Photoionization Cross Sections Research Articles

Effect of magnetic field on donor impurity-related photoionisation cross-section in multilayered quantum dot

ABSTRACT The effect of the magnetic field on the electron states in the AlGaAs/GaAs/AlGaAs multilayered quantum dot with and without shallow-donor impurity is under study. The magnetic field dependence of the impurity-related transition energies and the corresponding photoionisation cross-section is calculated, taking into account the excited states. The problem is solved using the effective mass approximation and rectangular potential profiles for wells and barriers using diagonalisation and finite element methods. The results obtained by both methods coincide with high accuracy. It is shown that the transition energies and photoionisation cross-section strongly depend on the impurity position and the magnetic field induction. As the magnetic field induction increases, all the peaks of photoionisation cross-section shift to the region of higher energies in the case of off-centre impurity and shift to lower energies in the case of on-centre impurity. The effect of merging two photoionisation cross-section peaks with increasing magnetic field induction is revealed, leading to a significant increase in the value of impurity photoionisation cross-section.

Inner-shell photoabsorption and photoionisation cross-sections of valence excited states from asymmetric-Lanczos equation-of-motion coupled cluster singles and doubles theory

The asymmetric-Lanczos-based equation-of-motion coupled cluster formalism to compute photoabsorption and photoionisation cross-sections of valence excited states [B.N.C. Tenorio, M.A.C. Nascimento, A.B. Rocha, and S. Coriani, J. Chem. Phys. 151 (18), 184106 (2019). doi:10.1063/1.5125125] has been adapted to enable the calculation of photoabsorption and photoionisation spectral signatures from inner-shell electrons of such states. Since excited-state properties depend on both the electronic character of the molecular wavefunction and on the nuclear dynamics, we computed the photoionisation spectra using both ground and excited state optimised geometries. The total cross-section profiles were generated for the first two electronically excited states of water, ammonia, ethylene, and uracil by two different methodologies: an analytic continuation procedure based on the Padé approximants and by the Stieltjes imaging procedure. A comparison with literature results, whenever available, is presented. Remarkable differences were observed between the results of the core-ionisation cross-sections of the valence excited states yielded by the two quadrature approaches, at variance from previous studies on the valence photoionisation cross-sections of ground states and of valence excited states. Their origin remains unclear.

Shake loss intensities in x-ray photoelectron spectroscopy: Theory, experiment, and atomic composition accuracy for MgO and related compounds

The relative intensities of XPS core levels, scaled by their photoionization cross sections, are regularly used to determine sample atomic composition. Cross sections, however, give the intensity to all possible final states for the core ionizations, not just to the main peak. This includes all intrinsic satellite structure (shake states and, for open shell systems, the different ionic multiplets). In practice, for solids, this is usually experimentally impossible to determine accurately because such a satellite structure sits on the inelastically scattered electron background and cannot be easily separated. Therefore, usually, only the intensity of the main peak is used. This limits the ultimate possible accuracy of XPS composition determination. The purpose of the present paper is to examine the contributions that a theoretical analysis of losses of intensity can make to improve quantitation. For an MgO single crystal, we show that the correct stoichiometry of 1:1 can be recovered using the theoretical analysis of the experimental MgO peak ratio intensities. For materials with a sufficient bandgap for the XPS main peaks to be separated from the scattered background, the intensity of main peaks can often be accurately determined. Thus, if one uses theory to calculate that fraction of the total intensity lost from a main peak into all its satellite structure, the intensity of just main peaks could then be used to more accurately determine relative atom % composition. This work tests this approach using a single crystal MgO (50% Mg, 50% O) standard. Ab initio electronic structure theory of representative MgO clusters is used to determine Hartree–Fock wave functions for the ground state and final ionized states corresponding to the main Mg 2p and O1s XPS peaks of the oxide. The sudden approximation, SA, is used to determine the fractional losses from these main peaks to shake satellites, which is found to be greater for O1s than Mg2p. This results in predicted “apparent composition” for stoichiometric MgO of 55.2% Mg, 44.8% O instead of the true 50% Mg, 50% O. Equivalent theory for CaO results in a predicted apparent Ca value of 53.4%. Experimentally, using Mg2s or 2p intensity ratio to O1s, we find values between 52.2% and 56.0% Mg using two crystals and four different instrument electron pass energies. The average value of the measurements is 54.5% Mg when corrected for the presence of an adventitious carbon overlayer and slight surface hydroxide. Though this agreement with theory may be somewhat fortuitous, given the potential experimental errors, which are fully discussed, it is similar to that in our earlier study on LiF. We also present preliminary experimental data on Mg(OH)2 and MgSO4, which show a similar trend of apparently higher than 50% Mg, but we have no theory values. We are not yet able to experimentally test for validation of the difference between apparent composition for MgO (55.2% Mg) and CaO (53.4% Ca), owing to significant carbonate formation at the surface of the single crystal CaO. An important conclusion is that the theoretical determination of shake losses, obtained with ab initio wavefunctions and the SA, is likely to be a useful way to calibrate the accuracy and reliability of compositions obtained from XPS intensities and merits further study.

