Cross Sections For Transitions Research Articles

Electronic Structures of Small Stoichiometric ZnxOx Clusters.

Stoichiometric ZnxOx clusters in the subnanometer size regime have been the topic of several computational and mass spectrometry studies that showed the particular stability of the stoichiometric species relative to nonstoichiometric species (ZnxOy, x ≠ y). In the current study, we present the angle-resolved anion photoelectron (PE) spectra of stoichiometric ZnxOx-clusters (2 ≤ x ≤ 5), which are interpreted with supporting computational studies that include natural orbital ionization calculations on detachment transition cross sections. All spectra show evidence of Dxh ring structures, which had been predicted to be the most stable structures in previous computational studies. However, a new lowest energy isomer is reported for the Zn2O2 anion and neutral, a bent chain, which is readily reconciled with the most intense feature in the Zn2O2- PE spectrum. The computed PE angular distributions (PADs) associated with the lowest energy cluster structures identified computationally agree with the experimental results, with the exception of Zn5O5-, the experimental PAD of which suggests that strong vibronic coupling may be introducing anomalies. While the lowest lying electronic state of the Zn2O2 chain structure is a triplet state, all neutral ring structures (including Zn2O2, the anion of which also populates the ion beam), favor a singlet electronic state. The computed singlet-triplet splitting of the Dxh structures increases monotonically with x. Overall, we find that the properties of the ring structures evolve smoothly, rather than in the punctuated manner typically seen in the small cluster size regime.

Study on temperature-sensitive factors relevant to thermographic features of Tb3+-doped phosphate glasses

Intensities of the 5D4 luminescence with the 5D3 level populated by GSA and ESA transitions of Tb3+ in ternary phosphate glass were measured at different temperature between 300 K and 675 K. Acquired 5D4 luminescence intensities related to GSA and ESA excitation transitions from thermally coupled 7F6 and 7F5 levels were treated to assess luminescence intensity ratios (LIR) as a function of temperature and resulting values of relative sensitivity SR. To understand origin of marked dissimilarity of SR values observed in some few Tb3+-doped hosts reported thus far an in-depth analysis of the excitation energy flow in systems under study is made. Spectra of absorption cross section and temperature dependent absorption coefficient for ESA transition were evaluated. Rates of temperature dependent cross relaxation process governing energy transfer between 5D3 and 5D4 were calculated. It was supposed that temperature-dependent absorption of colour centres in the blue region may affect adversely relative thermal sensitivity.

Isotopic effects on the rotational (de-)excitation rate coefficients of ortho-CH3Cl colliding with He

Chloromethane, CH3Cl, is the first organohalogen molecule to be detected in the interstellar medium. Using the recently generated accurate three-dimensional potential energy surface (3D-PES) for the weakly bound CH335Cl–He complex, we deduced that of CH337Cl–He. Both 3D-PESs were incorporated later into dynamical computations code for nuclear motions treatments of each isotopologue colliding with He. We determined the state-to-state (de-)excitation cross sections for transitions among the 105 lowest rotational levels of both ortho-CH335Cl and ortho-CH337Cl (up to jkɛ = 206±). For total energies up to 100 cm−1, we used the quantum close-coupling methodology, and we adopted the coupled state approach for higher energies (up to 2500 cm−1). Rate coefficients for temperatures ranging from 5 to 150 K are then deduced after averaging these cross sections over a Maxwell–Boltzmann velocity distribution. Computations show that isotopic effects are weak, validating the use of collision rates of ortho-CH335Cl for ortho-CH337Cl and vice-versa. However, we expect that our results will be helpful to accurately determine the abundances of these isotopologues in the interstellar medium and, more generally, for understanding the complex and still not fully elucidated chemistry of chlorine containing molecular species there.

Semiclassical analytic theory of electronic energy transfer in 3D atomic collisions.

