Binary and ternary recombination of para- H and ortho- H with electrons: State selective study at 77-200 K

Measurements in H afterglow plasmas with spectroscopically determined relative abundances of H ions in the para-nuclear and ortho-nuclear spin states provide clear evidence that at low temperatures (77-200 K) para- H ions recombine significantly faster with electrons than ions in the ortho state, in agreement with a recent theoretical prediction. The cavity ring-down absorption spectroscopy used here provides an in situ determination of the para/ortho abundance ratio and yields additional information on the translational and rotational temperatures of the recombining ions. The results show that H recombination with electrons occurs by both binary recombination and third-body (helium) assisted recombination, and that both the two-body and three-body rate coefficients depend on the nuclear spin states. Electron-stabilized (collisional-radiative) recombination appears to make only a small contribution. © 2012 American Institute of Physics.