Cross Sections For Transitions Research Articles

Calculation of electron-impact excitation from metastable states of the Sr atom

Electron-impact excitation of Sr from metastable states is studied in the framework of an elaborate close-coupling approximation, using accurate target wave functions. An interpretation is given of measured cross sections, including their near-threshold resonance structure. The cross sections for transitions induced from the 5s5p{sup 3}P metastable level to the 5s5d{sup 3}D, 5p{sup 23}P, 5s6s{sup 3}S, 4d5p{sup 1}D, 5s4f{sup 3}F, 4d5p{sup 3}F, and 5s6p{sup 3}P levels are calculated in Born-Ochkur and two-state and six-state close-coupling approximations. We also consider transitions from the 5s{sup 21}S ground state to these levels. The calculations are based on a nonrelativistic Hamiltonian. In the configuration-interaction approximation, accurate wave functions are used for the outer electrons, with a frozen core. The results are found to be in good agreement with the most recent experimental data. {copyright} {ital 1997} {ital The American Physical Society}

The40Ca(t,p)42Careaction at triton energies near 10 MeV per nucleon

Two nucleon transfer reactions have been used to probe for correlations in nuclei but there have been doubts about the assumptions made and hence about the success of the analyses. An experimental study of the ${}^{40}\mathrm{Ca}{(t,p)}^{42}\mathrm{Ca}$ reaction at incident triton energies of 10\char21{}15 MeV/nucleon was made to determine if the one-step, two-nucleon transfer mechanism was dominant at these energies. Measurements were made with 28, 33, and 37.3 MeV incident energy and particular care was taken to measure absolute differential cross sections for transitions to several residual states in ${}^{42}\mathrm{Ca}.$ The analysis of all the data used the results of full finite-range calculations and care was taken to calculate all the one-step and two-step reaction channels in a consistent manner starting from a shell-model description of the relevant Ca isotopes with a single, large basis set of single-particle wave functions. Although a complete prediction of the data was not achieved, it is concluded that incident triton energies about 10 MeV/nucleon are still not enough for one-step, two-nucleon transfer to dominate the $(t,p)$ reaction mechanism. Even with a full analysis, it was not possible to reliably extract the reaction amplitudes by fitting the data because of the effects of the discrete potential ambiguity when describing elastic scattering. Hence it is not possible to extract accurate data about two-nucleon correlations in nuclei from $(t,p)$ reactions at these energies. Additionally, some measurements on the ${}^{8}\mathrm{Ca}{(t,p)}^{50}\mathrm{Ca}$ reaction with 37.3 MeV incident energy are presented. Using a similar analysis, calculations predicted the magnitude of the differential cross-section data and the structure of the angular distributions. The ${J}^{\ensuremath{\pi}}$ of the second excited state of ${}^{49}\mathrm{Ca}$ was shown to be ${7/2}^{\ensuremath{-}}.$

Implementation of the sudden approximation eikonal method in ion - diatom collisions

We describe an implementation of the vibro-rotational sudden approximation eikonal (SEIKON) method to treat collisions between ions (atoms) and diatomic molecules at impact energies such that a semiclassical eikonal approximation can be applied to treat the relative motion of the colliding species, and a close-coupling molecular expansion is able to accurately represent electronic transitions. Our method, which does not make use of Franck - Condon-type approximations, allows us to evaluate vibrationally resolved charge-transfer and excitation cross sections, as well as total cross sections for transitions to the vibrational continuum. The implementation is illustrated with calculations of cross sections for single-charge transfer, vibrational excitation, and transfer dissociation in , and DT collisions. The validity of the Franck - Condon approximation for these reactions is studied.

Radiative proton capture to discrete states in 31N at 98 and 176 MeV

A high resolution magnetic pair spectrometer for medium energy photons has been used for studies of the exclusive (p,γ) reaction on 12C at 98 and 176 MeV. We present angular distributions of the differential cross section for transitions to the ground state and the first three excited states of 13N. The distributions from these four states are compared with predictions from a microscopic continuum shell model calculation using a realistic finite-ranged effective interaction with tensor components. In general, reasonable agreement is obtained for reactions to states described by p shell single particle wave functions, but poor agreement is obtained for transitions described by sd shell single particle wave functions. Nevertheless we find, however, that the dominant reaction mechanism is a direct capture process. We also compare the (p,γ) data with existing data on the exclusive (p,π) reaction at 185 MeV revealing notable differences between the two reactions.