Absolute Photoionization Cross Section of the Simplest Enol, Vinyl Alcohol

The absolute photoionization cross section of vinyl alcohol was determined by multiplexed photoionization mass spectrometry of the Norrish type II photodissociation of butanal at 308 nm. The measured cross sections at 10.005 and 10.205 eV are 7.5 ± 1.9 and 8.1 ± 1.9 MB, respectively. A higher signal-to-noise ratio photoionization spectrum of vinyl alcohol was recorded via the pyrolysis of 2-chloroethanol and scaled to the absolute cross sections measured using the Norrish type II method. From a comparison of our spectrum with previously reported photoelectron spectra we conclude that vinyl alcohol is mainly ionized by direct ionization in the energy range of 9-9.6 eV, whereas autoionization is responsible for the steady rise in the photoionization spectrum above the end of the Franck-Condon envelope at 9.9 eV.

Near-Threshold Measurement of the Photoionization Cross-Section of the Lithium 2P3/2 State in a Magneto-Optical Trap

Near-Threshold Measurement of the Photoionization Cross-Section of the Lithium 2P3/2 State in a Magneto-Optical Trap

Molecular Orbital Insight into the Near-Threshold Photoionization Cross Sections of Monocyclic Substituted Aromatic Compounds

Molecular Orbital Insight into the Near-Threshold Photoionization Cross Sections of Monocyclic Substituted Aromatic Compounds

Photoionization cross section ratio of nitrogen-site carbon in GaN under sub-bandgap-light irradiation determined by isothermal capacitance transient spectroscopy

The ratio of the photoionization cross sections () of carbon substituting at the nitrogen site [CN (0/−)] in n-type GaN, which is detected as a hole trap H1 ( + 0.85 eV) under sub-bandgap-light irradiation (390 nm), is determined with isothermal capacitance transient spectroscopy (ICTS). The current-injection ICTS and the sub-bandgap-light-excited ICTS were compared for the same p+-n junction diode, whereby the hole occupancy ratio () was obtained. Analysis of the dependence of on the temperature gave of 3.0 was then used to estimate the charge state of CN (0/−) under sub-bandgap-light irradiation.

Measurement of the photoionization cross section of the lithium 2P 3/2 state in a magneto-optical trap with a UV-light-emitting diode

We report the measurement of the photoionization (PI) cross section of the lithium 2P 3/2 state in a magneto-optical trap (MOT) using a UV-light-emitting diode (LED). Losses from the lithium MOT induced by additional radiation ionizing the atoms in the 2P 3/2 state have been measured as a function of the intensity of the LED. The experiments have been conducted using a fiber-coupled LED with a flat-top output intensity distribution after the fiber. This makes it possible for us to reduce influences of spatial misalignment between the photoionizing radiation and the MOT density distribution. The PI cross section of lithium in the 2P 3/2 state at 345.5 nm is determined to be (20.1 ± 0.9) Mb. The applied technique allows us to determine the PI cross section with three times better accuracy in comparison with previous measurements. Our result is in good agreement with previously reported experimental results and it differs from the published theoretical data.

Magneto-optical properties of impurity associated photoionization cross-section in laser-driven delta-doped quantum wires

Within the structure of parabolic band and effective-mass approximations, the effects of magnetic and intense terahertz laser fields on the donor impurity-associated photoionization cross-section and nonlinear optical properties in delta-doped quantum well wires are studied using the variational method. The binding energy and photoionization cross-section as functions of doping concentrations, magnetic and terahertz laser fields are computed. The linear, third-order nonlinear and the corresponding total absorption coefficients and relative refractive index changes with a function of incoming photon energy for various doping concentrations, magnitude of terahertz laser and magnetic fields are investigated. The numerical results bring out that the donor impurity binding energy, photoionization cross-section, and nonlinear optical properties in the structure are strongly dependent on doping concentrations. In addition, it has been observed that the magnetic and intense terahertz laser fields cause considerable changes in the binding energy, photoionization cross section and nonlinear spectral properties of the system.

