Ionization Threshold Energy Research Articles

Photopolymerization of long-chain diacetylene monocarboxylic acid in Langmuir-Blodgett films studied by UV photoelectron spectroscopy and X-ray absorption near-edge structure

The UV photopolymerization of cadmium salt Langmuir-Blodgett (LB) films of a diacetylene long-chain compound tricosa-10, 12-diynoic acid was studied with UV photoelectron spectroscopy (UPS) and X-ray absorption near-edge structure (XANES), and the results are compared with those for evaporated films of the same compound in free acid form. The UPS spectra showed a lowering of the ionization threshold energy from 6.7 eV (monomer) to 5.1 eV (polymer in both blue and red forms) owing to the formation of a delocalized π band, which is similar to the results for evaporated films. The XANES spectra indicate that polymerization does not proceed regularly in the topmost surface layer of the LB films probed by XANES, possibly owing to disordered surface structure. This is in contrast to the results for evaporated films, where a clear change of XANES spectra is observed owing to the formation of a delocalized π ∗ band.

Photon emission from avalanche breakdown in the collector junction of GaAs/AlGaAs heterojunction bipolar transistors

The base-collector junction of GaAs/AlGaAs single heterojunction bipolar transistors has been observed to emit light at avalanche breakdown. The spectral distribution curve exhibits broad peaks at 2.03 and 1.43 eV, with the intensities dependent upon the reverse current. These observations suggest that electrons, excited to the upper conduction band by the field, lose their energy by impact ionizing electron-hole pairs and producing the 2.03 eV light, which corresponds to the threshold energy for electron impact ionization. The band-edge emission is the result of direct-gap free-carrier recombination and self-absorption of the high-energy transition.

Bielectron pairing mechanism for carriers of like sign in crystals (review)

Semiconducting zinc diphosphide (ZnP2) crystals have been found to exhibit a very distinct IHS, which consists of eight lines from n = 4 to n = ii [i0]. It has become a widely held view that such spectrally inverted phenomena can be consistently interpreted as the observation of a new type of optical interband transitions in crystals, i.e., transitions between two conduction bands or two valence bands [~]. They can be dubbed second-order transitions if the usual interband transitions between the upper valence band and the lower conduction band, which results in fundamental absorption, are classified as first-order transitions. I It turned out that second-order transitions can also be accompanied by a hydrogen-like line structure. In contrast to excitons, however, this structure converges in the long-wavelength region of the spectrum, forming an inverse hydrogen-like series of absorption and emission lines. This unusual spectrum is du~ to a new quasiparticle, a bielectron or a bihole, which is a bound state of two carriers of like sign. Figure i shows a portion of the band structure of a crystal, illustrating the possibility of observing second-order transitions and IHS provided that E 0 18 eV, exceeding the threshold ionization energy of the cations (Ba2+). Radiative transitions of an electron from the valence band of 2pF- to the cation band, which is accompanied by the emission of photons with an energy of 5-7 eV may be one mechanism of the decay of this state. This new mechanism of fast luminescence (less than 10 -9 sec), and effect of radiation-induced "floating" of holes in the ionic BaY 2 crystal [13, 14], is a brilliant example of second-order phototransitions.

Role of the space charge in above-threshold ionization

A study was made of the influence of the space charge of a cloud of residual ions on the energy and angular distributions of electrons in the case of above-threshold optical ionization. This influence accounted for the characteristic reduction in the intensity of the first (with energies closest to the threshold ionization energy) peaks in the energy distribution of electrons. The mechanism of this influence could be electron trapping by the charged cloud of ions remaining in the interaction region. The space charge of the ions affected also the angular distribution of electrons escaping from the ionization region.

Transition Probability of Impact Ionization by Holes in Silicon

AbstractAn analytical expression is developed for the transition probability per unit time for impact ionization by holes. The approach is based upon time dependent perturbation calculations and the Bloch functions for the states of electrons and holes. The threshold energy of ionization by holes is calculated for the valley L1 for the electron band structure of silicon. The results obtained for the probability are in good agreement with experimental results for threshold energy and theoretical results for the ionization probability.

L-subshell-ionization cross sections of tungsten by electron impact near the threshold region.

