Controlling the Internal Excitation of H3 + Produced in a Duoplasmatron Ion Source

Abstract We have explored a combination of two methods to control the internal excitation of H 3 + produced in a duoplasmatron ion source. The H 3 + was formed starting from a gas of H 2 . The first control method varied the H 2 pressure in the ion source to collisionally relax any internally excited (H 3 + ) * . The second method added Ar to the source to chemically destroy (H 3 + ) * with internal excitation energies E int ≥ 0.57 eV, using the endoergic reaction H 3 + + Ar → ArH + + H 2 . To infer the H 3 + internal temperature, T int , representative of internal excitation under the hypothesis of thermodynamic equilibrium, we used merged-beams rate coefficient measurements of the endoergic deuterating reaction H 3 + + D → H 2 D + + H 2 and a semi-empirical theoretical model. We found that using collisional relaxation alone, we could vary T int over the range ≈ 1300 - 2400 K. Combining collisional relaxation and chemical destruction, we reduced the minimum to T int ≈ 1130 K. Over this temperature range, the fraction of H 3 + where all vibrational modes are in their v = 0 level varies from ≈ 0.88 at the lowest temperature to ≈ 0.44 at the highest temperature. This combination of cooling methods offers a potentially powerful means for studying reactive scattering processes as a function of the internal excitation of the H 3 + .