A previously developed semiclassical theory of nonadiabatic energy transfer is used to analyze electronic excitation and quenching in three-dimensional atomic collisions. The predicted transition probabilities, cross sections, and rate coefficients are compared with the quantum scattering calculations for O + O and N + N, for the same interaction potentials and nonadiabatic coupling, and with the experimental data where available. The theory predictions are in very good agreement with quantum scattering, at the conditions when the energy transfer is dominated by a single pair of adiabatic potentials. Closed-form analytic expressions for the cross sections and rate coefficients are obtained, for both the strongly and weakly coupled cases. The results quantify and illustrate the effect of the interaction potentials and their coupling on the energy transfer. The analytic cross sections and rate coefficients are in good agreement with the numerical predictions. The same approach has been used to predict the rate coefficients of electronic excitation and quenching in collisions of N + O atoms. The fidelity of these predictions may be improved considerably if accurate potentials for the excited electronic states of N + O and their coupling are available. The applicability of the semiclassical theory for the prediction of the rates of heavy particle impact excitation in atom-molecule collisions is discussed.

Oscillator strengths, intracenter absorption and photoionization cross sections of optical transitions of shallow donors in silicon

Oscillator strengths, intracenter absorption and photoionization cross sections of optical transitions of shallow donors in silicon

Mixed quantum/classical calculations of rotationally inelastic scattering in the CO + CO system: a comparison with fully quantum results.

An updated version of the CO + CO potential energy surface from [R. Dawes, X. G. Wang and T. Carrington, J. Phys. Chem. A 2013, 117, 7612] is presented, that incorporates an improved treatment of the asymptotic behavior. It is found that this new surface is only slightly different from the other popular PES available for this system in the literature [G. W. M. Vissers, P. E. S. Wormer and A. Van Der Avoird, Phys. Chem. Chem. Phys. 2003, 5, 4767]. The differences are quantified by expanding both surfaces over a set of analytic functions and comparing the behavior of expansion coefficients along the molecule-molecule distance R. It is shown that all expansion coefficients behave similarly, except in the very high energy range at small R where the PES is repulsive. That difference has no effect on low collision-energy dynamics, which is explored via inelastic scattering calculations carried out using the MQCT program which implements the mixed quantum/classical theory for molecular energy exchange processes. The validity of MQCT predictions of state-to-state transition cross sections for CO + CO is also tested by comparison against full-quantum coupled-states calculations. In all cases MQCT gives reliable results, except at very low collision energy where the full-quantum calculations predict strong oscillations of state-to-state transition cross sections due to resonances. For strong transitions with large cross sections, the results of MQCT are reliable, especially at higher collision energy. For weaker transitions, and lower collision energies, the cross sections predicted by MQCT may be up to a factor of 2-3 different from those obtained by full-quantum calculations.

Fine-structure Transitions of Ne+, Ar+, Ne2+, and Ar2+ Induced by Collisions with Atomic Hydrogen

We calculate cross sections for fine-structure transitions of Ne+, Ar+, Ne2+, and Ar2+ in collisions with atomic hydrogen by using quantum-mechanical methods. Relaxation rate coefficients are calculated for temperatures up to 10,000 K. The temperature-dependent critical densities for the relaxation of Ne+, Ar+, Ne2+, and Ar2+ in collisions with H have been determined and compared to the critical densities for collisions with electrons. The present calculations will be useful for studies utilizing the infrared lines [Ne ii] 12.8, [Ne iii] 15.6, [Ne iii] 36.0, [Ar ii] 6.99, [Ar iii] 8.99, and [Ar iii] 21.8 μm as diagnostics of, for example, planetary nebulae and star formation.

Photodissociation cross sections and rates of NaO

ABSTRACT Photodissociation of NaO may be important for the sodium chemistry in various astrophysical regions. This work produces the photodissociation cross sections and rates of NaO over the temperature range from 0 to 15 000 K. First, the state-resolved cross sections for transitions from the ground and first excited states of NaO are investigated using ab initio potential energy curves and transition dipole moments. The temperature-dependent cross sections were then obtained by assuming a Boltzmann distribution to describe the population of the initial state. Detailed comparisons with experimental cross sections at 200 and 300 K reveal that the X 2Π → 1 2∆ and X 2Π → 2 2Σ− transitions may be the main photodissociation pathways for NaO in the wavelengths of about 2400–2580 Å, while the X 2Π → B 2Σ− transition may play a dominant role in the wavelengths of about 3534–4230 Å. Finally, photodissociation rates in the interstellar, solar, and blackbody radiation fields were determined. In the interstellar and solar radiation fields, the X 2Π → B 2Σ− transition dominates at low temperatures and the A 2Σ+ → 2 2Σ+ transition dominates at high temperatures. The total photodissociation rates in ultraviolet-rich and visible-rich radiation fields are almost insensitive to the temperature. The photodissociation cross sections and rates of NaO should be useful for investigating the chemical evolution of the sodium element in planetary exospheres, atmospheres of cool stars, and envelopes of evolved stars.

