Detachment Cross Sections Research Articles

Dissociative electron attachment and intramolecular electron transfer in linear haloalkenes

Intramolecular electron transfer through alkyl chains has been investigated by measuring the cross section for halide detachment following resonant π* electron capture in linear n-halo-1-alkenes, for halo = chloro, bromo and n=2–6. The magnitude of the cross section decreases with increasing chain length for all the haloalkenes, with the exception of the halopropenes, but at a considerably faster rate for the chloro than for the bromo compounds. The decrease in cross section for the chloroalkenes occurs at a rate consistent with the decrease in electron-tunneling rates in hydrocarbons with through-bond interactions. For the bromoalkenes it appears that σ*–π* coupling is quite strong and thus the results are not consistent with an electron transfer interpretation. Excluding the propenes, the energy of the cross section maximum is essentially constant for the chloralkenes while it decreases with chain length for the bromoalkenes. Hartree–Fock calculations have been used to determine the equilibrium geometries of various conformers of the n-halo-1-alkenes for n=2–5. The 4- and 5-bromo-1-alkenes show considerably smaller conformational energy differences than do the chloro compounds. Electron attachment energies have been calculated for the most stable conformers at Hartree–Fock, density functional, and Mo/ller–Plesset second order levels. Trends in calculated attachment energies parallel experimental trends in the energies of dissociative attachment maxima only for the Mo/ller–Plesett second order calculations. At the Hartree–Fock level the singly occupied molecular orbitals of the haloalkene radical anions show a somewhat greater admixture of C–halogen σ* and C=Cπ* character in the bromo compounds than in the chloroalkenes, but the distinct difference in σ*–π* coupling in the bromoalkenes compared to the chloroalkenes is represented accurately only in the calculations that include electron correlation.

Quantum wavepacket study of electron detachment from by electron impact

Electron detachment from negative hydrogen ions by low-energy electron impact is considered as an especially simple example to test the validity of a sequence of approximations that allow quantum wavepacket computations to be implemented with desktop facilities. In the present approach, the motion of the electron initially localized in the loosely bound ion state is treated quantum mechanically while the incident electron is assumed to move along a classical trajectory. In addition, the initial azimuthal symmetry is assumed to be conserved during the collision in the co-rotating frame such that the quantum motion is restricted to two degrees of freedom. The wave-dynamical approach is used directly to determine the electron detachment cross section. It also provides a valuable means to clarify physical aspects of the considered process. As examples, we show that the effective detachment rate is strongly affected by projectile-induced polarization effects and that non-adiabatic features are important for a complete account of the electron detachment process. The results of the wave-dynamic calculations are in excellent agreement with recent experimental data.

Single- and double-electron loss of H- in collisions with SF6, CH4, and CO.

Measurement of absolute, total cross sections for single- and double-electron detachment of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ projectiles in sulfur hexafluoride, methane, and carbon oxide has been made. The relative energy range under investigation was from 1 to 5 keV. The resulting cross sections for the single-electron loss of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ in ${\mathrm{SF}}_{6}$ exhibit a flat behavior, whereas for methane and carbon monoxide the single detachment cross section increases with the energy. A free-collision model for electron loss was applied, and very good agreement was found for ${\mathrm{SF}}_{6}$, while for the other targets there is a departure of the trend of the experimental data. A comparison with the results from other research laboratories is given. The measured double-electron detachment cross sections are roughly one order of magnitude smaller than the single detachment cross sections. \textcopyright{} 1996 The American Physical Society.

Electron-impact detachment from negative ions.

Electron-impact detachment cross sections have been measured from threshold to about 30 eV for ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$. The purpose was to investigate the cross section near threshold and to reinvestigate earlier claims of resonances due to short-lived states of the doubly charged negative ion. Initial results for ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ showed no resonances [L. H. Andersen, D. Mathur, H. T. Schmidt, and L. Vejby-Christensen, Phys. Rev. Lett. 74, 892 (1995)]. Here we show that there is no detectable resonance structure for ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ either. The applied experimental technique is discussed in detail. Classical and semiclassical models for the detachment process are presented and compared with the data. \textcopyright{} 1996 The American Physical Society.

