Absolute cross sections and rate constants for the endothermic proton transfer reaction of rotationally state selected HBr+ ions with CO2 were measured as a function of the rotational energy for different center-of-mass (c.m.) collision energies. State selection of HBr+ ions was achieved via a (2+1) resonance enhanced multiphoton ionization (REMPI) process. By choosing different pump lines the mean rotational energy of HBr+ was varied from 1.4 to 25.1 meV. All experiments were performed in a linear ion guide apparatus. Collision energies considered ranged from 0.28 to 0.85 eV (c.m.).
At the highest collision energies the cross section decreases by about 60% from 10 Ų to 4 Ų with increasing rotational energy. In contrast to that, at the lowest collision energies the cross section was about 2.5 Ų, independent of the rotational energy. Two explanations for these effects are presented. Further on the exothermic proton transfer reaction of HBr+ ions with CO was analyzed. Here the rotational excitation leads to a decrease of about 30% of the cross section, independent of the collision energy.