Electron vortices generation of photoelectron of H2+ by counter-rotating circularly polarized attosecond pulses

Molecular-frame photoelectron momentum distributions (MF-PMDs) of an H2+ molecule ion in the presence of a pair of counter-rotating circularly polarized attosecond extreme ultraviolet laser pulses is studied by numerically solving the two-dimensional time-dependent Schrödinger equation within the frozen-nuclei approximation. At small time delay, our simulations show that the electron vortex structure is sensitive to the time delay and relative phase between the counter-rotating pulses when they are partially overlapped. By adjusting time delay and relative phase, we have the ability to manipulate the MF-PMDs and the appearance of spiral arms. We further show that the internuclear distance can affect the spiral vortices due to its different transition cross sections in the parallel and perpendicular geometries. The lowest-order perturbation theory is employed to interpret these phenomena qualitatively. It is concluded that the internuclear distance-dependent transition cross sections and the confinement effect in diatomic molecules are responsible for the variation of vortex structures in the MF-PMDs.

MQCT 2024: A program for calculations of inelastic scattering of two molecules (new version announcement)

This is a revised and updated version of the package MQCT. The package includes routines for the calculations of ro-vibrational state-to-state transition cross sections for molecule + molecule collisions, using the mixed quantum/classical theory approach. One important addition to the package is an efficient and accurate adiabatic-trajectory approximation method (AT-MQCT), which enables calculations for larger and more complex molecules. An efficient parallel IO is implemented for state-to-state transition matrix, which now permits to handle very large matrixes, but also helps to truncate these matrixes by neglecting insignificant matrix elements and to balance the load on different CPUs during the matrix calculations and during the trajectory propagation. A stable recursive approach is added for computation of Wigner 3j-symbols, which expands the range of rotational states of molecules up to j∼ 100. Examples of input files are provided for all ten system types covered by the package, along with updated user manual. New version program summaryProgram title: MQCT 2024CPC Library link to program files:https://doi.org/10.17632/sg36r35njz.2Developer's repository link:https://github.com/MarquetteQuantum/MQCTCode Ocean capsule:https://codeocean.com/capsule/1260072/tree/v1Licensing provisions: GNU GPL v3.0Programming language: FORTRANJournal reference of previous version: Comput. Phys. Commun. 252 (2020) 107155Does the new version supersede the previous version?: YesReasons for the new version:1)Recently, we developed and tested the adiabatic-trajectory method [1–3] within MQCT framework, which appears to be both efficient and accurate, and is likely to become a “workhorse” for inelastic scattering calculations in molecule + molecule systems. In the new version of the package this method is made available as an option, AT-MQCT.2)Since MQCT trajectory calculations became more affordable with AT-MQCT, we started doing calculations with larger and larger rotational basis sets and quickly encountered several new problems. The major one was an extremely slow serial IO of the state-to-state transition matrix to a single file, which is now replaced by a parallel IO to multiple files written to a separate directory. Very large matrixes (up to 1010 non-zero matrix elements) are now handled efficiently.3)Another problem was an incorrect calculation of Wigner 3j-symbols for the rotational states around j∼ 40 and above. In the revised version a new stable subroutine is employed that uses a recursive method and works properly at least up to j=100.4)Several “bugs” were caught and fixed, with one major in the Monte-Carlo sampling of MQCT trajectories. Now users can add a batch of new trajectories to the previously sampled batch. In addition, users can monitor the convergence of Monte-Carlo sampling in detail. These options work for all levels of theory, including AT-MQCT. Several errors were found in the equations for calculations of state-to-state transition matrix using the expansion of potential energy surface, and in symmetrization of matrix elements for the case of identical collision partners. All these have been corrected.Summary of revisions: Several technical revisions concern the treatment of diatomic molecules in the code: A bug was found and fixed in the PES interface that converts the units of vibrational coordinate (the diatomic bond distance). Now the PES can be expressed in either Angstrom or Bohr, as specified in the input file. Integration of matrix elements over the vibrational coordinate is modified to use an equidistant grid by default (Gauss-Legendre method is still available as option). And the same for integration over Euler angles α and γ (for all system types). Integration over β uses Gauss-Legendre quadrature only. The format of ro-vibrational wavefunctions pre-computed externally and supplied as input is made more intuitive, as described in the manual. Finally, a new method for the sampling of orbital quantum number ℓ is implemented, which skips some values of ℓ and obtains results by interpolation of the data at the retained values of ℓ (for all system types, as described in the manual). Last version of the code is always available at https://github.com/MarquetteQuantum/MQCT.Supplementary material: Updated and expanded MQCT user guideNature of problem: Calculations of rotationally and vibrationally inelastic scattering of two molecules, with possible applications in astrophysics and atmospheric chemistrySolution method: Mixed quantum/classical theory (MQCT) approach References[1]B. Mandal, A. Semenov and D. Babikov, J. Phys. Chem. A 124 (2020) 9877-9888[2]B. Mandal, C. Joy, A. Semenov and D. Babikov, ACS Earth Space Chem. 6 (2022) 521-529[3]B. Mandal, C. Joy, D. Bostan, A. Eng and D. Babikov, J. Phys. Chem. Lett. 14 (2023) 817–824.

