Total Detachment Cross Sections Research Articles

Total detachment cross sections of C−, CH−, C2−, and C2H−incident on N2at keV energies

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Cross sections and transport properties of negative ions in rare gases

We have used a combination of a simple semi-analytic theory - Momentum Transfer Theory (MTT) and exact Monte Carlo (MC) simulations to develop momentum transfer cross sections of negative ions in collisions with noble gases based on the available data for reduced mobility at 300K as a function of E/N. At very low energies, we extrapolated obtained cross sections using Langevin's cross section and supplemented it by the total detachment cross section that was used from the threshold around 6 eV up to 100 eV. Other possible reactive processes have not been taken into account. A good agreement for the mean energy and diffusion coefficients is an independent proof of the validity of the cross sections that were derived for the negative ion mobility data.

Electron detachment of negative ions: The influence of the outermost electron and its neutral core atom in collision with He, Ne, and Ar

A systematic investigation of the absolute total detachment cross sections of atomic negative ions is presented and a simple semiclassical model developed based on the experimental findings. The total electron detachment process for several anion species, namely ${\text{C}}^{\ensuremath{-}}$, ${\text{O}}^{\ensuremath{-}}$, ${\text{F}}^{\ensuremath{-}}$, ${\text{S}}^{\ensuremath{-}}$, ${\text{Si}}^{\ensuremath{-}}$, ${\text{Cl}}^{\ensuremath{-}}$, ${\text{Ge}}^{\ensuremath{-}}$, and ${\text{Na}}^{\ensuremath{-}}$, in collisions with He, Ne, and Ar, in the velocity range of 0.2--1.8 a.u., were studied. In the intermediate-velocity regime the experimental data are well reproduced by the proposed model, which assumes independent contributions from the outermost quasifree electron and from the neutral core atom of the negative ion.

Negative atomic halogens incident on argon and molecular nitrogen: electron detachment studies

During the last years we have measured total detachment cross sections of atomic and cluster anions colliding with gases in the velocity range of 0.2 to 1.8 a.u. In particular, we measured negative atomic halogens incident on argon and molecular nitrogen. These last data are for the first time analyzed using the simple semi-classical model that we have developed. For that purpose, the values of elastic plus inelastic cross sections for impact of free electrons on Ar and N2, the latter showing a shape resonance, convoluted with the anion's outermost electron momentum distribution yielded the overall shape of the anion cross sections. Inclusion of a velocity independent additive term, interpreted as an effective area of the collision region, led to accurate absolute cross section values. The high affinity of the halogens and the existence of a not well described resonance in the e-N2 collision, are characteristics that may be used to delimit the scope and validity of the model.

Strong similarities between small cluster anions and electrons colliding with argon

Total destruction cross sections for negative aluminum clusters $\mathrm{Al}^{\ensuremath{-}}{}_{n}$ with $n=2--4$, and total detachment cross sections of the aluminum anion ${\mathrm{Al}}^{\ensuremath{-}}$, incident on Ar are measured. These values, and previously measured cross sections of $\mathrm{C}^{\ensuremath{-}}{}_{n}$ and $\mathrm{Si}^{\ensuremath{-}}{}_{n}$ with $n=1--4$, are compared with total (elastic plus inelastic) cross sections for electron impact on the same target. Similarities between the experimental cross sections for anions and electrons, observed in the velocity range of $0.2\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1.8\phantom{\rule{0.3em}{0ex}}\mathrm{a.u.}$, led to a model in which the overall shape of the dependence on velocity of the anionic cross sections is well described by a convolution of the anion's outermost electron momentum distribution with the free electron-impact cross sections, and their absolute values obtained by fitting a cluster-size dependent additive constant. A geometrical interpretation is then proposed.

Electron-impact detachment and dissociation of C4− ions

CRYRING was used to study collision processes between an electron and a negative ion cluster C4−. The total detachment cross sections for the production of the neutral 4C, 3C, 2C, and C fragments were measured. The cross sections for pure detachment, and for detachment plus dissociation leading to the production of C3+C, 2C2, and C2+2C were extracted using a grid. It was found that the pure detachment process overwhelmingly dominates all other fragmentation processes. The threshold location for the detachment channel is found to be around 6.0 eV. Although the doubly charged negative ion C42− has received little previous attention, a defined near-threshold resonance observed in the detachment cross section curve, has been associated with the short-lived state C42− (0.7 fs lifetime).

