Measurement of the recombination of photoproduced PAH ions

Abstract

A new technique, Flowing Afterglow with Photo Ions – FLAPI, has been developed for measuring the rate coefficient for the recombination of complex ions with electrons. The method is based on the FALP-MS apparatus at the Université de Rennes I. A helium plasma is generated by a microwave discharge in a He buffer gas and downstream a small amount of argon gas is injected to get rid of helium metastables. A very small amount of neutral PAH molecules is added to the afterglow plasma by evaporation from a plate coated with the PAH to be studied. PAH ions are then produced by photoionization of the parent molecule using a pulsed UV laser (157 nm). The laser beam is oriented along the flow-tube and so a constant spatial concentration of photoions is obtained. The electron concentration along the flow-tube is measured by means of a movable Langmuir probe. The decay of the ion concentration in time is measured at a fixed position using a quadrupole mass spectrometer which is triggered by the laser pulse. Anthracene ion recombination has been studied using this technique and we have obtained the preliminary recombination rate coefficient (1.1 ± 0.5) × 10−6 cm3 s−1.