Plasma-relevant fast electron impact study of difluoromethane

Difluoromethane () is a commonly used etching gas in manufacturing very-large-scale integrated circuits as the fluorine source. Electron impact excitation cross sections for are important input data for modeling the reactive etching plasmas. In this work, we present the experimental generalized oscillator strengths for the , , and transitions on the absolute scale. The measurement was realized by employing the crossed-beam based relative flow technique at an angle-resolved electron energy loss spectrometer, operating at an incident energy of 1500 eV and a resolution of 80 meV. The dipole-forbidden transition at around 9.7 eV is identified for the first time. The measured data are fitted with the well-known Lassettre formula to yield analytical expressions and optical oscillator strengths at for dipole-allowed transitions. The analytical expressions enable integrations over momentum transfer to obtain the Born integral cross sections for electron impact excitations from the threshold to 5000 eV. The integral cross sections for dipole-allowed transitions are scaled to a more reliable level by using the BE-scaling method. The experimental data can benchmark theoretical calculations and supplement the molecular database for plasma modelers.

Microscopic calculation of the β− decays of 151Sm, 171Tm, and 210Pb with implications to detection of the cosmic neutrino background

The electron spectral shapes corresponding to the low-Qβ−-decay transitions Sm151(5/2g.s.−)→Eu151(5/2g.s.+), Sm151(5/2g.s.−)→Eu151(7/21+), Tm171(1/2g.s.+)→Yb171(1/2g.s.−), Tm171(1/2g.s.+)→Yb171(3/21−), Pb210(0g.s.+)→Bi210(1g.s.−), and Pb210(0g.s.+)→Bi210(01−) have been computed using beta-decay theory with several refinements for these first-forbidden nonunique (ff-nu) β− transitions. These ff-nu β− transitions have non-trivial electron spectral shapes with transition nuclear matrix elements (NMEs) computed by using the microscopic Interacting Boson-Fermion Model (IBFM-2) for the decays of 151Sm and 171Tm, and the nuclear shell model (NSM) for the decay of 210Pb. Within the respective Q windows, the computed ff-nu electron spectral shapes deviate maximally at sub-percent level from the universal allowed shape, except for the transition Pb210(0g.s.+)→Bi210(1g.s.−), where the maximal deviation is some 2.7%. This confirms that the so-called ξ approximation is fairly good for most of these low-Qβ− transitions and thus the allowed shape is a rather good first approximation. Our computed spectral shapes could be of interest for experiments aiming to measure the cosmic neutrino background (CνB), like the PTOLEMY experiment. We have also derived CνB cross sections for the ground-state transitions of the considered nuclei at the β endpoint. Our findings indicate that more work on the atomic mismatch correction is needed in the future in order to extract reliable and precise CνB cross sections for any nuclear target.