L-subshell-ionization cross sections ${\ensuremath{\sigma}}_{3}$/${\ensuremath{\sigma}}_{2}$ and ${\ensuremath{\sigma}}_{2}$/${\ensuremath{\sigma}}_{1}$ for ${L}_{3}$, ${L}_{2}$, and ${L}_{1}$ levels for tungsten have been measured by electron impact at incident electron energies from about one to three times the ionization threshold energy of the ${L}_{1}$ subshell. Ionization cross sections for the ${L}_{1}$, ${L}_{2}$, and ${L}_{3}$ levels were deduced from the measured intensity radiations L${\ensuremath{\beta}}_{3}$, L${\ensuremath{\beta}}_{1}$, and L${\ensuremath{\beta}}_{2}$ and using known values for the fluorescence yields, radiative transition probabilities, and Coster-Kronig parameters. The x-ray spectra were obtained with a curved crystal spectrometer (50 cm radius) and recorded on photographic film. Comparison is made with experimental measurements of Chu-Nan Chang and theoretical calculations of Gryzinski and McGuire. A good agreement is achieved for electron-beam energies ${E}_{0}$\ensuremath{\ge}1.25 times the energy ${E}_{1}$ of ${L}_{1}$-level ionization.

Sulfenic acids in the gas phase. Preparation, ionization energies and heats of formation of methane-, ethene-, and benzenesulfenic acid

Methane-, ethene-, and ethynesulfenic acids were generated in the gas phase by flash-vacuum pyrolysis of the corresponding tert-butyl sulfoxides at 400 °C and 10-4 Pa. Benzenesulfenic acid was prepared from phenyl 3-buten-1-yl sulfoxide at 350 °C and 10-4 Pa. The sulfenic acids were characterized by mass spectrometry Threshold ionization energies (IE) were measured as IE(CH3SOH) = 9·07 ± 0·03 eV, IE(CH2=CHSOH) = 8·70 ± 0·03 eV, IE(HCCSOH) = 8·86 ± 0·04 eV, and IE(C6H5SOH) = 8·45 + 0·03 eV. Radical cations [CH3SOH].+, [CH2=CHSOH].+, and [HCCSOH].+ were generated by electron-impact-induced loss of propene from the corresponding propyl sulfoxides and their heats of formation were assessed by appearance energy measurements as 685, 824, and 927 kJ mol-1, respectively. Heats of formation of the neutral sulfenic acids and the S-(O) (C), S-(O) (Cd), S-(O) (Ct) and S-(O) (CB) group equivalents were determined. The experimental data, supported by MNDO calculations, point to sulfenate-like structures (R-S-OH) for the sulfenic acids under study.

UV photoelectron spectroscopic study of the photopolymerization of long-chain diacetylene monocarboxylic acid in Langmuir—Blodgett films

Photopolymerization of diacetylene monocarboxylic acid in Langmuir—Blodgett films was studied by UV photoelectron spectroscopy. Upon polymerization, the ionization threshold energy decreased by 1.6 eV from 6.7 (monomer) to 5.1 eV (polymer). No change of the threshold was found between the blue and red forms of the polymer. This suggests that the polymer chain is in the acetylenic form in both forms. The mechanism of spectra sensitization by dyes is also discussed.

Non-localised impact ionisation using a modified lucky drift theory

The lucky drift theory developed by B.K. Ridley (1983) is modified to include the effect of field variation over the 'dead space'. The new theory is applicable to small-geometry logic devices for which the total energy available from the supply voltage is comparable to the ionisation threshold energy. It is shown that the new theory predicts a substantially lower level of multiplication than do conventional theories based on an ionisation coefficient.

Theory of initial yields of ions generated by electrons in binary mixtures. II

Eggarter [J. Chem. Phys. 84, 6123 (1986)] showed a method of calculation of the yield of ionization in a mixture by electrons, and carried out a numerical solution for the Ar–H2 mixture. At sufficiently high electron energies, the yields NAr and NH2 of each component species closely follow the relation NAr/NH2 =K⋅CAr/CH2, where CAr and CH2 represent the concentration fractions, and K is a constant. We now present a theoretical interpretation of that relation, and show the microscopic meaning of the constant K; it is the ratio of the ionization cross sections of the two component species, evaluated for an electron of a certain kinetic energy, which is much greater than the ionization threshold energy. The interpretation rests on the fact that the Spencer–Fano electron degradation spectrum depends on electron energy only mildly and smoothly except near the initial source energy and near the first ionization threshold energy. The same relation should hold approximately for any combination of ordinary molecules, although for the Ar–H2 case the relation is obeyed especially close. We also discuss the connection of our treatment with earlier treatments based on certain assumptions about the radiation-energy partition among the mixture components.

