of two pairs of structural isomers were deposited on Au(111) and observed via scanning tunneling microscopy in ultrahigh vacuum. We observe exclusively cyclic pentamers of isatin (1H-indole-2,3-dione), whereas its structural isomer, phthalimide (isoindole-1,3-dione), self-assembles primarily into close-packed arrays, with alternate structures that include kinetically locked disordered clusters and tetramer networks. Removal of the phthalimide NH group and its replacement with a CH2 group produces 1,3-indandione, which, despite the loss of the hydrogen-bond donor site, self-assembles into similar structures: close-packed areas and tetramer networks. The equivalent analog for isatin, 1,2-indandione, does not form pentamers and instead forms only close-packed areas and disordered regions. By iteratively altering the chemical structure, we demonstrate the influence that the chemical structure has on the resulting two-dimensional self-assembly.

Self-assembled monolayers of 1H-indole-2,3-dione (isatin), 3-methyl 2-oxindole, and 7-fluoroisatin are observed on the Au(111) surface via scanning tunneling microscopy (STM). We observed that isatin forms pentamers with density functional theory providing support for a cyclic structure stabilized by both N–H···O and C–H···O hydrogen bonds between neighboring molecules. The C–H···O bond is made between the 7-position C–H acting as the hydrogen bond donor and the 3-position carbonyl as the hydrogen bond acceptor, and calculations show that the isatin pentamer structure is 12 kJ/mol more stable than the dimer. When the 3-position carbonyl is removed and replaced with a methyl group (3-methyl 2-oxindole), we observe a monolayer with a mixture of catemer chains and pentameric clusters that are qualitatively different from those of isatin. Pentamer formation is completely broken when the 7-position hydrogen is removed and replaced with fluorine; the monolayer of 7-fluoroisatin is composed of a mixture of close...

A series of experiments and electronic structure calculations were performed to identify metastable 1,1′-ferrocenedicarboxylic acid supramolecular structures formed during solution deposition in a vacuum on a Au(111) substrate, as well as to observe their evolution into more stable species under mild annealing conditions. Electrospray ionization mass spectrometry measurments were performed to determine which species are likely to be present in the rapidly evaporating droplet, and these experiments found that a hexamer can exist in solution during deposition, albeit as a metastable species. The molecular clusters present after solution deposition were observed and analyzed using ultrahigh-vacuum scanning tunneling microscopy, and the initial monolayer contains four basic classes of structures: ordered dimer domains, tilted dimer rows, square tetramers, and rectangular chiral hexamers. Electronic structure calculations indicate that the chiral hexamers consist of a central dimer surrounded by four molecules...

A mechanism is described for the loading and unloading of samples from an instrument housing in a way that achieves strong mechanical contact between the sample and housing. A single linear–rotary magnetically coupled feedthrough is required to effect transfer. An additional load-lock mechanism requires only a single linear magnetic feedthrough, which remains in the UHV part of the chamber. The load-lock itself has no motion feedthroughs, which simplifies design and maximizes the pump-down speed. 3D-printing technology allows for intricately detailed parts to be used in both the sample transfer and load-lock mechanisms.

The preparation of 7-Fc(+) -8-Fc-7,8-nido-[C2 B9 H10 ](-) (Fc(+) FcC2 B9 (-) ) demonstrates the successful incorporation of a carborane cage as an internal counteranion bridging between ferrocene and ferrocenium units. This neutral mixed-valence Fe(II) /Fe(III) complex overcomes the proximal electronic bias imposed by external counterions, a practical limitation in the use of molecular switches. A combination of UV/Vis-NIR spectroscopic and TD-DFT computational studies indicate that electron transfer within Fc(+) FcC2 B9 (-) is achieved through a bridge-mediated mechanism. This electronic framework therefore provides the possibility of an all-neutral null state, a key requirement for the implementation of quantum-dot cellular automata (QCA) molecular computing. The adhesion, ordering, and characterization of Fc(+) FcC2 B9 (-) on Au(111) has been observed by scanning tunneling microscopy.

Monolayers of indole-2-carboxylic acid and indole-3-carboxylic acid on gold are studied using ultrahigh-vacuum scanning tunneling microscopy. Both molecules form symmetric, cyclic, hydrogen-bonded pentamers, a structure that is stabilized by the presence of a weak hydrogen-bond donor (NH or CH) adjacent to the carboxylic acid on the five-membered ring. In addition to pentamers, indole-2-carboxylic acid forms hexamers and catemer chains, while indole-3-carboxylic acid monolayers are generally disordered. Density functional theory calculations show that pentamers and hexamers have stability comparable to dimers or short catemers. The coexistence of all of these structures likely arises from the nonequilibrium conditions present in solution during pulse deposition of the monolayer.

