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The rate coefficients for the ternary association reaction of vibrationally cold N 2 + ions with N 2 and He were measured in an afterglow plasma in the temperature range of 140-250 K: k N 4 + = (1.02 ± 0.39)(300/ T ) (1.28±0.08) × 10 -29 cm 6 s -1 . The rotational and vibrational state populations of N 2 + ions were probed in situ by time-resolved cavity ring-down spectroscopy. The rate coefficient for Penning ionization of N 2 by helium metastable atoms was also determined: k m = [(2.70 ± 0.17) + (84.3 ± 5.9)e -(877±36)/ T ] × 10 -11 cm 3 s -1 .

We present studies on the thermalisation of H3+ ions in a cold He/Ar/H2 plasma at temperatures 30-70 K. We show that we are able to generate a rotationally thermalised H3+ ensemble with a population of rotational and nuclear spin states corresponding to a particular ion translational temperature. By varying the para-H2 fraction used in the experiment we are able to produce para-H3+ ions with fractional populations higher than those corresponding to thermodynamic values. At 35 K, only the lowest rotational states of para and ortho H3+ are populated. This is the first step towards experimental studies of electron-molecular ion recombination processes with precisely specified quantum states at astrophysically relevant temperatures.

The results of an experimental study of the H3(+) and D3(+) ions recombination with electrons in afterglow plasmas in the temperature range 50-230 K are presented. A flowing afterglow apparatus equipped with a Langmuir probe was used to measure the evolution of the electron number density in the decaying plasma. The obtained values of the binary recombination rate coefficient are αbinH3(+) = (6.0 ± 1.8) × 10(-8)(300/T)(0.36±0.09) cm(3) s(-1) for H3(+) ions in the temperature range 80-300 K and αbinD3(+) = (3.5 ± 1.1) × 10(-8)(300/T)(0.73±0.09) cm(3) s(-1) for D3(+) ions in the temperature range 50-300 K. This is the first measurement of the binary recombination rate coefficient of H3(+) and D3(+) ions in a plasma experiment down to 50 K.