On the relation between electronic structure and molecular dynamics. II. Sensitivity of collision induced rotational excitation of H2 by He to the electronic wave function

A formalism for relating dynamic observables of collision processes to the electronic structure of the colliding species is illustrated for the case of He collision-induced rotational excitation of H2. Expressions are presented for the functional derivative of transition cross sections and rate constants with respect to the electronic wave function. Regions of electronic coordinate space in which the electron probability densities have particularly strong influence on one or more inelastic transitions are identified. The possible role of functional derivatives of this sort for guiding electronic structure calculations to produce intermolecular potentials is discussed.

Quantum simulations of energy transfer and state-to-state transitions in collision of an atom with a large anharmonic cluster: He+Ar13

A time-dependent self-consistent field approach is used to simulate a He atom colliding with an Ar13 cluster. Direct energy transfer during the collision, and energy redistribution among the vibrational degrees of freedom of the anharmonic cluster following the collision, are studied. An important advantage of the method used is that quantum state-to-state transition cross sections can be computed for large systems. The following main results are obtained: (1) The process can be interpreted in terms of a direct collision, followed by post-collision energy redistribution in Ar13 , a description that appears only when the cluster vibrations are not described by the eigenstates of this system. A time scale of one picosecond is found for the post-collision intracluster energy distribution. (2) The long-time final state distribution of Ar13 is less state selective than the distribution immediately after the impact, but it is also not completely statistical. (3) There are state-to-state transitions having cross sections of observable magnitude. (4) The dominant transitions are those involving zero, one, and two “phonon” excitations. Some of the “two phonon” excitations have cross sections comparable to strong “single phonon” transitions. (5) Different types of modes show different propensities for excitations in the collision, in close relation to the geometric character of the modes. The results show that the TDSCF approximation is a powerful tool for treating both direct collision dynamics and collision-induced dynamics in scattering of large anharmonic systems.

The polarization-angular structure and elliptical dichroism of the cross sections for three-photon bound-bound transitions in atoms

Using the electric dipole approximation, we present, in invariant form, the cross section of an arbitrary three-photon transition between the discrete states of an atom with total angular momenta Ji and Jf. The cross section contains scalar and mixed products of the photon polarization vectors, and invariant atomic parameters dependent only on the photon frequencies. We determine the number of independent atomic parameters at fixed values of Ji and Jf and obtain their explicit expressions in terms of the reduced composite dipole matrix elements. The polarization dependence of the cross sections is expressed in terms of the degrees l and ξ of linear and circular photon polarizations. We analyze the phenomenon of dissipation-induced circular dichroism in three-photon processes, i.e., the difference Δ of the cross sections for opposite signs of the degree of circular polarization of all the photons. We study in detail the case of two identical photons and the phenomenon of elliptical dichroism, when Δ∼lξ holds and dichroism occurs only when the photons are elliptically polarized, with 0<|ξ|<1. Finally, we discuss the dissipation-induced effects of atom polarization in three-photon processes involving linearly polarized or unpolarized photons.

Luminescence detection of multiphoton ionization-fragmentation of the molecular CrO 4 2− anions adsorbed on the surface of dispersed SiO2

It is shown that luminescence detection of multiphoton ionization-fragmentation of the molecular CrO 4 2− anions adsorbed on the surface of dispersed SiO2 is possible under excitation with the fundamental frequency of a Nd:YAG pulsed laser (λ=1.064 μm). The structure and the process of formation of the adsorbed complexes under thermal activation of the surface and the nature of luminescence transitions in CrO 4 2− anions are studied in detail. It is shown that luminescence is excited as a result of the recombination of photoelectrons and ionized chromate ions. Multiphoton ionization of the ions occurs under three-photon resonance conditions. The resonance level is an antibonding state of the adsorption complex formed with the participation of an oxygen vacancy on the SiO2 surface. The dynamics of the multiphoton luminescence excitation process includes autoionization (stimulated by intercomplex electronic excitation) in superexcited states, fragmentation of chromate anions, and annealing of surface oxygen vacancies. The rate equations for three-photon-resonance multiphoton ionization are studied. The cross sections for two-and one-photon transitions on the nonresonance steps of multiphoton absorption are obtained. It is concluded that the nonlinear polarizability of the donor-acceptor adsorption bond in “chromate anion-oxygen vacancy” complexes is very important.