Traditionally, the goal of self-assembly and supramolecular chemistry is to engineer an equilibrium structure with a desired geometry and functionality; this is achieved through careful choice of molecular monomers, growth conditions, and substrate. Supramolecular assemblies produced under nonequilibrium conditions, in contrast, can form metastable structures with conformations quite different from those accessible in equilibrium self-assembly. The study of nonequilibrium growth of clusters potentially impacts the study of nucleation in atmospheric aerosols, nucleation in organic crystallization, and mesoscale organization for systems ranging from biological molecules to molecular electronics. In our experiments, we prepare surface monolayers of small organic and organometallic molecules through direct injection of a solution onto a substrate in high vacuum. During this process, the rapid evaporation of small solution droplets in high vacuum can lead to nonequilibrium growth conditions. The resulting structures are then characterized by scanning tunneling microscopy. Among the features observed in these experiments are cyclic, hydrogen-bonded pentamers. For carboxylic acids, the two-molecule ring dimer is the common binding motif. Large, cyclic hydrogen-bonded systems are uncommon, especially so for rings with five members. Despite this, pentagonal clusters appear to be a general phenomenon for systems containing adjacent strong and weak hydrogen-bonding elements on five-member aromatic rings. Regular pentamers have been observed as metastable structures for ferrocenecarboxylic acid, indole-2-carboxylic acid, and isatin (1-H-indole-2,3-dione). Electronic structure calculations confirm the relative stability of these structures with respect to the dimer or catemer conformations which are observed in the solid-state crystal structures. For ferrocenecarboxylic acid, cyclic pentamers undergo further self-assembly, resulting in long-range order in conjunction with local 5-fold rotational symmetry. This system is the first reported self-assembled molecular quasicrystal, and it remains the only example of a hydrogen-bonded quasicrystal. This supramolecular structure forms as a result of the cocrystallization of hydrogen-bonded cyclic pentamers with intercalated molecular dimers. The shared bonding to a single dimer is responsible for locking the adjacent pentamers in specific distances and orientations, which produces the quasicrystal. Careful analysis of experimental data provides evidence that, in some cases, metastable clusters are formed in solution and then subsequently adsorb on the surface. This is a unusual mechanism for supramolecular assembly, and it has important implications for understanding questions in crystal growth, namely: what the initial stages of crystal growth are as molecules are first precipitating from solution; what role the solvent plays in determining crystal structure; and whether solvent-mediated clustering is important in the broader phenomenon of solid-state polymorphism.

Self-assembled monolayers of ferrocenecarboxylic acid (FcCOOH) contain two fundamental units, both stabilized by intermolecular hydrogen bonding: dimers and cyclic five-membered catemers. At surface coverages below a full monolayer, however, there is a significantly more varied structure that includes double-row clusters containing two to twelve FcCOOH molecules. Statistical analysis shows a distribution of cluster sizes that is sharply peaked compared to a binomial distribution. This rules out simple nucleation-and-growth mechanisms of cluster formation, and strongly suggests that clusters are formed in solution and collapse into rows when deposited on the Au(111) surface.

A model scanning probe microscope, designed using similar principles of operation to research instruments, is described. Proximity sensing is done using a capacitance probe, and a mechanical linkage is used to scan this probe across surfaces. The signal is transduced as an audio tone using a heterodyne detection circuit analogous to that used in the theremin (one of the first electronic musical instruments, invented in the early 20th century). The instrument is useful for demonstrations and hands-on activities that introduce fundamentals of scanning probe microscopy and, by extension, nanoscience and nanotechnology. The details of instrument construction are provided, along with instructions for assembly and troubleshooting.

In chemical reactions at the gas–surface interface, the heterogeneity in structure of reaction sites plays a critical role in determining surface reactivity. This Perspective describes reaction mechanisms in such systems and details the use of in situ scanning probe microscopy to investigate reactions of gas-phase radicals with self-assembled alkanethiolate monolayers on gold surfaces. For both atomic hydrogen and atomic chlorine reagents, the presence of defects in the alkanethiolate surface order has a substantial influence on what reactions can occur and the speed at which they do so. Data acquired from a series of images were modeled using kinetic Monte Carlo simulations, and a surface radical reaction model was developed to explain the observed evolution of surface structure as the reactions proceed.