Electron Impact Detachment of Weakly Bound Negative Ions

A method is presented for accurately calculating the total electron impact detachment cross section for weakly bound negative ions. The results are compared to recent experiments for electron impact detachment of H 2 and B 2 . Cross sections differential in energy are presented which elucidate some of the dynamics of the detachment process. A scaling law for the cross section is proposed. [S0031-9007(98)08313-6] PACS numbers: 34.80.Kw, 34.10. + x There have been several recent experiments measuring the electron impact detachment of atomic and molecular negative ions [1‐8]. These experiments exemplify one of the more important processes in physics: the breaking of a target by a projectile giving at least three distinct bodies in the final state. This process is especially difficult to describe theoretically due to the necessity for describing two continua. In this paper, I describe an accurate method for calculating cross sections that takes advantage of the nature of the double continua for electron detachment of negative ions. The calculated cross sections have been dissected in order to gain an understanding of the dynamics governing the detachment process. The insight thus obtained leads to scaling rules for detachment from weakly bound negative ions (ions for which the weakly bound electron is most likely found outside of the region that contains the atomic electrons). There have been four previous methods used to describe electron impact detachment of negative ions. A brief description of these calculations may give some insight into the difficulties that must be overcome. The first method [9] used what is now called an R-matrix pseudostate method to calculate the detachment as an excitation to a positive energy pseudostate. This method did not achieve converged cross sections due to the slow decrease of cross section with increasing total angular momentum. The second method [5] utilized a classical, phenomenological technique. The total detachment cross section is estimated to be the cross section for a classical electron moving in a repulsive Coulomb potential to be absorbed by a partially absorbing sphere of radius r0. This radius and the absorbing fraction are fitted to the experimentally measured detachment cross section. The third method [10] used a fully quantum first order distorted wave theory. Electron wave functions separable in $ r1 and $ r2 (the two outgoing electron’s coordinates) are used for both the initial and the final states. The initial state is nearly separable in $ r1 and $

Doubly differential detachment cross sections for 0.5-MeV H- on He including projectile excitation to H(n=2).

Detailed theoretical results are presented for the electron-detachment cross section, doubly differential in both the electron momentum and angle, for the process 0.5-MeV ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$+He\ensuremath{\rightarrow}H(n=2)+${\mathit{e}}^{\mathrm{\ensuremath{-}}}$+${\mathrm{He}}^{\mathrm{*}}$. As discussed briefly elsewhere [C. R. Liu and A. F. Starace, Phys. Rev. Lett. 62, 407 (1989)], the laboratory-frame doubly differential cross sections (DDCS's) for electron detachment in the forward direction are shown to depend sensitively on the low-energy states of the H(n=2)-${e}^{\mathrm{\ensuremath{-}}}$ three-body system. In particular, the angular dependence of characteristic cusp and shape resonance features is presented. We find that the projectile frame DDCS for detached electron energies in the vicinity of the shape resonance peak is nearly isotropic. This is due in part to the $^{1}\mathrm{P}$ symmetry of the resonance feature, which limits the angular distribution to constant and ${\mathrm{cos}}^{2}$\ensuremath{\theta} terms, and in part to cancellation in the integral over momentum transfer on which the coefficient of the ${\mathrm{cos}}^{2}$\ensuremath{\theta} term depends.We also find that the rapid variation of these cusp and shape resonance features with angle in the laboratory frame requires that experimental angular and energy resolutions be accounted for in order to obtain good agreement with the measured results of M. G. Menendez and M. M. Duncan [Phys. Rev. A 36, 1653 (1987)] on the energy spectrum of detached electrons coincident with the formation of H(2p). The DDCS's for 0.5-MeV ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$+He\ensuremath{\rightarrow}H(1s)+${e}^{\mathrm{\ensuremath{-}}}$+${\mathrm{He}}^{\mathrm{*}}$ are also presented. When these latter results are added to those for producing H(n=2), the sum gives good agreement with the experimental data of M. G. Menendez and M. M. Duncan [Phys. Rev. 20, 2327 (1979)], which include all final states of the H atom, thereby confirming the important contribution the H(n=2) states make to the total-detachment cross section. We present evidence on the sensitivity of these results to the choice of the average excitation energy of the helium target. Finally, we pinpoint the origin of our predicted Gailitis-Damburg oscillations in the DDCS near threshold as stemming from a rapid decrease of an analytically known phase appropriate for long-range dipole fields.