Exact nonrelativistic expressions for the tensor for scattering of light by atoms

Exact nonrelativistic analytical expressions are derived for dipole two-photon transitions between arbitrary multiplets of the hydrogen atom and positive hydrogenlike ions. The result is expressed in terms of a single Gauss hypergeometric function and polynomials whose degrees increase linearly with the number of nodes of the bound states of the quantum system. The cross sections of elastic scattering of light by K-and L-shells of the hydrogen atom are given as an example. It is demonstrated that by expanding the discrete-spectrum wave functions in ultraspherical polynomials it is also possible to obtain analytical expressions of the cross sections of two-photon transitions between states described by the Simons model potential. The basis consisting of Chebyshev polynomials is shown to be the best expansion basis, and the coefficients of such an expansion are given for a broad range of parameters of the problem. Calculation of the polarizability of the 5S-state of the rubidium atom is chosen as an example. Finally, the results are compared with the experimental data and the theoretical results of other researchers.

High-momentum proton removal from16Oand the(e,e′p)cross section

The cross section for the removal of high-momentum protons from ${}^{16}\mathrm{O}$ is calculated for high missing energies. The admixture of high-momentum nucleons in the ${}^{16}\mathrm{O}$ ground state is obtained by calculating the single-hole spectral function directly in the finite nucleus with the inclusion of short-range and tensor correlations induced by a realistic meson-exchange interaction. The presence of high-momentum nucleons in the transition to final states in ${}^{15}\mathrm{N}$ at 60--100 MeV missing energy is converted to the coincidence cross section for the ${(e,e}^{\ensuremath{'}}p)$ reaction by including the coupling to the electromagnetic probe and the final state interactions of the outgoing proton in the same way as in the standard analysis of the experimental data. Detectable cross sections for the removal of a single proton at these high missing energies are obtained which are considerably larger at higher missing momentum than the corresponding cross sections for the $p$-wave quasihole transitions. Cross sections for these quasihole transitions are compared with the most recent experimental data available.

Stark components of lower-lying manifolds and emission cross sections of principal intermanifold and inter-Stark transitions of trivalent neodymium in strontium fluorovanadate

A detailed characterization of the Stark components of the $^{4}\mathrm{I}_{9\mathrm{/}2}$ and $^{4}\mathrm{I}_{11\mathrm{/}2}$ manifolds have been performed using the room-temperature fluorescence spectra for the $^{4}\mathrm{F}_{3\mathrm{/}2}$${\ensuremath{\rightarrow}}^{4}$${\mathrm{I}}_{9\mathrm{/}2}$ and $^{4}\mathrm{F}_{3\mathrm{/}2}$${\ensuremath{\rightarrow}}^{4}$${\mathrm{I}}_{11\mathrm{/}2}$ transitions of ${\mathrm{Nd}}^{3+}$ in strontium fluorovanadate, ${\mathrm{Sr}}_{5}$(${\mathrm{VO}}_{4}$${)}_{3}$F, crystals. The emission cross sections of the lower-lying intermanifold transitions as well as the major inter-Stark transitions, ${\mathrm{R}}_{1}$\ensuremath{\rightarrow}${\mathrm{X}}_{1}$(886.2 nm), ${\mathrm{R}}_{1}$\ensuremath{\rightarrow}${\mathrm{X}}_{5}$(942.8 nm), and ${\mathrm{R}}_{1}$\ensuremath{\rightarrow}${\mathrm{Y}}_{1}$(1065.2 nm) of ${\mathrm{Nd}}^{3+}$:${\mathrm{Sr}}_{5}$(${\mathrm{VO}}_{4}$${)}_{3}$F, have been also determined and compared with those of ${\mathrm{Nd}}^{3+}$:YAG.

