Negative Ion Resonances Research Articles

Negative ion resonances in the autoionizing region of zinc: excitation of the 4s4p3P0,1,2 states

Temporary negative ion resonances above the ionization threshold of zinc atoms have been observed in angular differential electron excitation functions and photon excitation functions of the lowest optically forbidden 3P state. These resonances involve the opening of the 3d-shell or double excitation from the outermost 4s-shell and provide a sensitive test for channel coupling and electron correlation effects.

Reaction threshold and decoherence: current induced desorption of CO on Cu(111) in STM

The transfer of a CO molecule from the Cu(111) surface towards the STM tip, induced by the tunnelling current, occurs only if a threshold voltage of 2.4 eV is exceeded (Bartels L, Meyer G, Rieder K-H, Velic D, Knoesel E, Hotzel A, Wolf M and Ertl G 1998 Phys. Rev. Lett. 80 2004). We present a theory based on the Quantum Nano Dynamics (QND) approach, treating the competition between a coherent and a decoherent mechanism of current induced CO desorption via inelastic tunnelling and vibrational excitation. The keypoint of the theory is to regard the tunnelling process as driving the system in a transient excited state and to include explicitely many-particle relaxation effects in the excited state. Only a decoherent desorption mechanism via a transient negative ion resonance of the sample surface, which is created through electron injection and interacts with the CO molecule, provides an explanation for the existance of the voltage threshold for CO desorption and reproduces the quantum yield and the isotope effect as observed in the STM experiment.

Fluorescence polarization of helium negative-ion resonances excited by polarized electron impact

We have investigated helium (1s3d) 3D → (1s2p) 3P (588 nm) fluorescence produced by electron impact excitation in the vicinity of the (2s22p) 2P and (2s2p2) 2D negative-ion resonances at 57.2 and 58.3 eV, respectively. In contrast to previous work, we use spin-polarized incident electrons and report the relative Stokes parameters P1, P2 and P3 in the 55–60 eV region. Our failure to see discernable resonance effects in P2 indicates that even though the lifetime of these resonances is significant (∼10 fs), magnetic forces acting on the temporarily captured electron are small. Resonant structures in the values of P1 and P3 are observed because the polarization contributions of resonant states are generally different than those from direct excitation of the 3 3D state.

Reactions in nanofilms of trifluoroacetic acid (CF 3COOH) driven by low energy electrons

Electron stimulated desorption (ESD) of fragment anions and thermal desorption spectroscopy (TDS) is applied to study reactions in thin films of a few CF 3COOH/CF 3COOD monolayers following low energy electron impact (0–20 eV). In ESD, the fragment ions H −/D − and F − are observed via resonant features in the energy range 5–12 eV similar to the dissociative electron attachment (DEA) resonances present in gas phase CF 3COOH. Extended electron irradiation of the film at energies above 9 eV generates CO 2 and CHF 3/CF 4 as revealed by a combination of TDS and ESD experiments. The yield for these reactions continuously increases with energy indicating that they are initiated via electronically excited neutrals rather than negative ion resonances. No reactions induced by sub-excitation electrons (≈1 eV) could be observed using the present experimental techniques. In a very recent study on electron attachment to clusters of CF 3COOH [J. Langer, I. Martin, G. Karwasz, E. Illenberger, Int. J. Mass Spectrom. Ion Processes 249/250 (2006), 477], the ion–molecule complex CF 3COO −·H 2O appeared as strongest signal via a resonance located at 0.4 eV.