Single- and double-electron detachment in collisions of two negative hydrogen ions

Employing the crossed-beams technique in conjunction with a beam-pulsing method, we have measured absolute cross sections sigma tot for the process H-+H- to H0+... for CM energies between 2.5 and 100 keV. Combining the present results with our previously measured cross sections for double-electron detachment ( sigma 00) and triple-electron detachment ( sigma 0+). We obtain cross sections sigma 0- for the single-electron detachment process from the relation sigma 0-= sigma tot- sigma 00- sigma 0+ The experimental data of singleand double-electron detachment are well described by theoretical results based on the non-stationary tunnelling approach. Both cross sections are calculated for CM energies between 0.15 and 300 keV. A new two-electron simultaneous transition mechanism is introduced. The importance of this process for the theoretical assessment of the experimental results is discussed.

Detachment and charge transfer for collisions of negative ions with ozone

Cross sections for electron detachment and charge transfer have been measured for collisions of O−, S−, and halogen anions with neutral ozone for laboratory collision energies ranging from 3 to 500 eV. Both electron detachment and charge transfer are found to be large over the entire range of collision energies for O− and S−+O3 whereas the cross sections for the halogen anions colliding with O3 exhibit threshold behaviors characteristic of endothermic scattering processes.

Double-electron detachment cross sections in intermediate-energy H- plus noble-gas collisions.

Absolute measurements of the total double-electron detachment (DED) cross sections for 3- to 50-keV ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ions incident on helium atoms and for 5- to 50-keV ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ions incident on neon and argon atoms are reported in this paper. The present DED cross sections are in better agreement with the previously reported DED cross sections for helium targets than for either neon or argon targets. A semiempirical calculation of the double detachment cross sections for the intermediate-energy collision region is also presented in this paper.

Electron-impact detachment of D-: Near-threshold behavior and the nonexistence of D2- resonances.

${\mathrm{D}}^{\ensuremath{-}}$ has been stored in the heavy-ion storage ring ASTRID and merged with the electron beam from an electron cooler. The cross section for electron-impact detachment of ${\mathrm{D}}^{\ensuremath{-}}$ was measured for relative energies from 0 to 20 eV. No evidence is found for the earlier reported resonances that were ascribed to short-lived ${\mathrm{H}}^{2\ensuremath{-}}$ states. We present a simple classical "reaction-zone" model which essentially reproduces the data except in the region close to the threshold where the cross section is small.

Single-electron detachment cross sections for 5-50-keV H- ions incident on helium, neon, and argon atoms.

Absolute measurements of the total single-electron detachment (SED) cross sections ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{\ensuremath{-}}10}$ for 3--50-keV ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ions incident on helium atoms and for 5--50-keV ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ions incident on neon and argon atoms are reported in this paper. The present SED cross sections for helium and neon targets are of similar magnitude but have a different energy dependence. By contrast, the SED cross sections for argon targets are significantly larger than either of the other two target species reported in this paper. Comparisons are made with the previous measurements and calculations in the literature.

Coherent control of H- photodetachment in parallel static electric and magnetic fields.

Theoretical calculations of quantum interference effects occurring in photodetachement of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ with short-pulse lasers in the presence of a uniform static electric field [Q. Wang and A. F. Starace, Phys. Rev. A 48, R1741 (1993)] are examined in more detail and extended to the case of parallel static electric and magnetic fields. We show that modulation factors characterizing near-threshold cross sections resulting from detachment by relatively long laser pulses may be employed to set the parameters that are most effective for control of detachment cross-section magnitudes with short coherent laser pulses. Use of a static magnetic field to control detached electron wave-packet motion perpendicular to the static field axis is shown to increase greatly the magnitude of quantum interference effects on the cross sections (from 10% of the field-free cross section in an optimized static electric-field case to 50% in a parallel static electric and magnetic-field case). Theoretical dependence of calculated cross sections on laser pulse lengths, time delays, relative phases, frequencies, and classical electron orbit times are presented and discussed in detail.