Theory of two electron rearrangement processes in HCI-atom collisions

In the present report a development of the nonstationary Volkov-Keldysh approach is described. Two electron rearrangement mechanisms are analyzed in the following regions of collision velocities v similar q1/2 and v similar q (q is the projectile ion charge). Analytical behavior of transition amplitudes and cross sections is established for the limiting case of large q >> 1. Calculations are in good agreement with experimental data for absolute cross sections as well as for the ratio of double to single ionization in the range of q = 2 ÷ 90.

Capacitance and conductance of semiconductor heterojunctions with continuous energy distribution of interface states

Electrical conduction in semiconductor heterojunctions with interface states presenting a continuous density of states distribution inside the gap is studied theoretically. The defect states are given with their density of states function and capture cross sections for transitions to the valence and conduction band. Conduction is considered under steady-state dc applied voltage V and under small-signal ac conditions. The formal developments are based on the resolution of Poisson equation and the continuity equations for electrons, holes, and occupied defect states. The numerical integration yields the position dependent dc and ac components of the concentrations of electrons, holes, occupied defect states, and of the current densities and recombination rates. For any value of x, one obtains the occupation function ft as function of the defect energy Et and the transition rates to the valence and conduction band. This allows a detailed analysis of the response of the interface state system to the external applied voltage. The main electrical characteristics obtained in the final analysis are the dc current–voltage and the ac capacitance–frequency and capacitance–voltage curves. The method is applied to a InGaAs/InP heterojunction with interface DOS functions being either constant inside the gap or having gaussian shapes with a given mean value and standard deviation. Discrete levels are treated as a particular case. The I(V) curve has an ideality factor n different from one, reaching the theoretical value of 2 for discrete midgap states. Comparison of C(f ) curves for different DOS functions allows us to show the appearance of characteristic features depending on the explicit form of the DOS function and different from those of the discrete level case. The C(f ) curves, computed for different values of applied voltage V, show cutoff frequencies whose values increase with V. It is shown that this is due to the fact that different defect states are involved in the dynamical response of the system. The low frequency C−2(V) curves are not linear, as for the ideal junction case, and present structures which are correlated with the slope of the interface charge density are represented as a function of applied voltage V.

Characterization of Stark components and peak emission cross sections of intermanifold and inter-Stark transitions of Nd3+ in Ba0.25Mg2.75Y2Ge3O12

The Stark components of the lower-lying manifolds of Nd3+ in Ba0.25Mg2.75Y2Ge3O12 have been characterized using the well-resolved emission spectra for the 4F3/2→4I9/2 and 4F3/2→4I11/2 transitions at 300 K. The emission cross sections of these intermanifold transitions as well as the inter-Stark transitions of R1→X5 at 940.4 nm and of R2→Y3 at 1061.8 nm have been determined. Finally, these results are compared with those of the standard laser host, Nd:YAG, and the potential of Nd:BAMGAR as a laser host is discussed.

Spectroscopic and laser properties of SrLaGa 3O 7:Pr 3+ crystals

The spectroscopic properties of trivalent praseodymium (Pr 3+) doped SrLaGa 3O 7 (SLG) crystals have been investigated at various temperatures. Absorption, emission, excitation and lifetime measurements have been performed and discussed in the framework of the Judd-Ofelt approach. Stimulated emission cross sections for the strongest transitions of Pr 3+ ion have been calculated. Stimulated emission has been observed for the first time in Pr 3+ doped SLG crystal, we have obtained laser operation at 488 n, at cryogenic temperatures and at 645 nm at room temperature.

An analysis of rotational transition probabilities and cross sections using close coupling, hard shape, and classical trajectory methods

Classical and quantum transition probabilities were calculated applying the two (2D) and three (3D) dimensional close‐coupled and classical trajectory methods. The collision energy was 0.01 eV and, in this case, there are 12 classical accessible states for Xe−CO2 collisions. It was shown that for Δj≥8 the 2D and 3D transition probabilities agree quantitatively. The state‐to‐state total cross sections were also compared, in the same level of dimension, and the results showed similar trends. In addition, the hard shape quantum cross sections were compared with the 2D results and showed that the classical turning point cannot be used to obtain the semi‐axes of the ellipse, namely A and B. Moreover, the intensity of the hard shape cross sections showed to be different either for the 2D or 3D close coupling calculations. The comparison between the transition probabilities, carried out by the 2D and 3D quantum methods, showed the nature of the 2D results with accurate agreement for large Δj transitions.

