A new technique, developed by physicists from the Max Planck Institute for Nuclear Physics, Heidelberg/Germany, and the Weizmann Institute of Science, Rehovot/Israel, has been used to measure the dissociative recombination rate of the hydrogen molecular ion with a free electron as a function of the initial vibrational quantum state (Science, vol. 281, 3 July 1998).
The measurement of molecular reaction rates for a specific process on a quantum state-to-state basis is usually difficult. Dissociative recombination, a process where a molecular ion captures a free electron and breaks into fragments, is one of the most important processes in various ionized gas environments such as laboratory plasmas, planetary ionospheres and interstellar molecular clouds.
A new method used for such studies relies on storage of a fast (MeV) beam of molecular ions in the Test Storage Ring (TSR) located at the Max Planck Institute for Nuclear Physics in Heidelberg. The TSR ring is one of the few heavy-ion storage rings existing in the world and allows for the long-time storage (seconds to minutes) of fast atomic or molecular ion beams; it is principally used for the investigation of laser, atomic and molecular physics. While stored, the beam can be merged with a powerful electron beam, allowing for the study of atomic and molecular recombination processes with high resolution.
The main obstacle in the study of molecular recombination processes is the fact that when molecular ions are produced in laboratory, their internal energy (which is usually described as vibrational and rotational energy) can be very high, while the molecular ions present in planetary ionospheres and interstellar clouds in contrast have very little internal energy. Since the dissociative recombination process is known to be very sensitive to the internal energy, the experimental studies of this reaction were hampered by the difficulty to produce large amounts of internally cold molecular ions; direct comparisons with theoretical calculations were basically impossible. Exploiting the storage capability of heavy-ion storage rings, it is possible to store the molecular ions for a time which is long enough to allow for the complete deexcitation of the initial vibrational excitation. The joint Max Planck/Weizmann team has been using the TSR ring for the study of the recombination process of vibrationally cold molecular ions for the last few years.
In the recent experiment published in the last issue of Science, the team has now reached another important milestone in the understanding of molecular recombination processes. Using molecular fragment imaging techniques, the scientists have succeeded in extracting the recombination rate as a function of the initial vibrational quantum state of the molecular ion. While, as a function of the storage time, the recombination process is studied by merging the initially hot molecular ion beam with the electron beam, the respective relative populations of vibrational states in the stored beam are measured using the Coulomb Explosion Imaging technique. This technique produces "images" of the distribution of the position of the nuclei inside the molecules, which are directly related to the wave function of the nuclei, and as such the method provides snapshots of the internal motion of the nuclei inside the molecule. As both the Coulomb Explosion Imaging and the recombination process are measured simultanenously on the same beam, it is thus possible to extract the recombination rate as a function of the initial vibrational quantum state of the molecular ions.
The experiment, performed for the isotopically equivalent hydrogen molecular ion (HD^+), shows that even for the simplest molecular ion deviations of more than an order of magnitude between theory and experiment exist for specific vibrational quantum states. The results demonstrate the need for further theoretical work in order to understand the influence of vibrational excitation on the dissociative recombination process and can be expected to guide future quantum chemistry calculations. The experimental technique is general and does not depend on specific level schemes which are required for laser manipulation of vibrational states. The only requirements are the knowledge of the vibrational wave function and a vibrational cooling time which is shorter than the useable lifetime of the ions in the storage ring. Most heteronuclear ions satisfy this criterion.
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