Spontaneous Electron Emission Research Articles

Stability and Cooling of the C_{7}^{2-} Dianion.

We have studied the stability of the smallest long-lived all carbon molecular dianion (C_{7}^{2-}) in new time domains and with a single ion at a time using a cryogenic electrostatic ion-beam storage ring. We observe spontaneous electron emission from internally excited dianions on millisecond timescales and monitor the survival of single colder C_{7}^{2-} molecules on much longer timescales. We find that their intrinsic lifetime exceeds several minutes-6 orders of magnitude longer than established from earlier experiments on C_{7}^{2-}. This is consistent with our calculations of vertical electron detachment energies predicting one inherently stable isomer and one isomer which is stable or effectively stable behind a large Coulomb barrier for C_{7}^{2-}→C_{7}^{-}+e^{-} separation.

Оценка сродства к электрону по данным о временах жизни отрицательных молекулярных ионов p-кумаровой и кумарин-3-карбоновых кислот

The molecules of p-coumaric and coumarin-3-carboxylic acids were studied by the method of mass spectrometry of negative ions of resonant electron attachment. The average lifetime of negative molecular ions relative to spontaneous electron emission has been experimentally measured. With Arrhenius approximation, the value of the adiabatic electron affinity (EAa) was estimated. It is established that the theoretical values of EAa calculated by the B3LYP/6-31+G(d) method with minimal addition of diffuse functions as the difference of the total energies of the neutral molecule and the radical anion correlate well with the values of EAa obtained from the experiment.

Photoexcited state of the organic electron acceptor TCNQ molecular anion in the condensed state

The first and second excited states of 7,7,8,8-tetracyanoquinodimethane (TCNQ) anions in acetonitrile were studied employing femtosecond time-resolved photoluminescence and transient absorption spectroscopy. It was found that the second excited state, which is a superexcited state, undergoes ultrafast decay due to spontaneous electron emission with a time constant of 0.16 ps. The rationale for the fast electron emission in acetonitrile is discussed in terms of the interactions with the surrounding polar solvent molecules.

Spontaneous electron emission vs dissociation in internally hot silver dimer anions.

Referring to a recent experiment, we theoretically study the process of a two-channel decay of the diatomic silver anion (Ag2 -), namely, the spontaneous electron ejection giving Ag2 + e- and the dissociation leading to Ag- + Ag. The ground state potential energy curves of the silver molecules of diatomic neutral and negative ions were calculated using proper pseudo-potentials and atomic basis sets. We also estimated the non-adiabatic electronic coupling between the ground state of Ag2 - and the ground state of Ag2 + e-, which, in turn, allowed us to estimate the minimal and mean values of the electron autodetachment lifetimes. The relative energies of the rovibrational levels allow the description of the spontaneous electron emission process, while the description of the rotational dissociation is treated with the quantum dynamics method as well as time-independent methods. The results of our calculations are verified by comparison with the experimental data.

Spontaneous Electron Emission from Hot Silver Dimer Anions: Breakdown of the Born-Oppenheimer Approximation.

We report the first experimental evidence of spontaneous electron emission from a homonuclear dimer anion through direct measurements of Ag_{2}^{-}→Ag_{2}+e^{-} decays on milliseconds and seconds timescales. This observation is very surprising as there is no avoided crossing between adiabatic energy curves to mediate such a process. The process is weak, yet dominates the decay signal after 100ms when ensembles of internally hot Ag_{2}^{-} ions are stored in the cryogenic ion-beam storage ring, DESIREE, for 10s. The electron emission process is associated with an instantaneous, very large reduction of the vibrational energy of the dimer system. This represents a dramatic deviation from a Born-Oppenheimer description of dimer dynamics.

Spontaneous decay of small carbon cluster dianions (n=7-11)

SynopsisWe have studied the stabilities of small carbon dianion clusters, (n = 7 – 11) in the cryogenic electrostatic ion storage ring, DESIREE. We observe spontaneous electron emission for tens of milliseconds, where dianions containing an even number of carbons decay slower in comparison to their neighbouring cluster sizes. For all the systems considered here, we find that there is a component which is stable at least on the time scales of hundreds of milliseconds.