Theory and Application of Dissociative Electron Capture in Molecular Identification

The coupling of an electron monochromator (EM) to a mass spectrometer (MS) has created a new analytical technique, EM-MS, for the investigation of electrophilic compounds. This method provides a powerful tool for molecular identification of compounds contained in complex matrices, such as environmental samples. In particular, EM-MS has been applied to the detection of nitrated aromatic compounds, many of which are potent mutagens and/or carcinogens and are considered environmental hazards. EM-MS expands the application and selectivity of traditional MS through the inclusion of a new dimension in the space of molecular characteristics-the electron resonance energy spectrum. EM-MS also enhances detection sensitivity as well because the entire electron flux of the proper energy can be delivered into the negative ion resonance that is analytically most useful to solving the problem at hand. However, before this tool can realize its full potential, it will be necessary to create a library of resonance energy scans from standards of the molecules for which EM-MS offers a practical means of detection. Unfortunately, the number of such standards is very large and not all of the compounds are commercially available, making this library difficult to construct. Here, an approach supplementing direct measurement with chemical inference and quantum scattering theory is presented to demonstrate the feasibility of directly calculating resonance energy spectra. This approach makes use of the symmetry of the transition-matrix element of the captured electron to discriminate between the spectra of isomers. As a way of validating this approach, the resonance values for 25 nitrated aromatic compounds were measured along with their relative abundance. Subsequently, the spectra for the isomers of nitrotoluene were shown to be consistent with the symmetry-based model. The initial success of this treatment suggests that it might be possible to predict negative ion resonances and thus create a library of EM-MS standards.

Integral cross sections for electron-impact excitation of the 33S and 31S states of He near threshold

A joint experimental and theoretical study of near-threshold electron-impact excitation of the 33S and 31S states in helium is reported. A high-resolution electron spectrometer is used to study integral cross sections in the energy region of the n = 3 − 5 negative-ion resonances. Photons are detected from the decay of the two states and the observed intensities are normalized to theoretical predictions using a new B-spline R-matrix (close-coupling) method that allows for non-orthogonal orbitals to improve the target description. Remarkable agreement between experiment and theory is demonstrated in both the overall energy dependence of the cross section and the fine details of a wealth of resonance structures. A detailed list of the resonances in the individual partial waves, along with their widths, is presented.

Negative ion resonance of a molecule adsorbed on a metal surface covered with a rare gas monolayer

The characteristics of negative ion states formed by temporary electron attachment to adsorbed molecules depend on the environment of the molecule. Adsorption site effects on the energy positions and widths of the N 2 - 2 Π g resonance are theoretically investigated for a nitrogen molecule adsorbed on an Ar monolayer covering a Cu metal substrate. The molecule is shown to preferentially adsorb above the valleys between Ar atoms, with its axis parallel to the Ar plane. The N 2 - characteristics are determined by a scattering approach with a 3D microscopic description of the adsorbed Ar layer. The anisotropy of the potential around the molecular adsorption site lifts the resonance degeneracy. For both resonance components, the energy is lowered and the lifetime is increased by the interaction with the Ar layer. The importance of the effect of the Ar–vacuum interface reflectivity observed in an earlier study using a dielectric continuous medium description of the Ar layer is confirmed. Different adsorption sites and orientations are studied, revealing an adsorption site dependence of the resonance characteristics.

The role of cascade processes in electron-impact excitation of the (3p54s2)2P3/2, 1/2 autoionizing levels in potassium

The cascade population of the (3p54s2)2P3/2, 1/2 lowest autoionizing levels in potassium has been studied to shed light on recent experimental observations of a noticeable enhancement in the cross sections around 22 eV incident electron energy. The ejected-electron excitation functions of both levels have been re-measured in the range of 20–28 eV, and calculations of the decay parameters for the 3p54s4p levels have been performed in single-configuration Hartree–Fock and configuration-interaction approaches. The analysis of these results, together with other available experimental and theoretical data on the 3p subshell excitation in potassium, revealed a relatively small contribution (<3%) of radiative transitions from the (3p5 4s4p1P)2P, 2D levels to the apparent (including cascade contributions) excitation cross sections of the (3p54s2)2P3/2, 1/2 levels. However, the decay of negative-ion resonances associated with the upper doublets in the 3p54s4p and 3p53d4s configurations may lead to a significant cascade population of the (3p54s2)2P3/2, 1/2 levels in the impact energy region 21–28 eV.

