Knowledge Bank

The dissociative recombination rate constant of positive ions with electrons have been measured to be of the order of $10-6$ to $10-7cm3s-3$ for most positive molecular ions. These rates are important in model calculations of the interstellar chemical evolution. $H3$ in particular occupies a key role. The dissociative recombination rate constant of $H3$ was measured to be $(2.9\pm 0.3)\times 10-7cm3s-1$ at 205 K by using the stationary afterglow $method1$. Also the recombimiation cross sections were measured by using several other techniques such as the inclined beam. the merged beam, and the ion-trap techniques. All the cross sections and the rate constants obtained by the various techniques referred to above are reasommably consistent. Adams and Smith on the other hand obtained a much smaller value for the rate constant$(\sim 1011cm3s-1)$ by using the flowing afterglow/ Langitmuir probe(FALP) $technique2$. A theoretical calculation also suggested a small rate $constant3$. Also very recently Hus $etal4$ repeated the merged beam experiments and obtained the smaller cross section by an order of magnitude than the previous values. These authors attributed the faster rate constants previously obtained to vibrationally excited $H3$. Considering the astrophysical impact of these low values, we have carried out direct measurements of the decay of the infrared absorption signals of $H3$, which can monitor the ion abundance in a particular vibration-rotation state without ambiguity as a function of time. The decay curve was analyzed and found to fit very well to the form expected for a recombination decay. The signal decay is attributed to the dissociative recombination with electrons and the rate constant was determined to be $(1.8+0.2)\times 10-7cm3s-1$, which disagrees with the recent $values2,4$.