Interference effects on intense XUV pulse induced resonant transitions between core ionized states of helium

In the context of core-resonant ionization of helium atom the inclusion of the He–He+(2p) ionization channel sets off a quantum mechanical interference between two competing pathways, He–He+(1s)–He+(2p) and He–He+(2p) (He–He+(1s) and He–He+(2p)–He+(1s)), thereby significantly modifying the line shape of the photoelectron spectrum. The signature of this interference arises as an asymmetry in the line shape of the multi-peak photoelectron spectra. On account of the discrepancies over the magnitude of the two photon photoionization cross section for the He–He+(2p) ionization channel the calculations have been performed for three different magnitudes of this photoionization cross section and the findings confirm that asymmetry in the line shape is prevalent for all the chosen cross sections. However, the magnitude of asymmetry depends on the magnitude of the photoionization cross section for the He–He+(2p) ionization channel. It is also interesting to note that at the considered laser peak intensities in this work an additional ionization channel from excited ionic state of helium plays a significant role in influencing the line shape of the photoelectron spectrum. The effects of pulse duration on the line shape of the photoelectron spectrum and the possibility to control the interference induced asymmetry in the multi peak line shape has also been investigated.

Electron-Impact Excitation of Ge iii and Photoionisation of Ge ii

Abstract Energy levels, Einstein A-values and oscillator strengths are calculated for doubly-ionised germanium (Ge iii). We have undertaken a study of the electron-impact excitation of Ge iii and have also completed a study of the photoionisation of Ge ii. Ge iii target structures were generated using the relativistic GRASP0 package. Several scattering models were investigated to assess the uncertainty quantification of our results. Initially, the lowest lying 64 jj fine-structure levels were included in the close-coupling expansion resulting from 14 non-relativistic configurations. A second larger model was subsequently generated using the GASP package and incorporated the lowest 589 jj fine-structure levels from 17 non-relativistic configurations. Both the DARC and Breit-Pauli parallel suite of R-matrix collision codes were utilised in the scattering calculations to generate the collision strengths for incident electron-energies between 0 and 5 Rydbergs and subsequently the Maxwellian averaged effective collision strengths for temperatures in the range 1000 to 200000 Kelvin. The photoionisation calculations comprised of two studies, a DARC and a Breit-Pauli calculation incorporating the 250 lowest lying levels. Photoionisation cross sections for Ge ii are presented in the range 0-5 Ryds. Due to the lack of data available in the literature, the present radiative data and electron-impact/photoionisation cross sections are compared between the models presented in this paper. To assess their significance for astrophysics, the data are incorporated into the stellar atmosphere package sterne. Test models for the heavy-metal subdwarf LSIV−14○116 are not affected by the new data, but models for cooler stars may be sensitive to them.

Confinement enhanced spin–orbit interchannel coupling effect on the atomic photoionization

The photoionization cross-section of an atom trapped inside a fullerene anion in the near-threshold region is dominated by the Coulomb confinement resonances. These prominent structures facilitate the possibility of an enhanced spin–orbit interaction activated interchannel coupling (SOIAIC) leading to significant modifications of the photoionization parameters even for low-Z confined atomic systems. The existence of such confinement enhanced SOIAIC structures are demonstrated for the relativistically split 2p ionization channels of Ar trapped inside fullerene anion. Dramatic modifications are observed not only in the cross-sections but also in the angular distributions, and relative phases between the degenerate channels present. Similar structures are found for the case of confined Kr as well. A detailed analysis of these interesting features is carried out which showed the importance of correlation and relativistic effects on the photoionization dynamics of confined atomic systems.

A Fully Relativistic Approach to Photon Scattering and Photoionization of Alkali Atoms

A fully relativistic approach to calculating photoionization and photon-atom scattering cross sections for quasi one-electron atoms is presented. An extensive set of photoionization cross sections have been calculated for alkali atoms: lithium, sodium, potassium, rubidium and cesium. The importance of relativistic effects and core polarization on the depth and position of the Cooper minimum in the photoionization cross section is investigated. Good agreement was found with previous Dirac-based B-spline R-matrix calculations of Zatsarinny and Tayal and recent experimental results.

