Electron Impact Detachment of Weakly Bound Negative Ions

Abstract

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) 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][2][3][4][5][6][7][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 r 0 . 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 r 1 and r 2 (the two outgoing electron's coordinates) are used for both the initial and the final states. The initial state is nearly separable in r 1 and r 2 since it describes one electron weakly bound to the atom/molecule and the projectile electron in a continuum state. However, the final state is not expected to be separable in these coordinates since the final state wave function describes two highly correlated electrons that are both in the continuum. The method is somewhat sensitive to the potential in which the electron waves are calculated, but the theoretical cross sections are in qualitative agreement with experiment. The fourth method [11][12][...

Size-Dependent Electron-Impact Detachment of Internally ColdCn−andAlnDiner1, Toker2, Strasser3 et al. 2004Phys. Rev. Lett.170100View full textAdd to dashboardCite

“…The most important feature is related to the conflicting dependence of the electron detachment cross section for the C n ÿ clusters on their binding energies. On one hand, the odd-even oscillations in the cross section follow nicely the expectation that a stronger binding energy of the electron should result in a smaller cross section (a 1=E 2 b scaling is predicted theoretically based on either classical [8,13] or quantum mechanical [12] arguments). On the other hand, the overall average increase in the cross section as a function of n is in contradiction with the argument above, as the binding energy increases as well.…”

“…2(a)]. Explicitly, the general trend for the experimental cross sections is an increase with n, while the trend for the theoretical cross sections is a decrease with n. The same result is obtained if Robicheaux's scaling law [12] is used instead, and for n > 4 the two theoretical models coincide.…”

“…Electron-impact detachment is a process that has been extensively studied for atoms and small molecules [7][8][9][10][11][12][13][14][15]. Because of the high degree of electron correlation in the negative systems, these experiments are sensitive to the electronic structure.…”

“…Because of the high degree of electron correlation in the negative systems, these experiments are sensitive to the electronic structure. It has been shown that, for these simple species, the electron binding energy E b is the most important parameter in the determination of both the threshold behavior and the magnitude of the cross section above threshold, and a simple scaling law has been obtained [8,12,13]. Among the different theoretical methods leading to this simple scaling law, two can be singled out: The first is a classical phenomenological derivation, where the detachment cross section is estimated to be the cross section for a classical electron moving in a ''reaction zone'' [13].…”

“…Among the different theoretical methods leading to this simple scaling law, two can be singled out: The first is a classical phenomenological derivation, where the detachment cross section is estimated to be the cross section for a classical electron moving in a ''reaction zone'' [13]. The second method is based on the distorted-wave Born approximation [12].…”