The two fundamental limitations of the present computing paradigm are power dissipation from transistor switching and the architectural von Neumann bottleneck that segregates processing from memory. We examine a cellular architecture which radically intermixes memory and processing, and which is based on a transistor-less approach to representing binary information using the arrangement of charge within the molecule. Representing bits by molecular configuration, rather than a current switch, yields the limits of functional density and low power dissipation. Matching a new computational element to a new architectural framework could enable general purpose computing to evolve along a new roadmap.

International audience; The molecule {Cp*(dppe)Fe(C C-)}(3)(1,3,5-C6H3) (Fe3) was adsorbed on a single-crystal gold surface and studied using ultrahigh-vacuum scanning tunneling microscopy (STM). Both the singly oxidized Fe3(+) and doubly oxidized Fe3(2+) are mixed-valence ions, and localization of the charge at specific metal centers was observed as the appearance of pronounced asymmetry in STM images. Switching the tip sample bias voltage demonstrates that this asymmetry is electronic in nature. The nature of intramolecular structure and the degree of asymmetry produced in STM images varies according to the state of the scanning tip. Constrained density functional theory was used to simulate STM images for the neutral molecule and for both mixed-valence species, and simulated images agreed closely with observed results. In particular, changing the number of molecular electronic states contributing to contrast in the STM image produced a good match to the variation in structures measured experimentally.

We use scanning tunneling microscopy (STM) to study octanethiol self-assembled monolayers (SAMs) on Au(111) exposed to atomic hydrogen. While the overall net reaction is to remove octanethiol molecules from the underlying gold surface, the monolayer structure heavily influences the rate of this reaction and molecules located along surface defects are preferentially removed before those located in close-packed areas. Octanethiol molecules remaining on the gold surface can go through significant rearrangement: domain boundaries can change both size and structure, annealing into surrounding close-packed domains; film defects diffuse to the edge of close-packed areas; and molecules located along the edge of close-packed domains shift position, changing the size and shape of the remaining close-packed features. Monolayer reactivity increases with increasing hydrogen-atom exposure, and we compare the experimental results with kinetic Monte Carlo simulations. We find that the edges of defect sites are potentiall...

Au(111) surfaces are exposed to solutions containing both octanethiol and dithiocarbamate (DTC) molecules, and the resulting surface composition and structure are studied using scanning tunneling microscopy (STM). DTC adsorption and monolayer formation are favored when present at the same concentration as octanethiol in solution. Higher octanethiol concentration in solution results in the incorporation of thiol into the resulting monolayer, with a strong dependence on the chain length of the DTC molecules. For diethyldithiocarbamate, thiol adsorption is limited and close-packed thiolate monolayers are not formed even at 100:1 excesses of the thiol in solution. For didecyldithiocarbamate, higher thiol concentrations lead to the formation of full thiolate mononlayers and the complete displacement of DTC.

Scanning tunneling microscopy is used to study monolayers of 1-adamantanethiolate as they are exposed to gas-phase atomic hydrogen. H-atom reaction results in complete removal of the organic monolayer. The relaxation of the reconstruction present at the gold–sulfur interface results in the formation of gold-atom islands, as well as the addition of gold atoms to extant surface defects such as steps and pits. Characterization of these changes shows that for 1-adamantanethiolate monolayers, 0.18 ± 0.033 monolayers of gold adatoms participate in bonding with thiolate sulfur atoms. This results in a 1:1 Au:S ratio, in contrast to the 1:2 Au:S ratio reported for n-alkanethiolate monolayers. The difference in adatom density implies a qualitative difference in binding between n-alkanethiols and 1-adamantanethiols.

Two-component octanethiolate-dialkyldithiocarbamate (DTC) monolayers were formed on Au(111) surfaces and studied using scanning tunneling microscopy (STM). Octanethiolate monolayers exposed to DTC in solution results in the erosion of octanethiolate domain boundaries and areas along terrace step edges and the insertion of DTC into these areas; this is consistent with the broad literature of substitution in alkanethiolate monolayers. Conversely, a DTC monolayer exposed to octanethiol results in displacement of DTC and the eventual formation of ordered octanethiolate domains. The effects of temperature, solution concentration, and deposition time are investigated.

