Collision-induced double resonance studies of HN+2 and HCN

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

We have investigated collision-induced rotational transitions of HN+2 and HCN using infrared-microwave four-level double resonance spectroscopy. These two isoelectronic molecules were studied in collisions with He, Ar, and N2. For all cases studied, we have observed that the collision-induced rotational transitions exhibit collisional "selection rules.'' The selection rules can be explained using the symmetry properties (i.e., parity) of the dominant terms in the interaction potential. This represents the first observation of selection rules for rotational energy transfer of a molecular ion. This study has allowed us to directly compare the difference between ion–neutral and neutral–neutral collisions which cause rotational transitions. We have experimentally observed that ion–neutral and neutral–neutral collisions differ because of the presence of the Langevin force in the ion–neutral interaction potential, which has two unique effects. The Langevin force produces a charge-induced dipole in the collision partner which is parallel to the ion's electric field. This charge-induced dipole interacts with the electric charge of the molecular ion which creates an attractive force between the ion and neutral. This interaction therefore decreases the ion–neutral distance and produces strong collisions which randomizes the rotational states. The second effect occurs when the molecular ion has a permanent electric moment. The charge-induced dipole in the collision partner will interact with an electric moment of the molecular ion creating a long-range interaction. For HN+2, a molecular ion with a permanent dipole moment, this interaction produces "dipole-type'' collisional selection rules.

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