Theory of tip‐dependent imaging of adsorbates in the STM: CO on Cu(111)

AbstractThe processes of local electron injection or extraction in the scanning tunneling microscopy (STM) and spectroscopy (STS) lead to the creation of short‐lived excited states localized at the electrode surfaces. The dynamic relaxation of the transient negative or positive ion resonances, due to both local and long‐range interactions, is the clue to the understanding of numerous phenomena in STM/STS ranging from the “anomalously” large tip height corrugation amplitudes on clean metal surfaces to the observation of quantum mirages and features in the STS, which are not observed with the help of other surface spectroscopies. Quantum nanodynamics theory (QND) has been applied to calculate the interaction potential of a single CO molecule with the Cu(111) surface, with a transient negative ion resonance formed when an electron is injected from the tip, and the tunneling conductance on the clean and CO covered Cu(111) surface using a clean metal tip Al/Al(111) and a Pt(111) tip with an adsorbed CO molecule at the apex. Within QND and three‐dimensional scattering theory, regarding the tunneling as an excited‐state problem, we provide the explanation of the tip‐dependent STM image of a single CO molecule on Cu(111). The appearance of the CO molecule as an indentation, using a clean metal tip and as a protrusion with a tip terminated by a CO molecule, is understood as a result of tunneling through two competing channels. Tunneling via adsorbate‐induced ion resonances enhances the tunneling conductance. In contrast, tunneling via metal ion resonances only leads to attenuation of the conductance in the presence of the adsorbate. The current in the vicinity of the adsorbed CO molecule is reduced when a clean metal tip is used; i.e., CO appears dark in the STM image, because metal ion resonances on Cu(111) derive from the surface states with image state components coupling to plasmons and are therefore very diffuse. With a CO‐terminated tip, the major current channel is, for symmetry reasons, from the 2π‐derived orbital of the tip CO molecule, via the diffuse 2π‐derived orbital of the CO molecule on the sample, hence adsorbed CO appears bright. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Laser-driven coupled electron-nuclear dynamics: Quantum mechanical simulation of molecular photodesorption from metal films

In this paper we report dynamical simulations of laser-driven, coupled nuclear-electron dynamics for a molecule-surface system. Specifically, the laser desorption of a small molecule (NO) from a metal slab (Pt) in the so-called DIET limit (Desorption Induced by Electronic Transitions), is studied. The excitation of the metal electrons by a laser pulse followed by the formation of a negative ion resonance, its subsequent decay, and the simultaneous desorption of the molecule are all treated within a single quantum mechanical model. This model is based on an earlier theory of Harris and others [S. M. Harris, S. Holloway, and G. R. Darling, J. Chem. Phys. 102, 8235 (1995)], according to which a nuclear degree of freedom is coupled to an electronic one, both propagated on a single non-Born-Oppenheimer potential energy surface. The goals of the present contribution are (i) to make a conceptual connection of this model to the frequently adopted nonadiabatic ``multi-state'' models of photodesorption, (ii) to understand details of the desorption mechanism, (iii) to explicitly account for the laser pulse, and (iv) to study the photodesorption as a function of the thickness of the metal film, and the laser parameters. As an important methodological aspect we also present a highly efficient numerical scheme to propagate the wave packet in a problem-adapted diabatic basis.

Role of electronic band structure and resonances on electron-scattering. The case of the hydrogenated polycrystalline diamond

