Excitation Transfer of Helium 31P, D →33D by Atomic Collisions–Violation of Wigner's Spin Conservation Rule

Pulsed-laser selective-excitation has been applied to a helium discharge plasma to excite the 3 1 P level. From the observed population perturbations of 3 1 P , 3 1 D and 3 3 D levels the cross section has been determined for the excitation transfer by atomic collisions: for 3 1 P →3 3 D it is (1.40 ±0.11) ×10 -18 cm 2 , and for 3 1 D →3 3 D it is (1.25 ±0.26) ×10 -17 cm 2 . The possibility has been examined that these transfer processes are interpreted from the mixing of the 3 1 D 2 and 3 3 D 2 atomic wavefunctions that is due to the spin-orbit interaction; the former cross section is compared with the 3 1 P →3 1 D transfer cross section, and the latter is compared with the 3 1 D -level dis-alignment collision cross section, which is measured to be (1.63 ±0.09) ×10 -14 cm 2 . Discrepancies by one order of magnitude for the both cases indicate the obvious violation of Wigner's spin rule.