On the size of the negative alkali ions Li-, Na-and K-

The ground-state wavefunction of the negative alkali ions Li-, Na- and K-, consisting of a core with two outer electrons, has been calculated in a hyperspherical approximation replacing the core by a model potential. The states are examined with emphasis on estimating geometrical properties: the expectation value of the radii of the two electrons, the interelectronic distance and the hyperradius R=(r12+r22)12/. A comparison is made with corresponding quantities for H- and possible scaling properties are examined, and it is found that a recent scaling law for the total detachment cross sections is not of geometrical origin.

Electron detachment processes in keV H-, Li-, Na-, K--rare-gas collisions

Electron detachment processes have been measured for the (quasi-) two-electron negative ions with ns2 structures (n=1, 2, 3, 4), namely H-, Li-, Na- and K- in collisions with He, Ne and Ar at energies of 0.5-100 keV amu-1. Total detachment cross sections have been determined for n=1-4. Detachment with simultaneous ns-np excitation has been measured for n=2 and 3. Finally, cross sections for target excitation and/or ionisation have been estimated for 50 keV. For energies above 1 keV amu-1, the total detachment cross sections are found to scale with the cross section for H- at the same velocity as the inverse square of the detachment energies. Simultaneous ns-nsp excitation is found to be important ( approximately 30%) and exhibits a considerable alignment. Target excitation at these energies is relatively small in all cases except for Na--Ne, due to strong quasi-molecular effects in this system.

Theory of electron detachment in collisions of negative ions with atoms II. Survival probability, differential elastic cross section, and total detachment cross section for the H - –He system

The close-coupling theory of electron detachment, developed in the preceding paper I, is used to calculate the probability of survival of the negative ion, and the total probability of electron detachment. Neglect of a term reduces the formulas to more familiar ones, which are then used to calculate differential elastic cross sections and total detachment cross sections for the H - –He and D - –He systems. Good agreement with experi­mental results is obtained.

Electron detachment and charge exchange in H-scattering by atomic deuterium

Results of a differential time-of-flight energy-loss study of H- scattering by D (and D- scattering by H) are presented. Differential cross sections for charge exchange, detachment and excitation were measured in the 100-1000 eV laboratory energy range and 0-1 keV deg reduced angular range. Contrary to the low-energy (less than 1 eV) predictions of Mizuno and Chen (1969) no oscillatory structure was observed in these cross sections. The charge exchange and elastic DCS are found to be strongly absorbed by detachment. Integration of the low-energy DCS and comparison with the total charge exchange cross section of Hummer and co-workers (1960) yielded an absolute calibration. A total detachment cross section for energies below 500 eV was thus obtained. This cross section is not found to decrease with energy. Experimental evidence for detachment from the 2 Sigma gH-2 state into the 'parent' 3 Sigma u H2 state is presented.

A simple approximation for low-energy total detachment cross sections

The problem of electron detachment processes in low-energy H--He and H--Ar collisions is investigated within the zero-radius potential approximation of Demkov and Devdariani (1973), using ab initio data on the negative-ion molecular systems. It is shown that the strong-coupling region extends beyond the crossing point Rc of the neutral and ionic potential energy curves. Much information can be obtained from only the knowledge of the molecular quantities for R>Rc which can be obtained by standard molecular calculations. The scattering equations are solved in the sudden approximation allowing for a very simple approximation to the total cross sections; the results compare well with experimental data.

Electron detachment in low-energy collisions ofH−andD−with He

Measurements and calculations have been made of elastic scattering and electron detachment in collisions of ${\mathrm{H}}^{\ensuremath{-}}$ and ${\mathrm{D}}^{\ensuremath{-}}$ with He at energies of 5-120 eV. The measurements show no other inelastic processes occurring in this energy range. The mechanism responsible for electron detachment is assumed to be the crossing of the ${\mathrm{H}}^{\ensuremath{-}}$ bound state with the continuum of free states; the bound state is then assigned a complex energy. The measured elastic scattering differential cross section shows no structure except at $E\ensuremath{\theta}\ensuremath{\sim}200$ eV deg, where there is a region of downward curvature in the graph of $log\ensuremath{\sigma}$ vs $\ensuremath{\theta}$. This is interpreted as the threshold angle for electron detachment, and it is directly related to the crossing point. By empirically fitting the experimental differential cross section, the general features of the complex potential were obtained. By using the resulting potentials, the total detachment cross section was calculated and compared to the experimental results of Bailey, May, and Muschlitz. Only fair agreement is found at low energies, and poor agreement at higher energies (&gt;100 eV). The theory predicts an isotope effect in the elastic differential cross section, and this effect provides a test of the theory. A careful series of experiments at 20 eV displayed the effect. A preliminary measurement of the electron energy spectrum was also made.