Application of NIR – CRDS for state selective study of recombination of para and ortho H₃⁺ ions with electrons in low temperature plasma

We present a study of H₃⁺ recombination performed at 77 K on the two lowest rotational levels of this ion, which belong to its two different nuclear spin states of the studied ion. A near infrared cavity ring-down spectrometer (~1381 nm, CRDS arrangement) has been used to obtain the time evolution of concentration of both states. From the overall ion density decay during the afterglow we obtained the binary recombination rate coefficient α_bin (77 K) = 1.2×10⁻⁷ cm³s⁻¹. We have also observed ternary helium assisted recombination of both para and ortho H₃⁺. The process is very slow (at 77 K) and the obtained ternary recombination rate coefficient is in contradiction with the theoretical prediction. It is the first time that the binary and ternary H₃⁺ recombination rate coefficient was measured at a known population of para and ortho H₃⁺ ions in decaying plasma.