Near-threshold measurement of integrated Stokes parameters for Kr excited by polarized electrons

Abstract

We have made high-accuracy measurements of the integrated Stokes parameters for resonance fluorescence from polarized electron-impact excitation of the 4p 5 5p͓5/2͔ 3 D 3 and 4p 5 5p͓5/2͔ 3 D 2 states of Kr. We report measurements in the region within 0.6 eV of threshold, which is below the first cascade threshold. We also present theoretical calculations of these Stokes parameters using a recently developed relativistic Breit-Pauli Rmatrix code. In well LS-coupled systems, nonzero values of the integrated Stokes parameter P 2 signal relativistic effects ͑like continuum spin-orbit coupling, i.e., Mott scattering͒. A single value of P 2 ϭ4(4)ϫ10 Ϫ3 at 12.0 eV was previously reported in this energy range ͓Furst et al., Phys. Rev. A 47, 3775 ͑1993͔͒. We have now measured P 2 at six different energies in this region to comparable precision. These results are consistent with P 2 ϭ0 and with the theory. We discuss the effect of the electron-beam energy width on the accuracy of the measurements. Even when such effects are accounted for, serious discrepancies remain between theoretical and experimental results for excitation of the intermediately coupled 3 D 2 state. ͓S1050-2947͑99͒07907-X͔ PACS number͑s͒: 34.80.Dp

A cylindrically symmetric “micro-Mott” electron polarimeter

“…Polarized electron sources based on 780 nm photoemission from bulk ⟨110⟩ GaAs photocathodes that we have used in our lab, under quite similar vacuum conditions, typically have polarizations P = 28% ± 2%. [17][18][19][20] Our polarization measurements of the present source using neon optical electron polarimetry 21 were consistent with the above value, and so we base our reported values of S eff on this number. Although GaAs sources of this kind can have lower polarization, typically due to multiple resistive heat-cleanings of the photocathode, 22 we consider this a conservative estimate.…”

Angular Momentum Partitioning and Hexacontatetrapole Moments in Impulsively Excited Argon Ions

“…This demonstrates that the RB hypothesis holds for the L and S multipoles of the ion. Additionally, we found P 2 to be consistent with zero for all transitions at all the energies we investigated, implying that LS coupling of the target holds and that spin-orbit coupling between the ion and the continuum electrons is negligible [7]. Figure 2 also shows the value of t 11 ͑S͒ 1͞ p 2 corresponding to collisions in which pure exchange excitation of the 4p electron occurs.…”

“…This collision is particularly interesting because the residual target state is well LS coupled, unlike neutral Ar [11]. This simplifies the physical interpretation of our data, and reduces the difficulty for scattering theory in describing the target's final state, known to be a problem for neutral atoms [7,8]. Moreover, collision (1) involves simultaneous ionization and excitation of the target.…”

“…Electron-atom scattering is a prototype of the manybody long-range force problem. Our understanding of collisions with H, He, and the light alkali atoms is good [1][2][3], but calculations of scattering from the heavy noble gases are not yet satisfactory [4][5][6][7][8][9][10]. This is due primarily to the outer p-shell configuration in such atoms and the fact that relativistic effects can be important in certain collision channels.…”

“…We measure the fractional linear polarizations and circular polarization of the light (Stokes parameters P 1 , P 2 , and P 3 ) [13]. A description of our apparatus is given elsewhere [7]. Details of the transitions and interference filters we used to isolate them are given in Table I.…”