Rational Design of Stable Dianions and the Concept of Super-Chalcogens.

Super-atoms are homo/heteroatomic clusters that mimic the chemistry of atoms in the periodic table. While a considerable amount of research over the past three decades has revealed many super-atoms that mimic group I (alkali metals) and group 17 (halogens) elements, little effort has been made to identify super-atoms that mimic the chemistry of chalcogens, i.e., those belonging to group 16 elements. This is particularly important as super-chalcogens can form the building blocks of new materials, just as super-alkalis and super-halogens form a variety of super-salts with unique properties. Using first-principles calculations and various electron counting rules, some of which have been prevalent in chemistry for a century, we provide a route to the rational design of dianions that are stable in the gas phase. And unlike the group 16 atoms, these super-chalcogens can retain the second electron without spontaneous electron emission or fragmentation. A new class of super-chalcogenides with unique properties could be formed with these super-chalcogens as building blocks.

Colossal Stability of Gas-Phase Trianions: Super-Pnictogens.

Multiply charged negative ions are ubiquitous in nature. They are stable as crystals because of charge compensating cations; while in solutions, solvent molecules protect them. However, they are rarely stable in the gas phase because of strong electrostatic repulsion between the extra electrons. Therefore, understanding their stability without the influence of the environment has been of great interest to scientists for decades. While much of the past work has focused on dianions, work on triply charged negative ions is sparse and the search for the smallest trianion that is stable against spontaneous electron emission or fragmentation continues. Stability of BeB11 (X)123- (X=CN, SCN, BO) trianions is demonstrated in the gas phase, with BeB11 (CN)123- exhibiting colossal stability against electron emission by 2.65 eV and against its neutral adduct by 15.85 eV. The unusual stability of these trianions opens the door to a new class of super-pnictogens with potential applications in aluminum-ion batteries.

Colossal Stability of Gas‐Phase Trianions: Super‐Pnictogens

AbstractMultiply charged negative ions are ubiquitous in nature. They are stable as crystals because of charge compensating cations; while in solutions, solvent molecules protect them. However, they are rarely stable in the gas phase because of strong electrostatic repulsion between the extra electrons. Therefore, understanding their stability without the influence of the environment has been of great interest to scientists for decades. While much of the past work has focused on dianions, work on triply charged negative ions is sparse and the search for the smallest trianion that is stable against spontaneous electron emission or fragmentation continues. Stability of BeB11(X)123− (X=CN, SCN, BO) trianions is demonstrated in the gas phase, with BeB11(CN)123− exhibiting colossal stability against electron emission by 2.65 eV and against its neutral adduct by 15.85 eV. The unusual stability of these trianions opens the door to a new class of super‐pnictogens with potential applications in aluminum‐ion batteries.

Negative-electron-affinity diamondoid monolayers as high-brilliance source for ultrashort electron pulses

Diamondoids are nanometer-sized, hydrogen terminated diamond-like molecules consisting of fused adamantane units. The thiolated diamondoid [121]tetramantane-6-thiol shows negative electron affinity behavior, i.e. population of unoccupied states directly leads to spontaneous electron emission. We present time-resolved photoemission data from self-assembled monolayers of [121]tetramantane-6-thiol in order to shed light on the emission process: A photon energy threshold for electron emission of 5.6–5.8eV was observed, and the electron affinity was estimated to be −0.21 to −0.57eV for Ag and Au substrates, respectively. Electrons are emitted after excitation in the metal substrate through the molecular orbitals within a few femtoseconds.