On the Polarization Dependence of Two-Photon Processes including X-ray Scattering

The polarization dependence of the cross sections of two-photon transitions including X-ray scattering was analyzed. We developed the regular approach to the derivation of the polarization parameters of photoprocesses. Our approach is based on the tensor representation of the photon density matrix, which is written in terms of the unit vectors directed along the major axis of the polarization ellipse (ϵˆ) and the photon propagation (kˆ). Explicit expressions for the product of two photon density matrices were derived. As an example, the parametrization of the polarization dependence of the X-ray scattering by closed-shell atoms is given both in terms of (i) scalar products of photon vectors ϵˆ1,2, kˆ1,2 and (ii) X-ray Stokes parameters. We discuss the applicability of the atomic scattering for the measurement of the polarization of X-rays.

ARSENY: A program for computing inelastic transitions via hidden crossings in one-electron atomic ion–ion collisions with classical description of nuclear motion

The ARSENY program is intended to compute cross-sections of inelastic transitions via hidden crossings upon slow one-electron atomic ion–ion collisions using the approximation of classical description of nuclear motion in the collision energy region E<20 KeV/nucleon. Particular attention is paid to the calculation of the analytical properties of adiabatic potential energy curves of the two-center Coulomb problem for complex values of the internuclear distance. Complex branch points and hidden crossings are calculated and their significance for the dynamics of one-electron collisional systems is revealed. Benchmark calculations of cross sections for charge exchange, excitation, and ionization in slow He2++H(1s) collisions are presented and compared with experimental data. Program summaryProgram Title: ARSENYCPC Library link to program files:https://doi.org/10.17632/n43srxwdnm.1Licensing provisions: CC By 4.0Programming language: FORTRAN 90/95. Compilers: Intel(R) Visual Fortran Compiler 19.0.4.245 [IA-32]Nature of problem: The processes of charge exchange, excitation, and ionization in one-electron atomic ion–ion collisions in a low-temperature plasma are actively studied in astrophysics and tokamak Charge-eXchange Recombination Spectroscopy (CXRS) Edge diagnostics in ITER [1–7]. Numerical calculations of cross sections for charge exchange, excitation, and ionization for slow one-electron atomic ion–ion collisions within the approximation of classical description of nuclear motion have been conventionally implemented, in particular, by the electron-nuclear dynamics approach [8] and the close-coupled channel method using basis functions of the two-center Coulomb problem (see, e.g., [9–13]). However, with real-valued internuclear distance these approaches did not allow for the dynamical structure of discrete-discrete and discrete-continuous transitions in the two-center Coulomb problem. This disadvantage manifests itself in the disagreement between the theoretical and experimental cross-sections at small velocity of ions [13–15]. In a number of papers [1,14,16–20] the dynamical structure of the transitions is considered in terms of hidden crossing of potential curves, when using the analytic continuation to a complex plane of the internuclear distance. This approach is implemented in the ARSENY program aimed at computing cross sections of inelastic discrete-discrete and discrete-continuous transitions via hidden crossings for slow one-electron atomic ion–ion collisions within the approximation of classical description of nuclear motion announced in [1]. Benchmark calculations of cross sections for charge exchange, excitation, and ionization in slow He2++H(1s) collisions are presented and compared with experimental data.Solution method: The potential energy curves E=E(R) and the separation constant λ=λ(R) depending on the real-valued and complex-valued parameter R of the two-center Coulomb problem are calculated from the minimization condition for a quadratic functional corresponding to a pair of nonlinear equations with respect to a pair of unknowns E(R) and λ(R) by iterating three-term recurrent relations [21,22] using a mean least square method [23]. These equations are obtained from the known recurrent relations for the expansion coefficients of quasiradial and quasiangular spheroidal Coulomb functions. The series of branching points Rc sought for in the complex plane of internuclear distance R and the hidden crossings of complex potential energy curves E(R) are calculated by an iterative method using the known analytical behavior in the form of a square root of difference E(R)−E(Rc) in the vicinity of the unknown branch points Rc. Calculations of cross-sections of inelastic transitions via hidden crossings for the slow one-electron atomic ion–ion collisions within the approximation of classical description of nuclear motion are carried out by integration over the impact parameter of the nonadiabatic transition probability expressed in terms of the Stückelberg parameter determined integral of difference of imaginary parts of each pair of complex potential energy curves E(R) by a contour around the branching point Rc [18,19].Additional comments including restrictions and unusual features: In calculations of cross sections and required branch points, about 220 adiabatic states of the lower part of the discrete spectrum of the two-center problem are used.