Ultraviolet photoelectron spectroscopy of solid iodine

The ultraviolet photoelectron spectra of solid iodine have been measured. The observed spectral features show a large deviation from a one-to-one correspondence with the gas phase spectrum. These results are examined by an extended Hückel band calculation, which indicates that this deviation is caused by the effect of a strong intermolecular interaction on the electronic structure, namely the formation of broad bands (0.7 to 2.0 eV). The characters of the observed spectral features are assigned by the calculated density of states. The energy gap is estimated to be 1.6 eV from the observed energy difference between the Fermi level and the top of valence bands (5.52 and 6.34 eV relative to the vacuum level, respectively), by assuming that iodine is an intrinsic semiconductor. The large decrease of ionization threshold energy (2.92 eV) from the gas to the solid state is also discussed.

Rutherford backscattering studies of plasma-etched silicon

Near-surface modifications of Si induced by reactive ion etching (RIE), such as surface residues and a disordered Si near-surface region, have been studied by He ion channeling. The dependence of the surface residue layer thickness/composition and the Si surface disorder intensity on the etching gas (mixture) used, the plasma exposure time, and the applied rf power density were investigated. For the case of SiO2/Si selective etching (Si etch rate low), thicker surface residue layers were formed and the Si near-surface disorder was more intensive than for a nonselective RIE process. In selective SiO2/Si etching, both Si substrate disorder and surface residue layer thickness increase initially with plasma exposure time of the substrate. The intensity of Si substrate disorder was found to increase with greater self-bias voltage (caused by more rf power) applied during etching. A thermal annealing study showed that the observed Si disorder must be due to several distinct silicon defect states and is not equivalent to amorphized Si. The deposited C,F-layer thickness was found to depend on the ion bombardment possible during reactive ion etching; above a certain ion energy threshold C,F-layer deposition is suppressed. Various approaches to recover high-quality Si surfaces following RIE were evaluated. Due to the complexity of the near-surface modifications caused by plasma exposure of a Si specimen, simple thermal annealing is ineffective in restoring the surface properties. Promising recovery techniques for device-quality surfaces were found to be O2 RIE and O2 annealing post-RIE treatments. Possible difficulties of the channeling method in the characterization of dry-etched Si were demonstrated by the observation of ion-beam-induced surface impurity desorption and surface-roughness-related Si surface peak areal density changes.

Reactive sputtering of copper and silicon near the sputtering threshold

We have investigated reactive sputtering of copper and silicon by oxygen in the ion energy regime near the sputtering threshold, using low-energy ion-beam and quartz-crystal microbalance techniques. Surface oxidation is observed to increase the ion energy threshold for copper sputtering, and oxide film growth by low-energy ion bombardment of copper and silicon is characterized. It is demonstrated that oxidation of copper is primarily by oxygen ion bombardment, while silicon exhibits greater reactivity to neutral oxygen molecules. Copper oxide layers as much as 150 Å thick can be formed at ion energy below the copper sputter threshold at room temperature. No evidence for selective formation of CuO or Cu2O was observed over the range of conditions investigated.

Analytic representation of the dipole oscillator-strength distribution. II. The normalization factor for electron continuum states in atomic fields

We address ourselves to the question what analytic formula is the most suitable for fitting the dipole oscillator-strength distribution df/dε over a wide range of the kinetic energy ε of an electron ejected from an atom or molecule. A suitable expression will enable one to interpolate or extrapolate data reliably and to use them readily in applications such as the modeling studies in radiation physics. It is useful to distinguish two factors that together constitute df/dε. The first factor is defined in terms of the dipole matrix element with respect to a final-state eigenfunction whose amplitude near the origin is independent of ε. As we showed earlier, this factor is analytic at all finite ε except at a singularity at ε=−I, where I is the ionization threshold energy. The other factor arises from the energy-scale normalization of the final-state eigenfunction, and is the object of the present discussion from several angles. First, we present a survey of numerical data for the s,p, and d states with 0≤ε≤5 a.u. for all atoms with Z≤38, evaluated within the Hartree–Slater potential model. Next, we discuss analytic properties of the normalization factor, which include its relation to the phase shift, and its behavior near a resonance. We also elucidate the connection of the continuum normalization with the bound-state normalization. Finally, we illustrate the significance of our findings in the practical fitting of the df/dε data in the presence of a resonance, taking the valence-shell ionization of Ar as an example.