{Cp*(dppe)Fe(C≡C-)}(2)(1,3-C(6)H(4)) is studied both as a neutral molecule, Fe(II)-Fe(II), and as a mixed-valence complex, Fe(II)-Fe(III). Scanning tunneling microscopy (STM) is used to image these species at 77 K under ultrahigh-vacuum conditions. The neutral molecule Fe(II)-Fe(II) has a symmetric, "dumbbell" appearance in STM images, while the mixed-valence complex Fe(II)-Fe(III) demonstrates an asymmetric, bright-dim double-dot structure. This asymmetry results from localization of the electron to one of the iron-ligand centers, a result which is confirmed through comparison to theoretical STM images calculated using constrained density-functional theory (CDFT). The observation of charge localization in mixed-valence complexes outside of the solution environment opens up new avenues for the control and patterning of charge on surfaces, with potential applications in smart materials and molecular electronic devices.

Single-molecule electronic components are potential building blocks for next-generation electronic devices. One architecture for such devices is quantum dot cellular automata (QCA), in which binary...

The nature of the sulfur−gold bond in alkanethiol films self-assembled on Au(111) surfaces is widely contested, despite significant interest in these systems over the last 25 years. Recent theoretical and experimental studies have suggested gold adatoms are incorporated into the alkanethiol−gold interface. We have exposed alkanethiol self-assembled monolayers (SAMs) to gas-phase hydrogen atoms to remove the monolayer; the gold adatoms remain on the surface and form features that we observe using scanning tunneling microscopy (STM). The features include the formation of single-atom-thick gold islands, decreasing size of surface vacancy pits, and faceting of terrace step edges; compared to the alkanethiol-terminated surface, these features indicate a net increase in the amount of gold present on the surface, compared to the alkanethiol-terminated bulk structure. Varying the length of the alkane chain does not affect the total adatom coverage, as the adatom coverages for ethanethiol (0.172 ± 0.039), octaneth...

Octanethiol self-assembled monolayers were exposed to gas-phase hydrogen atoms, and the resulting changes in the order and chemical structure of the surface were monitored using scanning tunneling microscopy (STM). Extensive damage to the monolayer was observed in the form of both dark and bright features in STM images. These changes began along domain boundaries and moved into close-packed regions of the monolayer as hydrogen-atom exposure time increased. Increasing exposure also results in an accelerated rate of observed surface changes, indicating that the reactivity of the surface increases as a result of initial gas-surface reactions. Complex restructuring of the alkanethiol monolayer is observed, including defect formation and the disordering of the alkanethiol monolayer. However, in some cases the monolayer demonstrates the capability of self-healing, with local annealing and reordering of close-packed domains. This annealing and reordering likely results from increased mobility of surface-bound alkanethiolates in the vicinity of monolayer defects, or from diffusion and readsorption of transiently formed alkanethiol molecules.

Scanning tunneling microscopy is used to characterize the collision-induced migration of molecules within well-ordered octanethiol and nonanethiol self-assembled monolayers. A seeded molecular beam is used to create xenon atoms with a kinetic energy of 1.3 eV, and collisions with these atoms cause measurable changes in alkanethiol monolayer surface structure. Migration rates are calculated and compared for molecules in close-packed domains, at domain-boundary defects, and along the perimeter of vacancy-island defects. The number of nearest-neighbor molecules (within the 5 A lattice distance) is strongly predictive of molecular stability with respect to xenon bombardment, and the overall dependence of stability on nearest neighbors is well fit by a simple exponential curve. The incident direction of the molecular beam is not observed to influence the direction of molecular motion; however, in some cases, migration correlates strongly to surface lattice directions. Finally, there is no evidence that substrate restructuring or gold-atom diffusion accompanies alkanethiol migration under these non-equilibrium conditions.

Scanning tunneling microscopy (STM) is used to characterize partial monolayers of C 60 , C 70 , and C 84 adsorbed on the Au(1 1 1) surface at room temperature and under ambient conditions. A high degree of structural polymorphism is observed for monolayers of each of these fullerenes. For C 60 , three lattice packings are observed, including a previously unreported 7 × 7 R 21.8° structure that is stabilized by adjacent surface step defects. For C 70 , two lattice packings are observed, and analysis of molecular features in STM images allows molecular binding geometry to be determined. In one of the two observed lattice structures, C 70 molecules align their long axis along the surface normal, while in the other, molecules align parallel to the surface and along a gold lattice direction. The parallel geometry is also preferred for isolated and loosely packed molecules on the surface. C 84 exhibits a large number of lattice orientations and no long-range order, and likely binds incommensurately on Au(1 1 1). Time series of images of partial C 70 monolayers show progressive surface modification as a result of perturbation by the STM tip; this is in contrast to the behavior of C 60 , where alterations in surface structure at room temperature are thermally driven.

