Total Excitation Cross Sections Research Articles

Theoretical study of the 3p-excitation in Mg+ and Mg0-inert gas collisions

Excitation of the 3p-states of Mg+ and Mg0 colliding with inert gases is treated within the coupled-channel impact parameter approximation. We have calculated the total excitation cross section and the linear polarisation of the (3p→3s)-emission as a function of the collision energy as well as the impact parameter dependence of the excitation probability and of the excitation amplitudes (expressed in terms of theλ, χ− parameters). Qualitative agreement with the experiments can be obtained when including both localised molecular orbital transitions at small internuclear distances (mechanism (i)) and the direct interaction between the Mg+,0-valence electrons and the inert gas atom at large internuclear distances (mechanism (ii)). We present evidence that it is inadequate at impact energies below 1 keV to compute the phaseχ directly from the excitation amplitudes obtained in the coupled-channel impact parameter approximation.

The 1s–2s and 1s–2p excitation of helium ions by electron impact

The electron impact excitation of He+ to the n = 2 level is studied using two variants of the polarized orbital method, one due to Daskhan et al. and the other due to Temkin and Lamkin. The present results for 1s–2p total excitation cross sections are in fair agreement with the measured values. For the 1s–2s excitation, our values fail to reproduce the more elaborate theoretical prediction by Morgan near the threshold energies.

R-matrix calculation for the excitation of the lowest-lying autoionising level in lighter alkali metal atoms

Low-energy electron impact total excitation cross sections of the lowest-lying autoionising level generated due to the inner-shell electron excitation 1s22s22p63s2Se to 1s22s22p53s22P0 and 1s22s22p63s23p64s2Se to 1s22s22p63s23p54s22P0 complex transitions in lighter alkali metal atoms i.e. sodium (Na) and potassium (K) atomic systems respectively have been calculated using single configuration Hartree-Fock (HF) wavefunctions for both initial and final states within the R-matrix method in the low bombarding energy region. The present calculated theoretical results differ significantly in nature from all earlier predictions and also disagree with the estimated values of the contribution of the lowest-lying autionising level to direct ionisation on the basis of the classical scaling law reported by McDowell (1969).

Determination of electron-xenon total excitation cross-sections, from threshold to 100 eV, from experimental values of Townsend's α

Recommended values are presented for qm, qt and QT (momentum transfer cross-sections for elastic collisions, total cross-sections for elastic collisions and total collision cross-sections, respectively) of electrons in xenon for electron energies from 0 to 104 eV. Recommended values of ionisation cross-sections qi are also presented. Using these values of qm together with values of qi and Townsend ionisation coefficients alpha , recommended values of total excitation cross-sections qe are determined. The values of qm for electron energies from 5 to 20 eV are smaller than those of Frost and Phelps (1964) by a considerable amount. The values of qe for energies from about 20 to 100 eV are much larger than those of de Heer et al. (1979).

Excited state formation in the interaction of mass resolved ion beams with molecular and atomic targets (1 – 4000 eV, 200 – 800 nm)

A 60° magnetic mass spectrometer is used to inject singly and doubly charged ion beams of up to 250 nA current and 1 – 4000 eV energy into a chamber which is filled with atomic or molecular gas (0 – 1 Pa). The light produced by the interaction processes is analyzed in a 0.5 m monochromator and the photons are counted in a cooled multiplier - single photon counting arrangement. From the light-to-ion current ratio, a total cross section for excitation is defined and its energy dependence is measured. The measured curves are explained in terms of a multiple-crossing Landau-Zener-Stückelberg approach.

Excitation of the lowest autoionising states in alkalis

Total cross sections for electron impact excitation of the lowest autoionising levels in Li, Na and K are calculated in the Glauber approximation. Similar calculations are also performed in the first Born approximation as well as using the method of Crothers and McCarroll (1965). The results obtained from these various approximations are compared with earlier theoretical work.

Ionization yield in xenon due to electron impact

The ionization yield in xenon for complete energy degradation of electrons with initial energy up to 1 keV has been calculated by solving the generalized Fowler equation. The expression for the energy spectrum of secondary electrons from the O shell was obtained by using the empirical scaling functions to weight the Williams-Weizsäcker cross section for glancing collisions and the Mott cross section for knock-on collisions. The total ionization and excitation cross sections were taken from the recent evaluation reported by De Heer et al. Contributions from the inner-shell ionization and the Auger process were explicitly taken into account. The results are expressed in terms of the quantity W, the mean energy required to produce an ion pair. The W value is found to decrease with increasing incident energy, finally approaching a constant value of 23.1 eV for electrons with an energy above 200 eV. The results are generally in good agreement with the available experimental work.