Negative electron binding energies observed in a triply charged anion: Photoelectron spectroscopy of 1-hydroxy-3,6,8-pyrene-trisulfonate

We report the observation of negative electron binding energies (BEs) in a triply charged anion, 1-hydroxy-3,6,8-pyrene-trisulfonate (HPTS(3-)). Low-temperature photoelectron spectra were obtained for HPTS(3-) at several photon energies, revealing three detachment features below 0 electron BE. The HPTS(3-) trianion was measured to possess a negative BE of -0.66 eV. Despite the relatively high excess energy stored in HPTS(3-), it was observed to be a long-lived anion due to its high repulsive Coulomb barrier (RCB) ( approximately 3.3 eV), which prevents spontaneous electron emission. Theoretical calculations were carried out, which confirmed the negative electron BEs observed. The calculations further showed that the highest occupied molecular orbital in HPTS(3-) is an antibonding pi orbital on the pyrene rings, followed by lone pair electrons in the peripheral -SO(3) (-) groups. Negative electron BE is a unique feature of multiply charged anions due to the presence of the RCB. Such metastable species may be good models to study electron-electron and vibronic interactions in complex molecules.

Experimental Observations of the Virtual Anode Motion and Streamer Breakdown Mechanisms in a Pseudospark Discharge

A pseudospark (PS) discharge or transient hollow-cathode discharge is a high-voltage low-pressure discharge, which is characterized by the cathode being hollow and the cathode's interior connected to the anode-cathode (A-K) gap by a hole in the center of the cathode. Some initial electron emission from the cathode region is the starting point of the different ionization processes occurring in the subsequent phases until the final breakdown. These initial electrons create an avalanche of charged particles whose tail is constituted by ions, meanwhile the head is essentially formed by electrons. An avalanche moving from the cathode toward the anode establishes physical conditions in order to create a virtual anode into the A-K region. Successive avalanches produced by spontaneous electron emission at the cathode, secondary electron emission due to ion impact in the cathode, or impact of the ionized particles with the residual gas, give access to the virtual anode arrives to the cathode leading the final electrical breakdown. During breakdown, electron and ion beam activities are present as consequence of the precedent ionization processes. In this paper, we present experimental observations of the moving virtual anode as well as the ion and electron beams emitted from a PS discharge for seven different gas pressures. The experiments were performed in hydrogen at a pressure of between 10 and 70 Pa, with a cathode aperture of 5 mm. A pulsed-charged capacitor scheme is used to produce up to a 30 kV step across the electrodes.

Free electron laser spectrum control with a quasiperiodic wiggler configuration

Recently, we have studied an unusual wiggler magnet configuration comprising two conventional periodic wigglers. It was shown that if the periods of the two constituent wiggler components are not integrally but irrationally related then considerable radiation is generated at frequencies that are irrational multiples of the fundamental radiation frequency. The generation of these irrationally related harmonics modifies considerably the basic free electron laser (FEL) mechanism and as such they may be used to manipulate the FEL spectrum. In this contribution, we consider the spontaneous emission of electrons in such a quasiperiodic wiggler structure with and without the electron energy spread effects. Numerical examples are used to demonstrate that significant radiation can be generated simultaneously at both the usual FEL harmonics and irrationally related frequencies. When the wiggler parameters are appropriately adjusted, the usual integrally related FEL harmonics are suppressed considerably. This feature may be used to relieve the mirror degradation problem in some ultraviolet and X-ray FELs.

Free-electron lasers: Spontaneous emission of electrons with chaotic orbits

A single-particle computer model is used to calculate the electron trajectories in combined helical wiggler and guide fields. The analysis considers the effects of a realizable wiggler field. Special attention is focused on the stability of the electron orbits near magnetoresonane. Upon calculating Poincaré surface-of-section plots and Lyapunov exponents, it is shown that the electron dynamics is chaotic. Analyzing the spectra of spontaneous emission, it is found that regular and chaotic orbits might be discriminated by optical measurements: off resonance, the spectra consist of well-defined lines. On the other hand, near magnetoresonance, chaotic orbits are revealed by noisy spectra concentrated at low frequencies, and a sudden breakdown of the integral radiant energy.