We report a scanning tunneling microscopy (STM) investigation of a dinuclear organometallic molecule, trans-[Cl(dppe)2Ru(C[triple bond]C)6Ru(dppe)2Cl] (Ru2), absorbed on a Au(111) surface; this molecule is a potential candidate for use in molecular quantum-dot cellular automata (QCA) devices. Isolated Ru2 molecules were observed under ultra-high-vacuum conditions. Submolecular structure was clearly discernible in the STM images, with a bright feature corresponding to each of the two Ru-ligand complexes within the Ru2 molecule. Rotation and translation of the Ru2 molecules were observed to be induced by the STM tip under some tunneling conditions.

The structure and dynamics of partial monolayers of C60 on the Au(111) surface were observed by scanning tunneling microscopy (STM) under ambient conditions. At submonolayer coverages, C60 molecule...

Current-to-voltage preamplifiers for scanning tunneling microscopy were tested side-by-side using a consistent testing protocol, to optimize performance and understand potential design trade-offs. Current-to-voltage gain and voltage noise density were measured as functions of frequency for two different circuit architectures: the feedback transimpedance amplifier and the current-shunt electrometer. The effect of specific component choices—of the integrated circuits, resistors, and capacitors—was also investigated.

Molecular chlorine was photolyzed using circularly polarized radiation at 310 and 330 nm, and orientation moments of the chlorine-atom Cl(P-2(j)) photofragment distributions were measured by resonance enhanced multiphoton ionization using circularly polarized light with Doppler resolution. The product atoms were found to be strongly oriented in the laboratory as a result of both incoherent and coherent dissociation mechanisms, and the orientation moments contributed by each of these mechanisms have been separately measured. The experimental results can be explained by nonadiabatic transitions from the C (1)Pi (1u) state to higher states of Omega =1(u) symmetry, induced by radial derivative coupling. Ab initio calculations indicate strong Rosen-Zener-Demkov noncrossing-type radial derivative couplings between states of 1(u) symmetry. The observed angular distribution (beta parameter) indicates that 88% of Cl*(P-2(1/2)) fragments produced at 310 nm originate from a perpendicular transition to the C state. The orientation measurements suggest that 67 +/- 16% of Cl-35*(P-2(1/2)) atoms dissociate via the 1(u)((3)Sigma (+)(1u)) state, and 21 +/-6% dissociate via the 1(u)((3)Delta (1u)) state. (C) 2000 American Institute of Physics. [S0021-9606(00)01444-6].

We have studied the complete Cl-atom molecular-frame photofragment angular momentum distributions from the photodissociation of Cl-2 and ICl in the 320-560 nm region using time-of-flight mass spectroscopy with laser detection. The experimental signals were analyzed using the polarization-parameter formalism described in the preceding paper. These experiments study three distinct cases. The first case is the 470 nm dissociation of Cl-2 through the B (3)Pi(0+u) state accessed via a parallel transition, yielding Cl-atom photofragments with polarizations described by the single parameter a(0)((2))(parallel to) = -0.7+/-0.2. The second case is the 320 nm dissociation of Cl-2 through the C (1)Pi(1u) state accessed via a perpendicular transition, yielding Cl-atom photofragments with polarizations described by the two parameters a(0)((2))(perpendicular to)= -0.50+/-0.10 and a(2)((2))(perpendicular to) = -0.32+/-0.06. The third case is the dissociation of ICl in the 490-560 nm region in which dissociative states of both parallel and perpendicular character are accessed. In this wavelength region, the polarizations of the resulting Cl-atom photofragments are completely described by the approximately constant incoherent parameters, a(0)((2))(parallel to) approximate to + 0.4, a(0)((2))(perpendicular to) approximate to -0.2, and a(2)((2))(perpendicular to) approximate to -0.2, whereas the interference contributions to the polarization, the Im[a(1)((1))(parallel to,perpendicular to)] and Re[a(1)((2))(parallel to,perpendicular to)], oscillate sinusoidally with excitation wavelength in a fashion that is sensitive to the shapes of the dissociative surfaces. (C) 1999 American Institute of Physics. [S0021-9606(99)01707-9].