Elastic and inelastic scattering of low energy electrons by hydrogenated and deuterated polycrystalline diamond surfaces was studied by high resolution electron energy loss spectroscopy (HREELS). Energy loss spectra extending up to 800 meV and elastic/inelastic excitation functions, intensity variations of the elastic peak/energy-loss peaks as a function of the incident electron energy, have been measured quasi-continuously over the range 3–17 eV. Their compared behaviors are analyzed taking into account the influence of the electronic band structure of the sample. Electron elastic reflectivities are characterized by an intense broad peak at 13.5 eV incident electron energy, which results from the very high backscattering probability associated with the second absolute band gap of bulk diamond. Excitation functions of the stretching vibration mode of the hydrogen or deuterium terminations present a shoulder at about 4.5 eV and two broad peaks located around 8.0 and 12.8 eV incident electron energy. Significant resonant scattering processes contribute to the vibrational excitation at 4.5 and 8.0 eV: a strong progression of multiple excitation losses has in particular been observed in the energy loss spectrum acquired at the former energy, and a bell shape peak characteristic of a negative ion resonance has been observed at the latter energy in the stretching excitation functions. The peak at 12.8 eV is interpreted as a maximum induced by the resonant electron reflectivity enhancement, which results from the dramatic variation of the density of states corresponding to the diamond second absolute band gap.

Negative ions of ethylene sulfite

The formation of negative ions in molecular beams of ethylene sulfite (ES, alternately called glycol sulfite or ethylene glycol, C(2)H(4)SO(3)) molecules has been studied using both Rydberg electron transfer (RET) and free electron attachment methods. RET experiments with jet-cooled ES show an unexpected broad profile of anion formation as a function of the effective quantum number (n(*)) of the excited rubidium atoms, with peaks at n(max)(*) approximately 13.5 and 16.8. The peak at n(max)(*) approximately 16.8 corresponds to an expected dipole-bound anion with an electron binding energy of 8.5 meV. It is speculated that the peak at n(max)(*) approximately 13.5 derives from the formation of a distorted C(2)H(4)SO(3)(-) ion. We suggest that quasifree electron attachment promotes the breaking of one ring bond giving a long-lived acyclic anion and term this process incomplete dissociative electron attachment. Theoretical calculations of plausible ionic structures are presented and discussed. Electron beam studies of ES reveal the presence of multiple dissociative attachment channels, with the dominant fragment, SO(2)(-), peaking at 1.3 eV and much weaker signals due to SO(3)(-), SO(-), and (ES-H)(-) peaking at 1.5, 1.7, and 0.9 eV, respectively. All of these products appear to originate from a broad temporary negative ion resonance centered at approximately 1.4 eV.

Resonance excitation of the 3p 6-subshell in potassium: Contribution to the single ionization

The total autoionization cross-section of potassium has been studied as a function of incident electron energy over the range from the lowest autoionization threshold at 18.72 eV to 24.4 eV and at an energy resolution of 0.25 eV. The cross-section has been obtained by determining the normalized total intensities of ejected-electron spectra accurately measured at the magic angle of 54.7°. The structure observed in the autoionization function has been assigned to the negative-ion resonances based upon the lowest autoionizing configurations 3p 54s 2 and 3p 53d4s. The quartet levels from the 3p 53d4s configuration give the bulk of the autoionization contribution in the single ionization cross-section in the energy range 20.5–24.4 eV. Comparison of the present results with earlier available experimental and theoretical results is presented.

High resolution electron energy loss spectroscopy study of Fomblin Z-tetraol thin films

High resolution electron energy loss spectroscopy has been used to obtain vibrational spectra of Fomblin Z-tetraol lubricant films on a commercial magnetic hard disk. The energy loss intensities of the ν(CF2) stretching mode are roughly independent of scattering angle up to angles of Δθ<14° indicating that they are excited by impact scattering. As a consequence there is little information that can be gleaned from the spectra about molecular orientation on the surface. A negative ion resonance enhances the energy loss cross section of the ν(CF2) stretching mode at the impact energy of EI=4eV. It is possible that this resonance is associated with the known sensitivity of fluorocarbons to electron induced dissociation. Annealing the disk sample to T=700K causes a dramatic decrease in the intensity of C–F stretching modes and an increase of the loss features due to C–H stretching. This indicates that the Fomblin Z-tetraol has decomposed and is exposing the a‐CHx overcoat on the magnetic media surface.