Total dissociation cross section of CF4and other fluoroalkanes for electron impact

Various techniques from the field of surface science have allowed the development of a method for measuring the total absolute cross section for electron-impact-induced dissociation of a number of molecules. The technique is particularly reliable for the fluoroalkanes such as CF/sub 4/, CF/sub 3/H, C/sub 2/F/sub 6/, and C/sub 3/F/sub 8/. The total dissociation cross section for CF/sub 4/ has been measured for energies between threshold (approx.12.5 eV) and 600 eV. The magnitude of the cross section at its maximum is 5.5 x 10/sup -16/ cm/sup 2/. Less extensive data are presented for CF/sub 3/H, C/sub 2/F/sub 6/, and C/sub 3/F/sub 8/. Their cross sections at the maxima are 5.8 x 10/sup -16/ cm/sup 2/, 8.6 x 10/sup -16/ cm/sup 2/, and 1.18 x 10/sup -15/ cm/sup 2/, respectively. Arguments are presented which suggest that the total dissociation cross section for each of these gases is equal to the sum of the cross sections for excitation to all electronic and ionic states, i.e., the total cross section for electronic excitation. The results are discussed from the point of view of the Bethe theory. It is concluded that the Bethe asymptotic behavior is not yet attained in the energy range of more » these measurements. « less

Penning Excitation Cross Sections for the Individual Zn(II) States by He 21S and He 23S Metastable Atoms

The relative Penning excitation rates for the individual Zn(II) states by He 2 1 S atoms and those by He 2 3 S atoms were measured separately using Penning electron spectroscopy. The absolute Penning excitation cross sections σ s (M) and σ t (M) corresponding to them were determined by using the known total Penning excitation cross section of Zn atoms by He 2 3 S atoms and the Penning electron energy spectra for the He–Ar system. The values of σ s (M) and σ t (M) are as follows: in units of 10 -15 cm 2 , σ s (4s 2 S 1/2 )=3.5(1±0.3); σ t (4s 1 S 1/2 )=2.2(1±0.1); σ s (4p 2 P)=0.93(1±0.35); σ t (4p 2 P)=0.24(1±0.25); σ s (4s 2 2 D 5/2 )=σ s (4s 2 2 D 3/2 )=0.56(1±0.4); σ t (4s 2 2 D 5/2 )=σ t (4s 2 2 D 3/2 )=0.24(1±0.25).

Glauber generalized oscillator strength for allowed transitions of lithium atom

Minima and maxima in the generalized oscillator strength (GOS) are studied as a function of momentum transfer for allowed transitions in lithium atom using the Glauber approximation. The results of the present calculations have been compared with the first Born approximation. The results indicate that the Glauber approximation predicts the occurrence of these extrema and their positions shift with energy of the incident particle, in contrast to the results of Born approximation. The total excitation cross sections are also obtained using the Glauber approximation and are compared with the Born calculations and the available experimental data.

Electron-impact excitation of the lithium isoelectronic sequence

The Hartree-Fock wave functions of Weiss for the lithium isoelectronic sequence through $Z=10$ are used to calculate the generalized oscillator strengths (GOS) and total cross sections for excitation in length and velocity formulations of the Born approximation for all possible transitions between the initial state $n=2$ to the final state $N=3$. The present results for the total cross sections are compared with the available theoretical Coulomb-Born calculations and experimental data. The existence of regularities and systematic trends in the GOS and cross sections along the isoelectronic sequence are qualitatively discussed.

Ionization yield in krypton due to electron impact

The ionization yield in krypton for complete energy degradation of electrons with initial energy up to 1 keV has been calculated by solving the Fowler equation. The main expression for the energy spectrum of secondary electrons was obtained by utilizing the qualitative features of two nearly distinct domains of glancing collisions and knock-on collisions. Empirical scaling functions were used to weight the two kinds of contributions. The total ionization and excitation cross sections were taken from the recent evaluation reported by De Heer et al. Contributions to the ionization yield from M and N shells and from the Auger electrons were explicitly included. The results are expressed in terms of the quantity W, the mean energy required to produce an ion pair. The asymptotic value of W is obtained to be 23.6 eV for an incident energy above 200 eV. The results are in good agreement with the available experimental data.

Differential and total cross sections for proton impact excitation of He

An extended form of the Vainshtein-Presnyakov-Sobelman approximation is used to calculate differential and total cross sections for proton impact excitation of the n = 2 level in He. Agreement with experiment is very good.

