Spin dependent reactions in an electronically excited helium discharge

Abstract

The electron spin dependence of reactions involving helium metastable atoms is investigated using optical pumping and microwave diagnostic techniques. Wigner (1927) proposed that electron spin angular momentum is conserved in atomic collisions. This rule of elec-tron spin conservation is tested in the collision of two helium triplet metastable atoms in the afterglow of a helium discharge at 2 Torr: If the Wigner spin rule holds, the above reaction will be dependent upon the spin states of the colliding metastable atoms. A simple theory is developed showing the influence of spin conservation on the number of electrons produced in such reactions. The effect of electrons resulting from other collisions in the afterglow is introduced, and the two treatments are combined to yield a quantitative prediction of the results of experimental observation. Optical pumping is used to change the normal distribution of the spin states. Microwave diagnostic techniques are used to observe any corresponding change in the number of electrons in the helium afterglow. A very definite and measurable change in electron number density results, indicating that the Wigner spin rule holds in the collision of helium triplet metastable atoms. The experimental data lie within 20% of the theoretical predictions in the early afterglow period, with the agreement decreasing at later times. [see PDF for formulas]