Molecular chlorine (Cl2) was photodissociated in the wavelength range 270–400 nm with linearly polarized light, and the orientation of the excited-state chlorine atom Cl*(2P1/2) was measured by 2+1 resonance enhanced multiphoton ionization (REMPI) using circularly polarized light. The degree of orientation of the Cl* photofragment is found to oscillate as a function of photolysis wavelength. The oscillation is a result of quantum mechanical coherence arising from electronic states of different symmetry that correlate to the same separated-atom asymptote. A simple curve-crossing model using abinitio potential energy curves reproduces the general shape of the oscillation but fails to give a quantitative fit.

The reactions of atomic chlorine with CH4 and CD4 were studied at five collision energies ranging from 0.13 to 0.29 eV using resonance-enhanced multiphoton ionization of the CH3 and CD3 products. Core-extracted ion arrival profiles were used to determine methyl radical product speed distributions. The distributions contain products that are moving anomalously fast which energetically cannot result from the reaction of ground-state chlorine with ground-state methane. We attribute these products to reaction of ground-state chlorine with methane vibrationally excited in trace quantities into low-energy bending and torsional modes. Measurements of product spatial anisotropy are used to confirm this interpretation and to indicate that the possible reaction of spin–orbit excited chlorine is less important. These low-energy vibrations create large enhancements in reactivity over ground-state molecules, and consequently, vibrationally excited reagents dominate reactivity at low collision energies and contribute substantially at the highest collision energies studied. It is suggested that vibrationally excited reagents play an important role in the thermal kinetics of the reaction of chlorine with methane and may contribute significantly to explain the observed deviation from Arrhenius equation behavior. Scattering distributions of the products of both ground-state and vibrationally excited reactions are reported, and additional measurements of the internal state distributions of the CH3 and CD3 products reveal that the methyl radicals contain very little energy in rotation or vibration.

We have studied the photodissociation of ICl using linearly polarized light at 532 nm. This wavelength excites several electronic states. Using linearly polarized probe light, we have measured the angular momentum distributions of the 35Cl(2P3/2) and 37Cl(2P3/2) photofragments, which are found to differ significantly. These distributions are expressed in a multipole moment expansion having four terms. These terms can be classified as incoherent contributions from parallel and perpendicular transitions, and coherent contributions from interference that arises from these interacting states. Analysis of these angular momentum distributions using this formalism shows that the incoherent contributions from the dissociating surfaces are mass independent, whereas the coherent contributions are mass dependent.

The title reaction has been studied by observing the ethyl radical product by means of resonance-enhanced multiphoton ionization (REMPI) that proceeds at 242 nm via highly dissociative Rydberg stat...

The photoloc technique can permit the measurement of not only the state-to-state differential cross section but also its complete product polarization dependence for all moments of orientation and alignment with k⩽2. We have realized this possibility for the reaction Cl+C2D6→DCl(v′=0,J′=1)+C2D5 at a collision energy of 0.25 eV, for which we have measured the differential cross section, 1/σ(dσ00/dΩr), and the four polarization-dependent moments of the differential cross section, A1(1)stf, A0(2)stf, A1(2)stf, and A2(2)stf, in the stationary target frame (STF), which are defined by Aq(k)stf=(dσkqstf/dΩr)/(dσ00/dΩr). For the Cl+CD4→DCl(v′=0,J′=1)+CD3 reaction at a collision energy of 0.28 eV we have also determined 1/σ(dσ00/dΩr) and A0(2)stf. The laboratory speed distributions of the DCl(v′=0,J′=1) products are measured using 2+1 resonance-enhanced multiphoton ionization (REMPI) and the core-extraction technique. The polarization-dependent differential cross sections are determined from the dependence of the ...

We describe procedures for the measurement of the differential-cross-section moments of the velocity distribution of the state-selected products of photoinitiated bimolecular reactions using resonance-enhanced multiphoton ionization (REMPI) detection and some form of laboratory velocity selection such as time-of-flight mass spectrometry. The relative ionization probability of a single product molecule is presented in the form I=1+f(Θ,Φ,θe,θu,Aq(k)stf), where the angles Θ, Φ, and θe describe the orientation of the product’s laboratory velocity with the photolysis and probe laser polarizations, θu is the product laboratory scattering angle, and the Aq(k)stf are the stationary target frame (STF) differential-cross-section moments. The STF is a reference frame defined by the laboratory velocity and the scattering plane. From the ionization probability, I, we derive a method to measure all five parameters with k⩽2, the differential cross section, 1/σ(dσ00/dΩr), and the four polarization parameters A1(1)stf, A0...