A Quasiclassical trajectory study of collisional excitation in O( 3P)+CO 2

The quasiclassical trajectory method is used to calculate cross sections for vibrational and rotational excitation in collisions of CO 2 with O( 3P) at relative translational energies from 2.2 to 6.0 eV. Rotational excitation is dominant at all but the highest energies. The largest vibrational excit cross sections for collisions with CO 2(000) are to excited bending states such as (010), (020), etc., and to excited symmetric stretch states. The 000 → 001 cross section is found to be quite small (⩽2 × 10 −18 cm 2 ) over the entire energy range, but the cross sections for excitation of combination states such as ( NN′1) are found to be considerably larger. Indeed the total asymmetric stretch excitation cross section is similar in magnitude to that obtained for Ar + CO 2 in recent molecular beam experiments.

Total inelastic-scattering cross sections by optical-potential methods

Total excitation cross sections for electrons scattering from atomic hydrogen are calculated by using a complex optical potential. Distorted-wave propagators as well as free-particle propagators are used in computing the optical potential. It is found that using an attractive potential to generate the propagator results in poorer agreement with experiment than using a free-particle propagator. The effect of neglecting coupling between excited target states in computing the optical potential is also studied. It is concluded that it is necessary to include coupling between excited target states if the calculation is to be reliable.

A photon polarization study of the doublet emission in potassium ion-atom collisions

Polarization fractions for the K(4p2P3/2→4s2S1/2) emission line at 766.5 nm have been measured for K+-K collisions in the (0.35÷1.5) keV energy range, using a crossed ion-atom beam apparatus. From these experimental results, and from the absolute total excitation cross-sections available from previous measurements, cross-sections are obtained for collisional population of the magnetic sublevels of the K(4p2P1/2) state.

Penning excitation cross sections for the individual CdII states by He(23S) metastable atoms

The branching ratios of the Penning excitation for the individual Cd II states by He(23S) atoms were measured using Penning electron spectroscopy. Using the value of the total Penning excitation cross section measured by Schearer and Padovani (1970) the excitation cross sections for the upper and lower states of the Cd II 441.6 nm laser line are determined to be 0.54 and 0.14*10-15 cm2, respectively. Those of the Cd II 325.0 nm laser line are equal to the above values, respectively, within the limits of error of the determination. The excitation cross section for the CdII ground state is 2.8*10-15 cm2.

Analytic Approximations for Integrated Electron?Atom Excitations

Accurate calculations of atomic excitations require estimates of the effect of higher excitations on the effective (optical) potential coupling various reaction channels. The total cross section for a particular excitation is proportional to the maximum contribution of that excitation to the imaginary part of the elastic momentum-space optical potential, and is typical of the contribution to the potential in general. Analytic expressions relevant to the calculation of optical potentials are given Their validity is estimated by comparison with more-accurate calculations and with experimental excitation cross sections.

K-shell excitation probabilities in 32 MeV S on Ar collisions

In collisions of 32 MeV Sn+ on Ar, impact parameter dependent K-shell excitation probabilities PK(b) have been determined for n=5, 11, and absolute K-shell excitation cross sections sigma K for n=5-13. A comparison between PK(b) and the 2p pi -2p sigma rotational coupling model clearly shows the main contribution to the total K-shell excitation cross section sigma K to be due to rotational coupling. This is also true for S5+ where no static L vacancies are present prior to the collision. The increase of sigma K with projectile charge state n+ (for 5<or=n<or=13) is only 60%, suggesting a high probability for direct 2p pi excitation on the incoming part of the collision even for the lowest charge states. For impact parameters of b<or=2000 fm, which do not significantly influence the total cross section sigma K, the contribution of direct 2p sigma excitation is found to be important.

Theory of collisionally aided radiative excitation in three-level systems

A theory of collisionally aided radiative excitation (CARE) for three-level systems in the weak-field limit is presented. Cross sections for the excitation of three-level atoms by two off-resonant pulsed radiation fields in the presence of collisions with structureless perturbers are calculated. Analytic expressions for the cross sections as functions of atom-field detunings are obtained under usual classical-trajectory and rotating-wave approximations using perturbation theory for various regions of detunings. Examples for the resulting excitation line shapes are given mostly for van der Waals potentials. A dressed-atom picture of the CARE processes is discussed. Emphasis is put on an interesting effect arising from the interference between the stepwise and the direct channel of excitation. Such an interference effect manifests itself as modulations in the total excitation cross section as a function of relative interatomic speed in some cases.