Photodetachment of the 2p2(3Pe) state of H-

Results of semiempirical, adiabatic hyperspherical calculations are presented for the photodetachment cross section of the process H[sup [minus]] 2[ital p][sup 2]([sup 3][ital P][sup [ital e]])+[gamma][r arrow]H([ital n]=2)+[ital e][sup [minus]] at photon energies in the range from threshold to 0.125 a.u. above. We find the detachment cross section to have a maximum value of 482 Mb at 25.7 meV above threshold, in good agreement with results of close-coupling calculations by Jacobs, Bhatia, and Temkin [Astrophys. J. 242, 1278 (1980)]. In contrast to the close-coupling results, however, we find that the detachment cross section rises by a factor of more than 400 from its threshold value to its maximum.

Cross section for the photodetachment of Be-

The cross section for single-photon detachment of an electron from the metastable ${\mathrm{Be}}^{\mathrm{\ensuremath{-}}}$ ion has been measured at a photon energy of 2.076 eV to be 31\ifmmode\pm\else\textpm\fi{}3 Mb. The method of laser photodetached electron spectroscopy was used in the measurement and the asymmetry parameter that characterizes the angular distribution of the detached photoelectrons was also measured to be \ensuremath{\beta}=0.49\ifmmode\pm\else\textpm\fi{}0.02 at the same energy.

Slow collisions of Na- and K- with atomic hydrogen

Total cross sections for electron detachment and charge transfer have been measured for collisions of Na- and K- with atomic hydrogen for relative collision energies ranging from 2-20 eV. The cross sections for detachment are considerably larger than those for charge transfer and the experimental results for Na-+H are analysed with a model which employs available potential energy curves and an average width to describe the molecular anion in its unstable region. The charge transfer cross section for Na-+H is compared to the results of a perturbed stationary state calculation which uses the average width inferred from the detachment results and a term of the form H12= beta exp - gamma R to describe the long-range coupling between the A 2 Sigma and X 2 Sigma states of NaH-. This latter calculation underestimates the observed low energy cross section for charge transfer.

Study of weakly-formed negative ions by laser photodetachment and accelerator mass spectrometry

A method of study of weakly-formed negative ions by excitation in laser light and accelerator mass spectrometry is described. Measurement of the photoelectric detachment cross section of the extra electron as a function of the photon energy gives information on the electron affinity. Absolute cross sections for photodetachment of negative ions of La, Ce, Th and U have been measured at the fundamental wavelength (1064 nm) of a Nd: YAG laser. To demonstrate the feasibility of determination of the electron affinity by this method, measurements near the threshold of photodetachment in Te with known electron affinity (1.971 eV) were performed.

Multiphoton detachment of H-

Multiphoton detachment of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ is considered. An accurate one-electron model, which reproduces precisely the known ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ detachment energy and the low-energy e-H(1s) elastic-scattering phase shifts, is developed. Generalized cross sections, based on lowest-nonvanishing-order perturbation theory, are evaluated by an accurate and efficient numerical algorithm for the solution of the associated set of inhomogeneous differential equations. Two- to eight-photon detachment cross sections are determined and compared with a recent one-electron model and two-electron ab initio calculations. It appears that the present study provides consistent results with an accuracy for higher-order (ng3) photon-detachment cross sections comparable to that of lower orders (n=2,3). Average detachment rates for linearly polarized light are calculated and compared with recent experimental measurements. It is found that, for higher-order photon detachment, the rates for circular polarization are small in comparison with the rates for linear polarization.

Multiphoton detachment of negative halogen ions

Our approach, based on the plane wave approximation with correlation corrections, allows us to find a simple analytical expression for arbitrary negative halogen ion multiphoton detachment. The numerical results for two-photon detachment cross sections of F - are in good agreement with experiment.

Electric-field effects on H- photodetachment with excitation of H(n=2).

