|Charge transfer measurements in low-energy ion-atom collisions (Vol. 44 No. 1)|
We have used a radio frequency ion trap to study two charge transfer reactions:
(1) Resonant charge transfer: 3He2+ + 4He (1s2) → 3He + 4He2+,
(2) Single electron charge transfer: 3He2+ + 4He (1s2) → 3He+ + 4He+.
We have determined the resonant charge transfer (RCT) rate coefficient of 3He2+ with para -4He (1s2) at energies below 1 eV (reaction (1)). The rate coefficient is measured to be 5.9±0.6×10-10 cm3s-1 at an equivalent temperature of 1200K and is in reasonable agreement with recent calculations. This measurement extends our knowledge to a lower energy region thus adding to our understanding of the charge transfer process of 3He2+, α-particles, with He encountered in astrophysics and fusion research.
While this measurement extends the experimental results below eV energies for the first time, it however provides an interesting observation. The rate coefficient for resonant two electrons transfer (reaction (1)) is orders of magnitude larger than the rate coefficient for single electron transfer (reaction (2)) at comparable temperature reported in the literature. This may lead to the following fundamental questions. The electron spin in para-He is anti-parallel. The spatial wave function that represents the two electrons is symmetric. The probability density for the two electrons close together is finite. Can the proximity of the two electrons account for this relatively large two electrons resonant charge transfer rate coefficient? Is it possible that the two anti-parallel electrons couple to form a loosely bound electron pair that is responsible for this relatively fast two-electron transfer? Can we gain some physical insight by measuring the rate coefficient of the resonant charge transfer of 3He2+ with ortho-4He (1s2s) (metastable helium (23S1)) where the two electron spins are parallel?
C. Kyriakides, B.S. Clarke, W. M. O'Donnell, B. Zygelman and V.H.S. Kwong, ‘Resonant charge transfer of 3He2+ with 4He(1s2) at energies below 1 eV’, J. Phys. B: At. Mol. Opt. Phys. 45 235701 (2012)