Desorption rates and mechanisms for dissociatively chemisorbed molecules

The desorption kinetics of dissociatively chemisorbed diatomic molecules are examined from a kinetic-modeling point of view. A comparison is made between a one-step process, resulting in the usual second-order kinetics, and a two-step process which takes into account explicity recombination of atoms and subsequent desorption of molecules. The kinetics from the two-step mechanism are found to be equivalent to second-order desorption with a coverage-dependent activation energy which, in many cases, is a linear function of coverage. The two-step process leads to second-order kinetics with a constant activation energy only for special values of the model rate parameters, or if chemisorption is activated. The steady onate approximation for the intermediate in the two-step process is often adequate, but the transient period during which a steady state is reached also contains important kinetic information. The implications of these results for desorption kinetics and molecular beam reaction experiments are discussed. Work performed under the auspices of the Office of Basic Energy Sciences of the Department of Energy     

NEXAFS and SEXAFS studies of chemisorbed molecules

Near-edge x-ray absorption fine structure (NEXAFS) and surface extended x-ray absorption fine structure (SEXAFS) spectroscopies and their application to the determination of the adsorption geometry and bonding of low-Z molecules on surfaces are discussed. NEXAFS is characterized by intramolecular resonances and probes the internal structure of the molecule (intramolecular bond lengths and possibly bond angles) as well as its orientation relative to the surface. SEXAFS provides information about the adsorption site and the molecule-substrate distances. After demonstrating the full power of SEXAFS in the analysis of oxygen adsorption on Cu(110) and on Ag(110) an example is given of a complete structure determination for the formate species (HCO₂) on Cu(110) using NEXAFS and SEXAFS.     

A theoretical interpretation of photoelectron energy spectra for oxygen chemisorbed on nickel

Ultraviolet photoemission spectra (UPS) measured by Eastman and Cashion for oxygen moderately to heavily chemisorbed on nickel are uniquely different from UPS data for bulk nickel and relatively low coverages of oxygen adsorbed on nickel. A theoretical model is proposed suggesting that the unique structural features of the UPS data for oxygen moderately to heavily chemisorbed on nickel are evidence for the incipient formation of surface nickel oxide. The theoretical model is based on the self-consistent-field Xα scattered-wave SCF-Xα-SW) cluster method and “transition-state” theory recently developed by Slater and Johnson. The same theoretical model is used to explain the similarities (and differences) between the UPS data for oxygen chemisorbed on nickel and X-ray photo-electron spectra (XPS) for bulk nickel oxide.     

卟啉酞菁二元分子光谱及激发态弛豫的非简并四波混频研究

通过对卟啉酞菁二元分子TTPP-O-Pc和TTPP-O(CH_2)_5-O-Pc的发射光谱和激发光谱的系统研究,证实了(tetratolylphenyl porphyrin)TTPP到Pc的能量转移,区分了后一种材料中发射光谱的两组谱带的来源.利用非简并四波混频(NDFWM)方法研究了二元分子激发态弛豫过程,获得了这两种材料的激发态弛豫时间分别为3.6ps和4ps,以及电荷弛豫时间分别为18.5ps和82ps.     

Three-particles final state reactions

The first iteration solution of the Faddeev-equation is used to evaluate the transition amplitude for the (α, 2α) reactions. Reasonable approximations are adopted to calculate the off-energy shell effects. Good agreement with the experimental results is obtained.     

Carbon K-edge structure of chemisorbed molecules by means of fluorescence detection

We report the first X-ray absorption fine structure measurements above the carbon K-edge recorded by fluorescence detection (FD). Studies for ethylene chemisorbed on Cu(100) were carried out with high-flux ( > 10¹⁰ photons/s) synchrotron radiation using an ultra-high vacuum compatible proportional counter. Our results show that K-edge studies by means of FD allow the measurement of extremely dilute concentrations of C corresponding to about 0.02 monolayers on the surface or 20 ppm in the bulk.     

