Propensity Rules for Angular Momentum Transfer in Electron-Impact Excitation and Deexcitation

Abstract

Following the recent investigation of Shurgalin et al. [Phys. Rev. Lett. 81, 4604 (1998)], electronimpact induced 3s ! 3p and 4s ! 3p transitions in atomic sodium are studied theoretically. A propensity rule for the angular momentum transfer is supported for scattering angles below 60 ± and incident energies below 20 eV. Excellent agreement between the predictions from an R matrix with pseudostates calculation and the experimental data is obtained and provides credibility to the present study.PACS numbers: 34.80.Dp Ever since the pioneering discussion by Kohmoto and Fano [1], a parameter of particular interest in the detailed study of atomic collisions has been the so-called "orientation" of the target, which describes the sense of circulation of the active electron(s) around the atomic core. This parameter can be studied in scattered-particlepolarized-photon coincidence experiments, in which the circular polarization P 3 of the light emitted perpendicular to the scattering plane is determined [2]. For the simplest case of 1 S ! 1 P o excitation without spin exchange and relativistic effects, P 3 , the orientation O 12 and the angular momentum transfer L Ќ are related through P 3 22O 12 2L Ќ . For more complicated situations, the result becomes P 3 2fL Ќ [2]. The factor f describes depolarization effects due to atomic fine structure and hyperfine structure, cascading if more than one photon can be emitted, and possibly relativistic effects.Alternatively, the angular momentum transfer can be studied in the "time-reversed" experiment, starting with a laser-prepared state. Following early work by Hertel et al. [3,4], this method was further developed by the NIST group [5-7] who generated benchmark data for 3s ! 3p electron-impact excitation in sodium. Subsequently, the technique was widely applied [8][9][10][11].Interestingly, the observed pattern for the angular momentum transfer as a function of the scattering angle showed a characteristic behavior, essentially independent of the collision energy and even the actual target. For electron-impact induced S ! P o excitation, almost without exception (see Ref.[12] for a rare example), the value of L Ќ is positive for small scattering angles u and increases with u to a maximum beyond which the variation becomes more individual. (See Ref. [13] for an extensive compilation of the available data.) Triggered by these results, considerable effort has been directed into finding a solid theoretical foundation for this empirical propensity rule, the Kohmoto-Fano paper being an early example. Propensity rules for orientation have also been formulated and discussed for heavy-particle impact excitation [14], and efforts have concentrated on exploring the generality of these rules [15,16].For electron-atom collisions, a significant step beyond the Kohmoto-Fano work was the analysis by Madison and Winters [17]. After pointing out a phase problem in the Kohmoto-Fano paper, they discussed the orientation parameter in terms of the charge q of the projectile, with q 61 for positron and...

Electron impact excitation of the 31D state of helium - an electron-photon coherence study at small scattering angles

“…excitation in sodium (Bartschat et al 1999) indicates that the propensity rule of Andersen and Hertel (1986) holds provided that the collision can be well described by low angular At 60 eV and 40 • scattering angle, where a comparison can be made between the phases β ±2 directly measured in the triple coincidence experiment of Mikosza et al (1997) and the present γ ± , the present data are consistent with that of Mikosza et al…”

Unusual angular momentum transfer in electron-impact excitation of neon

“…Comparing deexcitation to the 3s ground state in superelastic collisions with excitation to the 4s state in inelastic collisions, their results ultimately remained inconclusive for the 4s state. Bartschat et al [6] explained the findings of Shurgalin et al by noting that the simple Andersen-Hertel model is not applicable to the Na (4s) case, due to the very large dipole polarizability of this state, which leads to an additional attractive potential that was neglected in the semiclassical argument.…”

Electron-impact excitation of the Kr 4p55s states at 15 eV: close-coupling versus distorted-wave treatments

“…On the other hand, the small-angle results for the angular momentum transfer L ⊥ are very interesting because of the differences in their predicted sign. In fact, the BP15 and BP31 results violate the propensity rule of a positive angular momentum transfer in electron-induced inelastic S → P transitions at small angles (see Andersen et al (1996) and Bartschat et al (1999) for recent discussions). An experimental test of these predictions requires the difficult measurement of the circular light polarization P 3 for this VUV transition, but its feasibility was demonstrated for this system at slightly higher collision energies (Khakoo and McConkey 1987).…”