The title reaction is investigated using a photoinitiated technique that allows for the measurement of HCl product vibrational and rotational distributions using resonance enhanced multiphoton ionization (REMPI), the determination of center-of-mass scattering through core extraction, and the deduction of the average internal energy of the C 2 H 5 product from HCl spatial anisotropy. Vibrational excitation of the ethane reagent is shown to produce only a 5% to 10% increase in integral reactivity. Production of forward-scattered HCl ( v = 1) is observed, which is similar to the behavior of the chemically related Cl + CH 4 ( ν 3 = 1) reaction.

The chemical reaction of atomic chlorine with mixed monolayers of alkanethiolates having different chain lengths was investigated. In situ scanning tunneling microscopy was used to acquire time-lapsed series of images, allowing the measurement of the effect of monolayer structure and composition on reactivity. The rate of chemical reaction is strongly site-dependent. In particular, the boundary between two different-length alkanethiolates greatly promotes the reactivity of nearby molecules, much more so than any other native defect typical of single-component alkanethiolate monolayers.

Scanning tunnelling microscopy reveals that molecules of ferrocenecarboxylic acid can self-assemble into quasicrystal monolayers containing highly unusual cyclic hydrogen-bonded pentamers; this molecular framework could form the basis of a large range of supramolecular assemblies. The range of systems in which non-periodic quasicrystalline ordering has been observed continues to increase, with small-molecule self-assembled monolayers now entering the arena. Specifically, Natalie Wasio and colleagues find that the hydrogen-bonding patterns of ferrocenecarboxylic acid drive the assembly of these molecules into unusual cyclic pentamers, whose five-fold symmetry facilitates a two-dimensional quasicrystalline arrangement. The authors anticipate that a large range of novel quasicrystalline supramolecular assemblies could be formed based on this molecular framework. The process of molecular self-assembly on solid surfaces is essentially one of crystallization in two dimensions, and the structures that result depend on the interplay between intermolecular forces and the interaction between adsorbates and the underlying substrate1. Because a single hydrogen bond typically has an energy between 15 and 35 kilojoules per mole, hydrogen bonding can be a strong driver of molecular assembly; this is apparent from the dominant role of hydrogen bonding in nucleic-acid base pairing, as well as in the secondary structure of proteins. Carboxylic acid functional groups, which provide two hydrogen bonds, are particularly promising and reliable in creating and maintaining surface order, and self-assembled monolayers of benzoic acids produce structure that depends on the number and relative placement of carboxylic acid groups2,3,4,5,6. Here we use scanning tunnelling microscopy to study self-assembled monolayers of ferrocenecarboxylic acid (FcCOOH), and find that, rather than producing dimeric or linear structures typical of carboxylic acids, FcCOOH forms highly unusual cyclic hydrogen-bonded pentamers, which combine with simultaneously formed FcCOOH dimers to form two-dimensional quasicrystallites that exhibit local five-fold symmetry and maintain translational and rotational order (without periodicity) for distances of more than 400 angstroms.

Scanning tunneling microscopy images of diferrocenylacetylene (DFA) coadsorbed with benzene on Au(111) show individual and close-packed DFA molecules, either adsorbed alongside benzene or on top of a benzene monolayer. Images acquired over a range of positive and negative tip–sample bias voltages show a shift in contrast, with the acetylene linker appearing brighter than the ferrocenes at positive sample bias (where unoccupied states primarily contribute) and the reverse contrast at negative bias. Density functional theory was used to calculate the electronic structure of the gas-phase DFA molecule, and simulated images produced through two-dimensional projections of these calculations approximate the experimental images. The symmetry of both experimental and calculated molecular features for DFA rules out a cis adsorption geometry, and comparison of experiment to simulation indicates torsion around the inter-ferrocene axis between 90° and 180° (trans); the cyclopentadienyl rings are thus angled with respect to the surface.

