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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.