A theory of the photodetachment of hydrogen negative ions in an external static electric field with the production of hydrogen in excited states is developed. We use a combination of the frame-transformation theory and the Green's-function method. The cross section in nonzero fields is expressed in terms of the photodetachment matrix elements for zero field. The latter are calculated using the adiabatic hyperspherical representation. For energies close to the threshold we use a smooth extrapolation of the zero-field photodetachment matrix elements down to threshold taking care to enforce the proper threshold behavior of cross sections for nonzero fields. The ripples in the total cross section due to interference effects are not noticeable for incident photons linearly polarized along the static field direction, but are relatively well pronounced for the case of circularly polarized photons propagating along the static field. On the other hand, all partial cross sections corresponding to photodetached electrons having magnetic quantum number m'=0 (with respect to a quantization axis parallel to the static electric field) show significant ripple structure regardless of the light polarization. The partial cross section for detachment with excitation of the n=2, m=1 Stark substate by circularly polarized photons exhibits a particularly distinct ripple structure. All cross sections in the region of the shape resonance increase with increasing electric field in accordance with experimental observations. This effect is stronger in the case of linear polarization. We also predict a fairly stable value for the width of the $^{1}$${\mathit{P}}^{\mathit{o}}$ shape resonance as a function of the electric-field strength over the range from 0.4 to 1 MV/cm. For electric-field strengths below 0.4 MV/cm, the ripple structure in the shape resonance makes it difficult to define a resonance width.

Short-pulse detachment of H- in the presence of a static electric field.

Quantum interference effects occurring in photodetachment of H[sup [minus]] in the presence of a uniform, static electric field are shown theoretically to be controllable through use of short laser pulses having characteristic times comparable to photodetached electron reflection times. In particular, calculated cross sections for single-photon detachment by two laser pulses that are delayed and phase shifted relative to one another are shown to oscillate as a function of the relative phases of the laser pulses at fixed photodetached electron energy.

Resonant two-color detachment of H- with excitation of H(n=2).

The cross sections for resonant two-color, two-photon detachment of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ with excitation of the degenerate H(2s) and H(2p) levels are calculated within a semiempirical adiabatic hyperspherical representation. The first photon, with energy ${\mathrm{\ensuremath{\omega}}}_{1}$=0.4017 a.u., is resonant with the well-known Feshback $^{1}$${\mathit{P}}^{\mathit{o}}$ resonance below the H(n=2) threshold. The second photon, with energy ${\mathrm{\ensuremath{\omega}}}_{2}$\ensuremath{\ge}0.126 05 a.u., scans the energy region above the H(n=2) threshold over which long-range dipole-field-induced cross-section oscillations are predicted to occur. Such Gailitis-Damburg oscillations have not yet been observed experimentally. Results for various pairs of light polarization for the two photons are presented. Our resonant two-color, two-photon detachment cross sections are 8--9 orders of magnitude greater than the corresponding nonresonant, single-color, two-photon detachment cross sections obtained by Liu, Du, and Starace [Phys. Rev. A 43, 5891 (1991)]. Unmistakable evidence of long-range dipole-field effects is presented over the 5-meV energy range above the H(n=2) threshold. Furthermore, the differential cross sections for right- and left-circularly polarized, copropagating photons and especially the circular dichroism differential cross sections are shown to have nearly a full cycle of a greatly enhanced dipole-field-induced oscillation extending over the region from threshold to \ensuremath{\approxeq}34 meV above.

P-wave photodetachment in a static electric field.

Photodetachment from negative ions initially bound in [ital s] states has been studied in a static electric field. A 10-keV ion beam was sent through a region with fields of up to 1.5 kV/cm applied parallel to the beam direction. A pulsed dye-laser beam, perpendicular to the ion beam, photodetached the ions. The relative cross sections were measured by detecting the resulting fast neutral atoms with a gated channeltron detector. Polarization-dependent electric-field oscillations were oberved on the [ital p]-wave detachment cross section of Au[sup [minus]] near the threshold at 18 620.2 cm[sup [minus]1]. A simple theory for calculating the oscillatory structure is presented. Good agreement is found between the data and the theory, for laser polarizations both parallel to and perpendicular to the static electric field.