Absolute Generalized Oscillator Strength Profile of NegativeIon Resonance at 8.9 eV in C2F6

A temporary negative ion resonance at 8.9 eV in C2F6 has beenclearly identified using angle-resolved electron energy lossspectroscopy at 2.5 keV impact energy, and can be attributed toelectron capture into 12a1u, 12a1g, 22a2u, and12eg virtual molecular orbitals. The corresponding absolutegeneralized oscillator strength profile is found to have a shapecharacteristic of non-dipole interaction with the maximum K~0.8 a.u. of momentum transfer.

Specific features of the resonant electron attachment to the chlorodibenzo-p-dioxin molecules

The processes of resonant dissociative electron attachment to the molecules of dibenzo-p-dioxin and its chlorinated derivatives containing one to four chlorine atoms (totally eight compounds) were investigated. It was established that 2,3,7-trichlorodibenzo-p-dioxin; 1,2,3,4-tetrachlorodibenzo-p-dioxin; 1,3,7,8-tetrachlorodibenzo-p-dioxin, and 2,3,7,8-tetrachlorodibenzo-p-dioxin molecules are chatacterized by positive electron affinities. At electron energies below 2 eV, the electron attachment is caused by the shape resonances. Based on the energy correlation between the negative ion resonance peaks at 3—4 eV and the UV band maxima, it was suggested that electron attachment in this energy region occurs by the mechanism of inter-shell resonance with the molecular singlet-excited states as parents. The possibility for the rearrangement processes resulting in oxy-anionic structures to occur is substantiated.

Role of electronic band structure and resonances on electron reflectivity and vibrational excitation functions: The case of hydrogenated diamond

We report on an high resolution electron energy loss study of electron surface reflectivity and excitation of the C-H stretch mode of hydrogenated polycrystalline diamond films at low energy. Their specular energy dependences demonstrate that (i) the reflectivity is dominated by the conduction band structure. In particular the second absolute band gap of bulk diamond results in a strong increase of the elastic backscattered intensity around 13.5 eV. (ii) The vibrational excitation is dominated by a negative ion resonance process around 8 eV and a band gap induced scattering enhancement around 13 eV.

Mechanisms of molecular manipulation with the scanning tunneling microscope at room temperature: chlorobenzene/si(111)-(7 x 7).

We report a systematic experimental investigation of the mechanism of desorption of chlorobenzene molecules from the Si(111)-(7 x 7) surface induced by the STM at room temperature. We measure the desorption probability as a function of both tunneling current and a wide range of sample bias voltages between -3 V and +4 V. The results exclude field desorption, thermally induced desorption, and mechanical tip-surface effects. They indicate that desorption is driven by the population of negative (or positive) ion resonances of the chemisorbed molecule by the tunneling electrons (or holes). Density functional calculations suggest that these resonant states are associated with the pi orbitals of the benzene ring.

Absolute collision cross sections for low energy electron scattering from NO: the role of resonances in elastic scattering and vibrational excitation.

Absolute measurements of elastic scattering and vibrational excitation of the NO molecule by low energy electron impact (0.4-2.5 eV) are presented. They show that previous estimates of these cross sections may be in error by as much as a factor of 3 and provide compelling evidence for a reassessment of the balance between elastic scattering and vibrational excitation at incident energies below 2 eV. They also confirm the critical contribution that intermediate negative ion resonances (NO-) make to the various scattering processes for this molecule at low incident energies.

Resonant Electron Scattering of 11-Mercaptoundecanoic Acid Self-Assembled Monolayer Adsorbed on Au (111)

We present a High-Resolution Electron Energy Loss study of an 11-mercaptoundecanoic acid self-assembled monolayer (SAM) adsorbed on Au(111). This work has to be placed in the general context of the study of thin organic layers by HREELS and of low-energy interaction mechanisms of electrons with large organic molecules. Evidence is presented here for two resonance phenomena, at ~1.5 eV and at ~6 eV, respectively. The first one is attributed to a negative ion resonance involving the carboxylic group, and the second one is the well-known resonance of CH2. From the angular distribution of the elastically scattered electrons the crystalline domain size of the monolayer is estimated to be about 100 A.