Mixed quantum/classical theory for rotational energy exchange in symmetric-top-rotor + linear-rotor collisions and a case study of the ND3 + D2 system.

The extension of mixed quantum/classical theory (MQCT) to describe collisional energy transfer is developed for a symmetric-top-rotor + linear-rotor system and is applied to ND3 + D2. State-to-state transition cross sections are computed in a broad energy range for all possible processes: when both ND3 and D2 molecules are excited or both are quenched, when one is excited while the other is quenched and vice versa, when the ND3 state changes its parity while D2 is excited or quenched, and when ND3 is excited or quenched while D2 remains in the same state, ground or excited. In all these processes the results of MQCT are found to approximately satisfy the principle of microscopic reversibility. For a set of sixteen state-to-state transitions available from the literature for a collision energy of 800 cm-1 the values of cross sections predicted by MQCT are within 8% of accurate full-quantum results. A useful time-dependent insight is obtained by monitoring the evolution of state populations along MQCT trajectories. It is shown that, if before the collision, D2 is in its ground state, the excitation of ND3 rotational states proceeds through a two-step mechanism in which the kinetic energy of molecule-molecule collision is first used to excite D2 and only then is transferred to the excited rotational states of ND3. It is found that both potential coupling and Coriolis coupling play important roles in ND3 + D2 collisions.

Оптическое возбуждение спин-триплетных состояний двухэлектронных доноров в кремнии

In this paper, we propose a method for resonant optical excitation of ortho states of two-electron donors in silicon, direct transitions to which from the ground state are extremely suppressed in case of a weak spin-orbit coupling. Excitation is proposed to be carried out using the points of anti-crossing of ortho and para states under conditions of uniaxial stress of the crystal. In these points the states cannot be unambiguously assigned to any group of states with a certain spin, as a result of which the optical transition becomes allowed. The structure of the energy levels of two-electron impurities is such that the excitation of such state almost unambiguously leads to the population of the underlying ortho-type state, which is expected to be very long-lived in the case of weak spin-orbit coupling. In the present work, theoretical estimates of the cross sections for optical transitions in the vicinity of the level anticrossing point as a function of strain for strong and weak spin-orbit coupling are made.

Collisional S-matrix for the vibrational dynamics of H+H2 by quantum computing

An algorithm and a system of quantum circuits are developed and applied to compute accurately the S matrix for the transitions between vibrational states of H2 for collisions with H. The algorithm was applied to 100 eV laboratory collision energy at a quantum circuit simulator. The effects of the discretized dissociative continuum to the transition cross sections are carefully studied and accuracy and convergence of the results with the chosen parameters of the algorithm and the collision system are verified by comparison with a solution of the time-dependent Schrodinger equation using the classical algorithm as well as comparison with a few results from the literature.