Dynamic Redistribution of Excess Charge During Photoemission in an Electron Bombarded Glass-Ceramic

ABSTRACTWe have measured the energy distribution of excess charge trapped by an electron irradiated mica-glass ceramic, Macor™. Photoelectron emission as a function of incident wavelength shows two distinctly different contributions to the measured density of electron trap states. First, there is a broad continuum of binding states which extends well above the Fermi level. This results in spontaneous electron emission which depletes the excess charge in these states rapidly at room temperature and more slowly at 170K. The free decay of charge emission can be described semiclassically. Second, localized trap states appear as peaks on the continuum distribution which occur at 2.7 and 3.1 eV. These correspond to two thermionic emission peaks which occur during heating at 550K and 595K respectively. Photoemission at high intensities using the 458, 488, and 514 nm wavelengths of an argon ion laser and the 632 nm wavelength of a He-Ne laser greatly enhances the depletion of excess charge from the sample. The laser-induced emission intensity is initially high and drops rapidly, followed by a more gradual decrease. The initial intensity measures the population of electrons with binding energies less than the photon energy. The later emission is related to the repopulation of these states by the dynamic redistribution of electrons from deeper states. This behavior may lead to a model for the mechanism of electron redistribution in charged insulating materials.

Contribution of broadening effects to chemisorptive emission

The effects of energy broadening on the spontaneous emission of electrons during chemisorption (“chemisorptive emission”) are considered using an approach formerly used with success by Hagstrum for the related process of ion neutralization. Broadening is assumed to arise from two sources: electron tunnelling occurs over a range of distances from the surface, and the lifetime of the initial state is finite. Experimentally determined electron yields for electronegative gases adsorbing on highly electropositive metals are used to determine appropriate broadening parameters, which generally agree well with resonance widths used by other authors for similar systems.

Extreme pressures IV. Compressoelectric effect : spontaneous emission of electrons

A new solid state mechanoelectric phenomenon, the compressoelectric effect, has recently been predicted (L. Levitt, Lett. Nuov. Cim, 12, 537 (1975)) from the theoretical increase in the Fermi energy of a metal with increasing density, due to applied pressure :deF = (h2 /3m) (3No z/8πA)2/3 p-1/3 dp(where z, A, and p are the valence, A.W., and density of the metal). In general, the Fermi energy under stress, [math], is a simple function of the density: [math]/eF = (p'/pO )2/3 . Therefore there exists some high density, pc , at a corresponding high pressure, pc , for which [math] must become so large that spontaneous compressoelectric of electrons from a metal surface will occur. This will be possible only when the Fermi energy, [math], becomes larger than the internal potential of the electrons, and [math] eint = eF + Oe, where O is the work function at ordinary pressure. When the critical value [math] is attained we have, therefore, pc /Po = [1 + (Oe/eF )]3/2 . If it is assumed that eint is constant, then O must be a decreasing function of p: — d O = 2eF βdp/3e, where β is the isothermal compressibility. For spontaneous electron emission we have Vc = Oe = eF [(Pc /Po )2/3 -1]. The critical pressure, pc, for spontaneous emission depends on the compressibility, β, which, at extreme pressures (> 104 atm) we have previously shown to be β = v/Bp = A/B pp, for all solids where B (in cm3 ) is a constant characteristic of the solid.

The study of freshly deformed metal surfaces with the aid of exo-electron emission

The emission of exo-electrons from metals is reviewed. Deformed metal surfaces produce counts in point-counters, counting tubes or electron-multipliers and this apparently spontaneous electron emission was at first ascribed to the latent heat developed during phase changes or interaction with oxygen. More recent investigations showed that the surface films on deformed metals contain special electronic energy levels associated with crystal imperfections in the oxide lattice. The energy levels can be excited thermally or optically, leading to electron emission at definite temperatures or wavelengths of light. The decay of emission depends on the atmosphere with which the metal surface is in contact.

