Microwave Afterglow Studies of Electron-Ion Recombination at Low Temperatures.

A specially constructed low temperature microwave afterglow-mass spectrometer apparatus employing microwave heating of the electrons has been used to study the dissociative recombination of the complex molecular ions Ne(,3)('+), H(,3)('+), and H(,5)('+) with electrons. The measured electron decays in recombination controlled afterglows are fitted by computer solutions of the continuity equation containing recombination and ambipolar diffusion loss terms. At (TURN)80K and (TURN)6 Torr of neon, Ne(,3)('+) is the dominant afterglow ion. The recombination coefficient, (alpha)(Ne(,3)('+)), has been determined over the range 80K (LESSTHEQ) T(,e) (LESSTHEQ) 4000K, and the result may be expressed as (alpha)(Ne(,3)('+)) = (1.1 (+OR-) 0.1) x 10('-6){T(,e)/300}('-0.36)cm('3)/sec with T(,+) = T(,g) = 80K. This trimer ion exhibits a dependence on electron temperature similar to that predicted and measured for diatomic ions but a larger (> 10('-6) cm('3)/sec) room temperature rate coefficient. Admixtures of (TURN)1 mTorr of H(,2) in 20 Torr of neon at 265K lead to an H(,3)('+) dominated afterglow. The recombination coefficient, (alpha)(H(,3)('+)) varies little over the range 256K (LESSTHEQ) T(,e(, ))<(, )400K, then approaches a T(,e)('-0/88) variation, i.e., (alpha)(H(,3)('+)) = (2.0 (+OR-) 0.3) x 10('-7){T(,e)/300}('-0.88) cm('3)/sec for (TURN) 400K(, )<(, )T(,e) (LESSTHEQ) 4000K, with T(,+) = T(,g) = 256K. The value of (alpha)(H(,3)('+)) at T(,e) = 300K is (1.5 (+OR-) 0.3) x 1('-7) cm('3)/sec, which is ('(TURN))35% lower than the results of a previous afterglow study. This T(,e)('-0.88) variation at higher temperatures is stronger than the T(,e)('-0.5) to T(,e)(' -0.24) variations derived from e('-) + H(,3)('+) recombination cross section experiments using merged beams and ion-storage techniques. For the cluster ion H(,5)('+), the measured recombination coefficient, expressed in the conventional manner, is (alpha)(H(,5)('+)) = (1.8 (+OR-) 0.3) x 10('-6){T(,e)/300}('-0.69) cm('3)/sec, for 128K (LESSTHEQ) T(,e) (LESSTHEQ) 4000K and T(,+) = T(,g) = 128K. (At these lower temperatures H(,5)('+) was the dominant afterglow ion at p(H(,2)) (TURN) 1 mTorr and p(Ne) = 14 Torr.) This T(,e) variation is somewhat stronger than measured (alpha)(T(,e)) variations of the dimer ions N(,2)('+)(.)N(,2) and CO('+)(.)CO, but the coefficient has the typically large value at room temperature ((TURN) 10('-6)) that most cluster ions do.