Combined experimental and theoretical study on photoionization cross sections of benzonitrile and o/m/p-cyanotoluene

Photoionization cross sections (PICSs) for the products of the reaction from CN with toluene, including benzonitrile and o/m/p-cyanotoluene, were obtained at photon energies ranging from ionization thresholds to 14eV by tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Theoretical calculations based on the frozen-core Hartree-Fock approximation and Franck-Condon simulations were carried out to cross-verify the measured PICS. The results show that the photoionization cross sections of benzonitrile and cyanotoluene isomers are similar. The generalized charge decomposition analysis was used to investigate the components of the highest occupied molecular orbital (HOMO) and HOMO-1. It was found that the HOMO and HOMO-1 of benzonitrile and cyanotoluene isomers are dominated by the features of the benzene ring, indicating that the substitution of CN and methyl has a minor influence on the PICS of the studied molecules. The reported PICS on benzonitrile and cyanotoluene isomers in the present work could contribute to the near-threshold PIMS experiments and determine the ionization and dissociation rates in interstellar space for these crucial species. The theoretical analysis on characteristics of molecular orbitals provides clues to estimating the PICS of similar substituted aromatic compounds.

Experimental production cross sections for synchrotron radiation induced L‐series X‐rays of Sn and Sb at energies across their Li (i = 1–3) absorption edges

We have measured the X‐ray production cross sections (XRPCS) for the Lp (p = l, α, η, β1,3,4,6, β2,15, γ1,5, γ2,3,4) emission lines of Sn and Sb using synchrotron induced mono‐chromatic radiation tuned across their respective Li (i = 1–3) sub‐shell absorption edges covering the range 4.0 keV–5.0 keV. These measured XRPCS have been compared with the corresponding values calculated using the non‐relativistic Hartree‐Fock‐Slater model based Li (i = 1–3) photoionization cross‐sections, the Dirac‐Fock model based X‐ray emission rates and three sets of fluorescence and Coster‐Kronig (CK) yields based on the Dirac‐Hartree‐Slater model, the semi‐empirical values tabulated by Krause and recently reported experimental values. The present measured Lp XRPCS for both Sn and Sb at different investigated photon energies are found to be in good agreement with those calculated using the experimental photoionization cross sections, fluorescence and CK yields. Further, a good agreement observed between the present measured and calculated branching ratios for both Sn and Sb indicates the reliability of the Dirac‐Fock model based X‐ray emission rates. The Lp XRPCS and intensity ratios for Sn and Sb calculated using the experimental photoionization cross sections, the DF model based X‐ray emission rates and the experimental fluorescence and CK yields are recommended for use in different applications.

Experimental L-series x ray production cross sections for Re by tuning synchrotron radiation across its Li (i=1–3) sub-shell ionization thresholds

In the present work, the Lk (k = l, α, η, β1, β3, β6, β4, β2,15, β5,7, β9,10, γ1,5, γ2,3,6,8, γ4) x ray production (XRP) cross sections have been measured for Re at energies across its Li (i = 1–3) sub-shell ionization threshold energies covering the region 10.5 keV–14.0 keV using synchrotron radiation. The measured values have been compared with the three sets of XRP cross sections calculated using the Dirac-Fock model based x ray emission rates, the non-relativistic Hartree-Fock-Slater model based Li (i = 1–3) sub-shell photoionization cross-sections and three sets of the fluorescence (ωi) and Coster-Kronig (fij) yields based on the Dirac-Hartree-Slater model based values, the semi-empirical values and the experimental values reported earlier by us. The present measured XRP cross sections are found to be in a good agreement with those calculated using the experimental ωi/fij values; whereas these are found to be significantly lower (~30%) than those calculated using the theoretical and the semi-empirical ωi/fij values. In view of the observed good agreement of the present measured Lk XRP cross sections with those calculated using the experimental fluorescence and Coster-Kronig yields and that of the measured branching ratios (ILl/ILα,; ILβ2,15/ILα,; ILη/ILβ1; ILγ1,5/ILβ1 and ILγ2,3/ILβ3) with the theoretical values calculated using the Dirac-Fock model based x ray emission rates, the use of these parameters is recommended for different analytical applications.