Development of a quantitative ring dye laser absorption diagnostic for free SiH radicals

A quantitative absorption diagnostic for ppm amounts of silylidyne radicals (SiH) was developed. The SiH radicals were produced in high diluted mixtures of silane (SiH 4) in Ar with and without additional H 2 at high temperatures. The experiments were performed in the reaction zone behind reflected shock waves at temperatures between 1300 and 3000 K and at pressures 0.9 ⩽ p ⩽ 2.5 bar. The SiH radicals were detected by time-resolved narrow-bandwidth laser absorption measurements near λ = 413 nm. A spectroscopic model was introduced to describe the absorption cross section of the SiH single line transitions in the A- X(0,0) band. The SiH absorption-over-time profiles could be transformed into concentration time histories by applying this model. The detection limit of the present quantitative ring dye laser absorption diagnostic for shock tube applications was about 1 × 10 12 cm −3 and is thus well suited for kinetic studies of elementary reactions involving SiH radicals.

Inelastic and quasielastic collisions of Rydberg atoms with the heavy rare-gas atoms.

A theory of inelastic n,l-changing and quasielastic l-mixing collisions of Rydberg atoms with the heavy rare-gas atoms is developed. It is based on the semiclassical impact-parameter approach combined with the normalized perturbation theory. Semiclassical formulas for scaled transition probabilities and cross sections as functions of the inelasticity parameter and the collision strength are analyzed. This approach gives a general description of collisional nl\ensuremath{\rightarrow}${\mathit{n}}^{\ensuremath{'}}$ transitions independently of the specific values of the principal quantum number, the relative velocity, and the type of colliding atoms. The energy and angular dependence of the electron-atom scattering amplitude is included in the theory by incorporating the impulse-approximation results in the weak-coupling limit. The Ramsauer-Townsend effect affects significantly the values of the cross sections, especially for the inelastic transitions with large energy transfer. The results obtained are used in calculations of the quenching cross sections for the Rb(nS,nD,nF) atoms in collisions with Ar, Kr, and Xe. Comparison of theory and experiment is made in a wide range of the principal quantum numbers and transition energy defects. \textcopyright{} 1996 The American Physical Society.

Excitation and ionization of neutral Cr and Mo, and the application to impurity influx

The atomic physics required to study impurity influx from a localized surface is examined and then applied to chromium and molybdenum. To this end, we have calculated term energies and oscillator strengths for the and transitions in neutral Cr and Mo in a multiconfiguration Hartree - Fock approximation. We have calculated excitation cross sections for these dipole transitions and for the transitions, which couple the ground to the metastable system, in a close-coupling approximation using the non-relativistic R-matrix method. Pseudo-resonances were removed from the latter transitions using a novel transformation and reduction method which eliminates those linear combinations of (N + 1)-electron bound states that are not required by orthogonality. We have calculated the direct ionization cross sections from the ground and metastable terms in a distorted-wave approximation, and find significant near-threshold sensitivity to the effective potentials used. We have incorporated this primary atomic data into the Atomic Data and Analysis Structure (ADAS) and solved the quasi steady-state generalized collisional - radiative population rate equations. We present results for the number of ionization events per observed photon, the ratio, over a wide range of electron temperatures and densities. The ratio can be used to directly relate spectroscopic emissivity measurements to impurity flux from a localized surface such as the target plates in a magnetic fusion divertor.

Barrier photo-emf studies of electronic transitions in α-RbAg 4I 5

A study has been made of the photo-emf of α-RbAg 4I 5 superionic crystals exposed to light from the impurity excitation region. The effects of local light irradiation and silver saturation on the photo-emf excitation spectrum were investigated. It was found that irradiation with light having energies 2.78 eV and 2.92 eV causes color centers formation in the crystals. Illumination intensity and time dependences of the photo-emf for the main excitation bands were measured at room temperature. The estimation of the photon capture cross sections for the electronic transitions gives values of the order of magnitude of 10 −17 cm 2. A mechanism for the origin of the photo-emf is proposed.