Photolysis of Cl2 initiates the title reaction at a sharply defined collision energy of 0.24±0.03 eV. Nascent product rotational state distributions for HCl (v=0) are determined using resonance enhanced multiphoton ionization (REMPI), center‐of‐mass scattering distributions are measured by the core‐extraction technique, and the average internal energy of the C2H5 product is deduced from the dependence of the core‐extracted signal on the photolysis polarization. The HCl product has little rotational excitation, but the scattering distribution is nearly isotropic. Although seemingly contradictory, both of these features can be accounted for by using the simple line‐of‐centers model presented to explain earlier results for the Cl+CH4 reaction. In contrast to the Cl+CH4 reaction, the data suggest that the Cl+C2H6 reaction proceeds through a loosely constrained transition‐state geometry. The reactions of atomic chlorine with ethane, C2H6, and perdeuteroethane, C2D6, yield virtually identical results. These findings, along with the low energy deposited by the reaction into the ethyl product (200±120 cm−1), demonstrate that the alkyl fragment acts largely as a spectator in this hydrogen abstraction reaction.

Comparison of state-to-state differential cross sections for methane in the ground vibrational state to methane with one quantum of asymmetric stretch excitation probes the effect of C−H stretch ex...

The mechanism for the reaction of atomic chlorine with vibrationally excited methane is investigated by measurement of correlated state and scattering distributions using the method of core extraction (see preceding paper). Laser photolysis of molecular chlorine creates monoenergetic chlorine atoms (≳98% Cl 2P3/2) that react with vibrationally excited methane molecules prepared by linearly polarized infrared laser excitation. The resulting HCl product population distributions are determined by (2+1) resonance‐enhanced multiphoton ionization (REMPI), and the differential cross section for each product rovibrational state is measured by core extraction. Approximately 30% of the product is formed in HCl(υ=1,J) with a cold rotational distribution; the remaining population is formed in HCl(υ=0,J) and is more rotationally excited. We observe a rich variation of the scattered flux that is dependent on the internal‐energy state of the product. The HCl(υ=1) product is sharply forward scattered for low J and becomes nearly equally forward–backward scattered for high J; the HCl(υ=0,J) product is back and side scattered. The reactions of Cl with C–H stretch‐excited methane (CH4) and C–H stretch‐excited CHD3 are found to have similar angular and internal‐state distributions. Observation of the spatial anisotropy of the HCl(υ=0, J=3) product shows that significant vibrational excitation of the methyl fragment does not occur.The measured spatial anisotropy is most consistent with a model in which backscattered HCl(υ=0, J=3) is formed in coincidence with slight methyl vibrational excitation and the forward‐scattered HCl(υ=0, J=3) is formed in coincidence with no methyl excitation. The approach of the attacking chlorine atom with respect to the C–H stretch direction can be varied by rotating the plane of polarization of the infrared excitation. A marked steric effect is observed in which Cl atoms approaching perpendicular to the C–H stretch preferentially yield forward‐scattered HCl(υ=1) product. On the other hand, the reaction is weakly dependent on the rotational quantum state of CH4(υ3=1,J), and on the rotational polarization. The data are consistent with a model that has a widely open ‘‘cone of acceptance’’ in which the impact parameter controls the internal‐state and scattering distributions of the HCl product.

We describe a method we call core extraction for measuring the speed distributions of products from photoinitiated bimolecular reactions for the purpose of determining state‐to‐state differential cross sections. Core extraction is demonstrated by determination of the state‐to‐state differential cross section for the reaction Cl+CH4(υ3=1)→HCl(υ=1, J=1)+CH3. The method of core extraction measures three‐dimensional projections of the velocity distribution using a time‐of‐flight mass spectrometer equipped with a mask to reject off‐axis scattered products. This three‐dimensional projection is then converted to a state‐to‐state differential cross section via simple transformations. Competition between instrumental resolution and signal in core extraction is discussed, and the behavior of our system is checked with simple velocity distributions that result from photodissociation of Cl2. Core extraction is compared with other methods for the measurement of state‐resolved differential cross sections.

We describe a thermal gas cracker designed to produce low fluxes of gas-phase radicals for use in radical-surface reaction studies. A resistively heated thin piece of highly oriented pyrolytic graphite is used as the pyrolysis filament, with the major advantage that this material remains inert at high temperatures. The instrument is built within an existing titanium sublimation pump, which simplifies construction and allows for self-pumping of the radical source. Thermal generation of Cl atoms from Cl(2) was chosen to test the effectiveness of the instrument. (35)Cl and (37)Cl were generated with a concomitant decrease in parent (70)Cl(2) and (72)Cl(2) species, as monitored by a residual gas analyzer. The cracking fraction of Cl(2) as a function of cell temperature is reported, with nearly full conversion achieved at high temperature.