Rotational Cooling of HD+ by Superelastic Collisions with Electrons
Résumé
Rotational cooling of HD+ by superelastic collisions (SEC) with electrons was observed at the Heidelberg test storage ring by merging a beam of rotationally hot HD+ ions with an electron beam at zero relative energy. Neutral fragments resulting from DR events were recorded at different electron densities using a high resolution imaging detector and a large-area, energy sensitive detector. The data allowed to deduce the time dependence of the population of three groups of rotational angular momentum states J built on the vibrational ground state of the ion together with the corresponding DR rate coefficients. The latter are found to be (statistical uncertainties only) langlealpharangle0,1,2 = 3.8(1), langlealpharangle3,4 = 4.0(2), and langlealpharangle5,6,7 = 9.0(1.3) in units of 10-8 cm3/s, in reasonable agreement with the average values derived within the MQDT approach. The time evolution of the population curves clearly reveals that rotational cooling by SEC takes place, which can be well described by using theoretical SEC rate coefficients obtained by combining the molecular R-matrix approach with the adiabatic nuclear rotation approximation. We verify the DeltaJ = -2 coefficients, which are predicted to be dominant as opposed to the DeltaJ = -1 coefficients and to amount to (1 - 2) · 10-6 cm3/s, to within 30%.