Collision studies with internally cold ion beams and merged electron beams in a cryogenic storage ring

Abstract

The electrostatic cryogenic storage ring (CSR) with wall temperature below 10 K and ultralow pressure, mimics the conditions of the interstellar medium. Infrared active molecular ions can radiatively relax towards their ro-vibrational ground state while being stored in the CSR. In this work, an electron-ion collision experiment is performed under these conditions and the conditions of electron-ion collisions in the merged beams are modelled. For this purpose, the electron beam trajectory is calculated in a magnetic field analysis program. The effect of the overlap geometry, drift tube potential, space charge variations and thermal energy spread of the electron beam have been studied individually. The collision energy distributions in the combination of such experimental conditions are provided. Based on these energy distributions and a narrow-peak cross section, the measured collision rate coefficient is simulated at several detuning energies. A correction method is developed to account for collision energy broadening from the full overlap geometry and drift tube potential. With these tools, dissociative recombination (DR) of HeH+ ions is investigated in the CSR and absolute DR rate coefficients are obtained for ro-vibrationally cold ions. The time dependence of the DR rate showed fast changes in the rate coefficient at increasing storage time. This could be explained from the radiative cooling model of the HeH+ ion. In addition, merged beams with an electron deceleration drift tube have been successfully used and the results compared to the operation without employing the drift tubes.