New Method to Study Ion-Molecule Reactions at Low Temperatures and Application to the H2++H2→H3++H Reaction

Studies of ion-molecule reactions at low temperatures are difficult because stray electric fields in the reaction volume affect the kinetic energy of charged reaction partners. We describe a new experimental approach to study ion-molecule reactions at low temperatures and present, as example, a measurement of the H2++H2→H3++H reaction with the H2+ ion prepared in a single rovibrational state at collision energies in the range E_col /k_B =5-60 K. To reach such low-collision energies, we use a merged-beam approach and observe the reaction within the orbit of a Rydberg electron, which shields the ions from stray fields. The first beam is a supersonic beam of pure ground-state H₂ molecules and the second is a supersonic beam of H₂ molecules excited to Rydberg-Stark states of principal quantum number n selected in the range 20-40. Initially, the two beams propagate along axes separated by an angle of 10°. To merge the two beams, the Rydberg molecules in the latter beam are deflected using a surface-electrode Rydberg-Stark deflector. The collision energies of the merged beams are determined by measuring the velocity distributions of the two beams and they are adjusted by changing the temperature of the pulsed valve used to generate the ground-state H₂ beam and by adapting the electric-potential functions applied to the electrodes of the deflector. The collision energy is varied down to below E_col /k_B =10 K, that is, below E_col ≈1 meV, with an energy resolution of 100 μeV. We demonstrate that the Rydberg electron acts as a spectator and does not affect the cross sections, which are found to closely follow a classical Langevin-capture model in the collision energy range investigated. Because all neutral atoms and molecules can be excited to Rydberg states, this method of studying ion-molecule reactions is applicable to other reactions involving singly charged cations.