Effects of plasma screening on radiative transition and photoionization of Si[formula omitted]–Si[formula omitted] in a dense plasma environment

The screening induced by a dense plasma environment is crucially important for determining radiative atomic data including bound–bound radiative decay rates and photoionization cross sections, which are vital for obtaining accurate opacity. Previous experimental investigations of ionization potential depression have shown that the widely used analytical models of screening struggle to capture the essential physics in the solid-density plasma. In the present work, we investigate how plasma screening affects the radiative atomic data of Si10+–Si13+ embedded in a dense plasma environment using a self-consistent field finite temperature ion sphere model. The single-electron effective potential of an embedded atom has a short-range feature that determines the available number of bound states. Plasma screening is effective in modifying the matrix elements of the radiative dipole transitions, thereby generally increasing the photoionization cross section near the ionization threshold and decreasing the bound–bound radiative decay rates. The enhancement of the photoionization processes stems from lowering the ionization threshold and the shape and virtual-state resonances, where the upper bound levels become delocalized after a certain critical screening strength. With increasing plasma density, the Rydberg series for the bound–bound radiative transitions disappear successively.

Relativistic effective charge model of a multi-electron atom

A relativistic version of the effective charge model (ECM) for computation of observable characteristics of multi-electron atoms and ions is developed. A complete and orthogonal Dirac hydrogen basis set, depending on one parameter—effective nuclear charge Z*—identical for all single-electron wave functions of a given atom or ion, is employed for the construction of the secondary-quantized representation. The effective charge is uniquely determined by the charge of the nucleus and a set of occupied single-electron orbitals for a given state. We thoroughly study the accuracy of the leading-order approximation for the total binding energy and demonstrate that it is independent of the number of electrons of a multi-electron atom. In addition, it is shown that the fully analytical leading-order approximation is especially suited for the description of highly charged ions since our wave functions are almost coincident with the Dirac–Hartree–Fock ones for the complete spectrum. Finally, we evaluate various atomic characteristics, such as scattering factors and photoionization cross-sections, and thus envisage that the ECM can replace other models of comparable complexity, such as the Thomas–Fermi–Dirac model for all applications where it is still utilized.

Photoionization of nS and nD Rydberg atoms of Rb and Cs from the near-infrared to the ultraviolet spectral region

We present calculations of the photoionization (PI) cross sections of rubidium and cesium Rydberg atoms for light with wavelengths ranging from the infrared to the ultraviolet, using model potentials from Marinescu et al (1994 Phys. Rev. A 49 982). The origins of pronounced PI minima are identified by investigating the free-electron wavefunctions. These include broad PI minima in the nS to ϵP PI channels of both Rb and Cs, with free-electron energy ϵ, which are identified as Cooper minima. Much narrower PI minima in the nD to ϵF channels are due to shape resonances of the free-electron states. We describe possible experimental procedures for measuring the PI minima, and we discuss their implications in fundamental atomic physics as well as in practical applications. Measurements of PI cross sections of Rydberg atoms may serve as a sensitive probe for many-electron interactions of the Rydberg electron in the atomic core region.

Photoionization of Rydberg atoms in optical lattices

We develop a formalism for photoionization (PI) and potential energy curves (PECs) of Rydberg atoms in ponderomotive optical lattices and apply it to examples covering several regimes of the optical-lattice depth. The effect of lattice-induced PI on Rydberg-atom lifetime ranges from noticeable to highly dominant when compared with natural decay. The PI behavior is governed by the generally rapid decrease of the PI cross sections as a function of angular-momentum (ℓ), lattice-induced ℓ-mixing across the optical-lattice PECs, and interference of PI transition amplitudes from the lattice-mixed into free-electron states. In GHz-deep lattices, ℓ-mixing leads to a rich PEC structure, and the significant low-ℓ PI cross sections are distributed over many lattice-mixed Rydberg states. In lattices less than several tens-of-MHz deep, atoms on low-ℓ PECs are essentially ℓ-mixing-free and maintain large PI rates, while atoms on high-ℓ PECs trend towards being PI-free. Characterization of PI in GHz-deep Rydberg-atom lattices may be beneficial for optical control and quantum-state manipulation of Rydberg atoms, while data on PI in shallower lattices are potentially useful in high-precision spectroscopy and quantum-computing applications of lattice-confined Rydberg atoms.