Electron impact excitation cross section calculations of the fine structure transitions of Mo and their applications in the diagnostics of the laser induced Mo plasma

The current work reports a detailed calculation of electron impact excitation cross sections for the fine structure transitions of Mo from the manifolds 4d 55s, 4d 45s 2, 4d 6 and 4d 55p to the manifolds 4d 55s, 4d 45s 2, 4d 6, 4d 55p, 4d 45s5p, 4d 55d, 4d 56s, 4d 45s6s, and 4d 57s, using the relativistic distorted wave approximation for the applications in plasma modelling. Multi-configurational Dirac–Fock wave functions are used in the calculations. The oscillator strength and cross section results are compared with the previous calculations and measurements. A comprehensive collisional radiative (CR) model is developed and used to characterize laser induced molybdenum plasma to ensure that the calculated cross sections can be used for various plasma modelling applications. The current CR model has taken into account the electron impact excitation and de-excitation processes using the calculated consistent cross sections. The electron induced processes are dominant kinetic processes in the laser induced plasma. Furthermore, the diagnostics of the laser induced Mo plasma is done by coupling the current CR model with the experimental laser induced breakdown spectroscopic measurements of Mal et al (2021 Appl. Phys. B 127 52). The plasma parameter, i.e., electron temperature has been calculated using nine measured intensities of the emission lines of Mo, with wavelengths 406.9, 423.3, 438.2, 453.7, 476.0, 550.7, 553.3, 557.0 and 592.9 nm. The results are also compared with the values reported in the Boltzmann plot at various delay times ranging from 0.5 to 5.0 μs.

Electron impact excitation of the Tl + ion: resonance and cascade transitions

Results of experimental and theoretical investigations into the electron-impact excitation of the resonance → 6s 2 1 S 0 (132.2 nm), as well as the cascade 6s7s 1 S 0 → (309.2 nm) and 6p 2 1 D 2 → (150.8 nm) spectral transitions from the ground 6s 2 1 S 0 level in the thallium ion, are presented. Crossed beams of electrons and Tl+ ions in combination with a spectroscopic method were used in the experiment. Measured cross sections have a distinct resonance structure arising mainly from the electron decay of atomic 5d 96s 26p 2, 5d 106s7s(1 S)np (n ⩾ 7), 5d 106s6d(1 D)np (n ⩾ 6), 5d 106s6d(1 D)nd (n ⩾ 6) and ionic 5d 96s 2 np, 5d 96s 2 nd, 5d 96snp (n ⩾ 6) autoionizing levels, as well as radiative transitions from the higher 6s7s 1 S 0, 6s6d 1 D 2, and 6p 2 1 D 2, 1 S 0 ionic levels. Relativistic distorted wave (RDW) calculations are performed to obtain the effective cross sections for the above transitions. The absolute values of the cross sections are found to be 1.39 × 10−16 cm2 at 300 eV for the λ132.2 nm resonance line, and 0.11 × 10−16 cm2 and 0.56 × 10−16 cm2 at 100 eV for the λ309.2 nm and λ150.8 nm lines, respectively. The contribution of the cascade transitions being studied to the effective cross section of the λ132.2 nm resonance line at the energy of 100 eV is about 30%. A calculation using the semi-empirical Van Regemorter formula is also performed to obtain the effective cross section of the resonance λ132.2 nm line. The absolute value of the cross section at 300 eV is found to be nearly the same as that given by the RDW calculation.

Valence-shell electronic excitations of CF4 studied by fast electron scattering

Oscillator strengths and cross sections of the valence-shell excitations in CF4 are of crucial importance in modeling the etching plasma and monitoring the greenhouse effect. In this work, generalized oscillator strengths of the valence-shell excitations in CF4 were measured by using an angle-resolved electron energy loss spectrometer, operating at an incident electron energy of 1.5 keV and a high energy resolution of 80 meV. The experimental GOSs are analytically fitted with the well-known Lassettre formula, and, thus, the optical oscillator strengths are determined at the limit of squared momentum transfer K2=0. The analytical GOS also enables an integration over K2, determining the Born cross sections with respect to the incident electron energy from the excitation threshold to 5000 eV. The BE-scaled integral cross sections for dipole-allowed transitions are obtained by adopting the BE-scaling method. Oscillator strengths and cross sections of the valence-shell excitations in CF4 provide an independent cross-check to the previous experimental and theoretical works. Moreover, the cross-checked data supplement and benchmark the molecular database for use in plasma models.