On the interpretation of the photoelectron spectrum of CO chemisorbed on Ni

A LCAO-SCF calculation on the NiCO molecule is reported. In addition to the ionization potentials the vibrational structure due to ionization is calculated. The computed ionization potentials are compared with those of recent semiempirical calculations on the CO — Ni system and with the photoelectron spectra of CO chemisorbed on nickel surfaces. We produce further evidence that the first peak in the photoelectron spectrum relates to the 5σ and 1π orbitals of CO and that the peak at higher ionization energy represents the 4σ orbital.     

Nuclear magnetic spin-rotational relaxation times for linear molecules

The stochastic differential equation study of nuclear magnetic relaxation by spin-rotational interactions is applied to the linear rotator model of the molecule. Inertial effects are included in the calculations, which are performed analytically. Expressions are derived for the spin-rotational contributions to the longitudinal and transverse relaxation times, and for the spin-rotational correlation time.     

Nuclear magnetic spin-rotational relaxation times for symmetric molecules

It is shown that the problem of calculating times related to nuclear magnetic spin-rotational interactions may be solved for the symmetric rotator model of a molecule by employing the method already proposed in a general manner for asymmetric molecules that undergo rotational thermal motion. Expressions are derived for the spin-rotational correlation time and for the contributions arising from spin-rotational interactions to the longitudinal and transverse relaxation times.     

Theory of nuclear magnetic spin-rotational relaxation for asymmetric molecules

A stochastic differential equation study of nuclear magnetic relaxation by spin-rotational interactions was successfully completed for spherical, symmetric rotator and linear molecules. This theory has now been extended to asymmetric rotator molecules by first establishing the conditions under which such an extension is mathematically feasible and then investigating the consequences of accepting these conditions.     

Polarized coincidence electroproduction scaling laws for the final state

We study the inclusive electroproduction of single hadrons off a polarized target. Bjorken scaling laws and the hadron azimuthal distribution are derived from the quark parton model.The polarization asymmetries scale when the target spin is along the direction of the virtual photon, and (apart from one significant exception) vanish for transverse spin. These results have a simple explanation; emphasis is given both to the general mathematical formalism and to intuitive physical reasoning.Through this framework we consider other cases: quarks with anomalous magnetic moment; renormalization group effects and asymptotic freedom; production of vector mesons (whose spin state is analysed by their decay); relation to large transverse momentum hadron production; and a covariant parton model calculation. We also look into spin-0 partons and Regge singularities.All of these cases (apart from the last two) modify the pattern of conclusions. Vector meson production shows polarization enhancements in the density matrix element ?₀₊; the renormalization group approach does not lead to any significant suppressions. They are also less severe in parton models for large p_T hadrons, and are not supported by the covariantly formulated calculation. The origins of these differences are isolated and used to exemplify the sensitivity that polarized hadron electroproduction has to delicate detail that is otherwise concealed.     

Nonlinear dielectric relaxation of polar molecules in a strong ac electric field: steady state response

The nonlinear dielectric relaxation ac stationary response of an assembly of rigid polar molecules acted on by strong superimposed external dc E0 and ac E1(t)=E(1) cos omegat electric fields is evaluated in the context of the noninertial rotational diffusion model. The calculation proceeds by expanding the relaxation functions f(n)(t) (the expectation value of the Legendre polynomials P(n)), which describe the nonlinear relaxation of the system, as a Fourier series in the time domain. Hence, an infinite hierarchy of recurrence equations for the Fourier components of f(n)(t) is obtained. The exact solution of this hierarchy can be obtained in terms of a matrix continued fraction, so allowing us to evaluate the ac nonlinear response. For a weak ac field our results are in complete agreement with previous solutions obtained by perturbation methods. Diagrams showing the behavior of the in-phase and out-of-phase components of the electric polarization are presented.     