Dependence of the Average Charge of an Ion on the Density of the Surrounding Medium

The experiments of Lassen revealed the existence of two "density effects" expressing a dependence of the average charge of a moving heavy ion on the density of the surrounding medium. These are as follows: (1) the "pressure effect" characterizing gases and (2) the "state of condensation effect" characterizing solids and liquids. According to Bohr and Lindhard, these two effects are phenomenologically analogous since they involve a relationship between the times separating successive collisions of the moving ion and the times required for the excited orbital electrons to readjust themselves either by radiation or by redistribution of their excitation energy. It is conjectured that the phenomenological similarity may not be complete since in determining the state of condensation effect one also has to take into account the effect of a polarizing field due to the reaction of the perturbed medium on the moving ion. This field causes spontaneous emission of electrons carried by the ion, thus increasing the effective charge of the ion. A relationship is established between various values of the polarizing field and the corresponding average charge of an incident heavy ion in which the orbital electrons are continually maintained in an excited state due to the recurrent collisions with the atoms in the surrounding medium. The value of the polarizing field is determined for a particular case of a heavy fission fragment in gold and by applying this value to the above relationship it is shown that the contribution of the autoionization is substantial. Therefore, this contribution should be taken into account in determining the cause of the "state of condensation effect." The "autoionization" is due to "distant collisions," i.e., to the portion of the medium relatively far removed from the particle track, whereas the effect of Bohr and Lindhard is due to "close collisions" with atoms adjacent to the track.

Electron Capture and Loss by Moving Ions in Dense Media

Several experiments have shown an anomaly in the stopping power which appears to be greater for incident heavy particles in condensed media than in gases. It is suggested that this anomaly is characteristic of incident ions, i.e., of particles that are not completely stripped of orbital electrons, and it seems to be due to the reaction of the perturbed medium upon the moving ion having charge $\mathrm{Ze}$, which reaction results in a field $F=(\frac{1}{\mathrm{Ze}})(\frac{\mathrm{dW}}{\mathrm{dz}})$ acting against the motion of the ion ($\frac{\mathrm{dW}}{\mathrm{dz}}$ is the energy loss per cm). Although $F$ is relatively weak in gases, it is substantial in condensed media and may cause the spontaneous emission of electrons carried by the ion (autoionization), thus increasing the ions' effective charge. The higher effective charge accounts for the higher stopping power in condensed media. It has been determined that when an incident helium atom passing through a liquid undergoes a collision, thus emerging in a singly ionized and excited state, it cannot exist in such a state since it becomes instantly doubly ionized. However, the same type of collision occurring in a gas leaves the atom singly ionized. The effective charge of the incident particle depends upon the values ${\ensuremath{\varphi}}_{0}$, ${\ensuremath{\varphi}}_{1}$, and ${\ensuremath{\varphi}}_{2}$, representing the probability of the particle's being neutral, singly ionized, and doubly ionized, respectively, where ${\ensuremath{\varphi}}_{0}:{\ensuremath{\varphi}}_{1}:{\ensuremath{\varphi}}_{2}={\ensuremath{\sigma}}_{10}{\ensuremath{\sigma}}_{21}:{\ensuremath{\sigma}}_{01}{\ensuremath{\sigma}}_{21}:{\ensuremath{\sigma}}_{01}{\ensuremath{\sigma}}_{12}$ (${\ensuremath{\sigma}}_{\mathrm{if}}$ represents the cross section for electron loss or capture; $i$ and $f$ represent the charge before and after the event, respectively). It is shown that ${\ensuremath{\sigma}}_{\mathrm{if}}$ is higher for a condensed medium than for a gas when $i<f$ and lower when $i>f$. ${\ensuremath{\sigma}}_{\mathrm{if}}$ has been calculated for He passing through argon gas at 400 kev, and gives roughly ${\ensuremath{\varphi}}_{0}:{\ensuremath{\varphi}}_{1}:{\ensuremath{\varphi}}_{2}=0.05:0.75:0.20$; thus, an average square charge is ${〈{\ensuremath{\epsilon}}^{2}〉}_{\mathrm{Av}}=1.55{e}^{2}$. If autoionization is taken into account, ${\ensuremath{\sigma}}_{\mathrm{if}}$ for He in liquid argon gives roughly ${\ensuremath{\varphi}}_{0}:{\ensuremath{\varphi}}_{1}:{\ensuremath{\varphi}}_{2}=0.04:0.70:0.26$, and thus ${〈{\ensuremath{\epsilon}}^{2}〉}_{\mathrm{Av}}=1.74{e}^{2}$.