Laser measurements of ion velocities in plasmas

Our present work addresses the behaviour of the dominant ground state N₂⁺ ion in a RF plasma struck in molecular nitrogen. We use a conventional parallel plate reactor in a plasma volume of ca. 1.7 l. Ions are excited by the output of a pulsed dye laser system tuned to a specific rotationally resolved line within the B-X transition in N₂⁺, probing the N´´=6 level in ν´´=O of the ground electronic state. Fluorescence is collected in a direction perpendicular to the laser beam axis as a function of the wavelength of excitation, resulting in a Doppler profile of the transition, and thus giving information on the velocity components of the absorbing ions in the direction of the laser beam. We carry out two types of experiment, with the beam either parallel or perpendicular to the driven electrode. In addition, the Doppler profile gives the ions´ velocity components parallel to the electrode