Laser-induced fluorescence studies of ion collisional excitation in a drift field: Rotational excitation of N+2 in helium

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Abstract
Initial results are presented for a new method of studying collisional excitation and deactivation processes of molecular ions. Translationally excited ions are prepared in the uniform electric field of a drift tube. Collisions with the inert buffer gas lead to rotational excitation (T‐R). Laser‐induced fluorescence (LIF) is used as a direct optical probe of the internal states of N+2 using the B 2Σ+u–X 2Σ+g transition at 391.4 nm. In this initial experiment, rotational excitation is observed for N+2 in collisions with helium at energies up to 0.054 eV (c.m.). The rotational state distribution can be described by a Boltzmann temperature corresponding to the center‐of‐mass collision energy, in good agreement with theory. Approximately ten collisions or less are required to obtain full equilibration of the rotational distribution. Applications of this new technique to the interpretation of ion–molecule reaction rates in drift tubes and to the study of ion‐neutral energy transfer processes are discussed.