Analytic Representation of the Generalized Oscillator Strength for Ionization

The energy distribution of secondary electrons resulting from ionizing collisions of fast charged particles with an atom or molecule is indispensable to microscopic modeling of radiation actions. In the Bethe theory, the distribution is determined from a more basic quantity, viz., the generalized oscillator strength for ionization. We examine analytic properties of the generalized oscillator strength for ionization from a fixed orbital, as a function of the squared momentum transfer $K^{2}$, or of the kinetic energy $\epsilon =k^{2}/2$ of an ejected electron. For fixed e, the generalized oscillator strength is analytic on the complex $K^{2}$ plane except at those points where $\Delta =[(K-k)^{2}+2I][(K+k)^{2}+2I]$ vanishes, I being the relevant ionization threshold energy (and quantities being measured in atomic units). On the basis of this observation and others, we show that the $K^{2}$ dependence of the generalized oscillator strength at fixed e should be well approximated by a polynomial in $K^{2}$ di...

Effect of low energy Ar+ ion implantation on silicon surface barriers

The electrical characteristics of Schottky diodes fabricated on silicon surfaces subject to low energy (10 keV) Ar + implantation have been studied as a function of Ar + ion dose. Significant changes in electrical characteristics are seen for ion doses as low as 5×10 11 cm -2 , well below the amorphization threshold dose for silicon. In conjunction with the ion energy threshold established earlier for silicon surface damage effects (about 25 eV), these results outline fundamental limits on the effect of ion-assisted dry etching processes on silicon surface barriers.

Theory of steady-state high-field hole transport in GaSb and AlxGa1−xSb: The impact ionization resonance

Calculations of the steady-state impact ionization rate of holes in GaSb and AlGaSb are presented based on a Monte Carlo simulation with the unique inclusion of a complete band structure (derived using the K*P method) and quantum effects such as initial state collision broadening. It is determined, by comparisons to existing experimental data, that the holes in the split-off band play the most important role in impact ionization in both GaSb and GaAlSb. The experimental data are fit quite well at low applied electric fields where the recently discovered resonance in the hole impact ionization rate occurs. It is found that the resonance data can be explained, using the parametrized Keldysh formula, if the impact ionization threshold energy in both the heavy- and light-hole bands is ∼1.40 eV, roughly twice the band-gap energy, The threshold energy in the split-off bands is taken to be equal to the split-off energy for Δ>

Ultraviolet Photoelectron Spectroscopy of Some Fundamental Vinyl Polymers and the Evolution of Their Electronic Structures

Abstract UV photoelectron spectra were measured for polypropylene (PP), poly(vinyl fluoride) (PVF), 1,2-polybutadiene (1,2-PBD), poly(vinyl chloride) (PVC), and poly(vinyl alcohol) (PVA). Their threshold ionization energies were found to be 8.5, 9.2, 7.5, 8.8, and 8.0 eV (1 eV≅0.1602 aJ), respectively. The evolution of their electronic structures (from those of small related molecules) were analyzed, mainly, by using the photoelectron spectra of these small compounds (including the new data of 1,2-propanediol). The data of previously measured polyethylene (PE) and polystyrene (PS) were analyzed together. With the results, the polymers were classified into three groups according to the increasing order of localization of the positive hole in the cationic state: (1) PE, PP, and PVF, (2) 1,2-PBD, PVC, and PVA, and (3) PS as a representative of aromatic-pendant polymers.

Do we observe the “adiabatic” ionizations of the water dimer? An interpretation of the threshold ionization energy and the nature of the ionic states

The physical meaning of the experimentally observed threshold ionization energy of the water dimer is discussed in terms of the proton-transfer potential energy surfaces of (H 2O) + 2 in the 1 2A″ and 1 2′ states within the framework of the Franck-Condon principle. The possibility that the real adiabatic ionizations are not observed in the photoionization experiments is suggested. The locations of the ionization sites and the relaxation mechanisms in the ionic states are also discussed on the basis of a Mulliken population analysis.

Effect of argon ion implantation dose on silicon Schottky barrier characteristics

The electrical characteristics of Schottky diodes fabricated on silicon surfaces subject to low-energy (10 keV) argon ion implantation have been studied as a function of Ar+ ion dose. Significant changes in electrical characteristics are seen for ion doses as low as 5×1011 cm−2, well below the amorphization threshold dose for Si. In conjunction with the ion energy threshold established earlier for Si surface damage effects (∼25 eV), these results outline fundamental limits on the effect of ion-assisted dry etching processes on Si surface barriers.