The atomic orientation J⊥+
generated by polarized-electron impact excitation of Hg (6s2)1S0 →
(6s6p)3P1 is studied using the electron–photon coincidence technique. J⊥+ shows a
significant dependence on the spin projection of the incident electrons. Experimental
results are compared with theoretical predictions employing a semi-relativistic five-state
R-matrix
(close-coupling) description and the relativistic distorted-wave approximation.
The experimental data show, in qualitative agreement with the calculations, that
the well established orientation propensity rules for light atoms are apparently
violated if the electron spin is initially down with respect to the scattering plane.
The applicability of the propensity rules to electron-impact excitation of heavy
atoms is analysed. The observed apparent violation is attributed to a quantum
mechanical interference caused by ‘intermediate coupling’ within the excited state.
A re-evaluation of the data supports the validity of the orientation propensity
rules.
Direct measurements of the three Stokes parameters (polarization components) P 1 , P 2 and P 3 of the VUV Hg transition 6s6p 1 P 1 → 6s 2 1 S 0 (185 nm) have been carried out at electron impact energies of 15 eV, 50 eV and 100 eV. Within the experimental uncertainty, no influence of the electron spin was discovered for scattering angles θ 30 • . At 15 eV excitation energy and scattering angles θ 80 • , increasing spin effects become apparent. The experimental data are compared to theoretical predictions from a first-order full-relativistic distortedwave model, a five-state Breit-Pauli R-matrix (close-coupling) approach, and a convergent close-coupling model, in which relativistic effects are accounted for by adding non-relativistic amplitudes using known intermediate-coupling coefficients. At scattering angles θ 15 • , all of the theories reproduce the experimental data well, whereas the CCC model exhibits the best overall agreement with experiment at large scattering angles.
We present a comparative picture of the (e, 3e) process on H − , He and Li + . These three targets have isoelectronic structure with increasing nuclear Coulomb interaction. The study has been done at high incident energy, E 0 ≈ 5.6 keV, very small scattering angle and low ejected electron energies. The actual scattering angle is adjusted so that the momentum transfer and the energies of the ejected electrons are the same in all three cases. The result is that the final states are identical except for the charge on the bare nucleus. The value of the momentum transfer relative to the reciprocal of the atomic size is found to profoundly affect the angular distribution of the five-fold differential cross section.
We have used the relativistic distorted-wave method to calculate the excitation of
the three lowest-lying singlet and triplet D states, for both calcium and strontium.
Results are presented for both the differential cross sections and Stokes
parameters, for electron impact at 20 and 40 eV. Two different sets of
wavefunctions were used, in order to assess the importance of configuration
interaction for these states.
We have carried out relativistic distorted wave calculations for the excitation of the 63P
1 and
61P
1
states of mercury from the ground 1S
0
state by spin-polarized electrons. The ground and excited states are represented
by multiconfiguration Dirac–Fock wavefunctions. We have corrected previously
published expressions for the generalized Stokes parameters (GSP) and
the complete set of independent scattering parameters. Results for GSP,
generalized STU parameters and the complete set of independent scattering
parameters for incident electrons with an energy of 8 eV are obtained and
compared with the available experimental data and theoretical semirelativistic
R-matrix
and distorted wave Born approximation calculations.
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