Unified theory of photoelectron and vibrational spectra of chemisorbed molecules on metal surfaces

Photoelectron spectra of diatomic molecules such as CO and N₂ weakly chemisorbed on metal surfaces exhibit a multitude of satellite lines due to multi-electron excitations in which the photoinduced hole is screened by charge transfer from the metal into the low-lying unoccupied level. Recently Heskett et al. [Surface Sci. 139 (1984) 558] noticed that in the systems which display the satellite lines, the molecular vibrational frequencies decrease with increasing molecular coverage, in contrast to what is expected from dipole-dipole coupling theory. They then claimed that instead of the 2π¹-metal bonding, the 5σ orbital is responsible for the bond to the substrate. This leads to a shift of the C-O stretching frequency to lower energy as the molecular coverage increases, since the 5σ orbital has an antibonding character with respect to the stretch vibration. Other experimental phenomena which should be taken into account are the negative shift and substantial broadening of the vibrational spectra of these molecules upon chemisorption on metal surfaces. Experimental results seem to reveal that the magnitude of negative shift and width increases with the strength of chemisorption, thereby suggesting that an unique mechanism is responsible for these vibrational properties. We propose a unified theory which enables us to understand these apparently unrelated electronic and vibrational properties in terms of the occupancy of the low-lying 2π¹ level in the neutral ground state and in vibrational excited states of the chemisorbed molecules. Also discussed is a change of the vibrational frequencies with coverage within the framework of the chemical frequency shift caused by charge transfer between the 2π¹ level and the metal.     

Thermal desorption spectra of chemisorbed atoms and molecules with lateral interactions and finite mobility: Nondissociatively adsorbed molecules

A model of thermal flash desorption of interacting chemisorbed atoms and molecules from a crystal surface is developed. The possibility of adatom diffusion along the surface is taken into account, but thermal equilibrium of the adatom system during the desorption process is not assumed. The whole range of adsorbate mobilities, including absolutely immobile species as well as adsorbates in thermal equilibrium, is investigated. The result is obtained in the form of a chain of differential equations for correlation functions of occupation numbers which is solved in various approximations. The numerical results differ from the results of previous calculations and are in better agreement with the experiments. The experimental thermal desorption spectrum of CO on W(210) is satisfactorily reproduced.     

The hadronic final state at HERA

The electron-proton collider HERA offers unique capabilities to study the properties of the hadronic final state. This paper summarizes selected results highlights from the colliding beam experiments H1 and ZEUS.     

Geometry and bond lengths of chemisorbed atoms and molecules: NEXAFS and SEXAFS

A brief review is given of the near edge x-ray absorption fine structure (NEXAFS) and surface extended x-ray absorption fine structure (SEXAFS) techniques which have been developed in conjunction with the availability of high brightness monochromatic synchrotron radiation. Concepts and applications of the techniques are discussed using selected examples of chemisorption problems involving atoms and molecules on surfaces. Extensive references to related work are given.     

Stieltjes electrostatic model interpretation for bound state problems

In this paper, it is shown that Stieltjes electrostatic model and quantum Hamilton Jacobi formalism are analogous to each other. This analogy allows the bound state problem to mimic as n unit moving imaginary charges \(i\hbar \) , which are placed in between the two fixed imaginary charges arising due to the classical turning points of the potential. The interaction potential between n unit moving imaginary charges \(i\hbar \) is given by the logarithm of the wave function. For an exactly solvable potential, this system attains stable equilibrium position at the zeros of the orthogonal polynomials depending upon the interval of the classical turning points.     

ELS study on chemisorbed state of CO on Cr(110)

The chemisorbed state of CO on a Cr(110) surface has been investigated at 300 K by electron energy loss spectroscopy (ELS) with the in-situ combined supplementary techniques. The ELS spectrum of the Cr(110) surface after CO adsorption is characterized by the peaks at 2, 4.4, 6–7, 9, 11, 14.5, 19 and 23 eV, and is found to be practically the same as that of the oxygen covered surface. The C-KLL Auger spectra obtained in the range 0.1–900 L CO agree with those of metal carbides. These results are considered to indicate that CO is dissociatively chemisorbed on the Cr(110) surface throughout the whole exposure region examined. The average sticking probability of CO on Cr(110) is 0.7 at below 0.5 L, and the maximum work function increase at 1 L is ~0.1 eV. The adsorbed state of O atoms produced from dissociative adsorption of CO is also discussed.