Experimental investigation of volume Li- production

An experimental investigation of dissociative attachment in lithium is described. Using laser excitation of a supersonic lithium beam a population of metastable vibrationally-excited lithium molecules has been created in collision-free conditions. Electron-ion pairs have been created by three-step photoionization of lithium atoms in the beam, and the ion signal monitored by pulsed extraction of the Li+ ions. By reversal of the extraction pulse lithium negative ions have been sought at the level of 1 in 2 x 104 electrons attaching to the vibrationally-pumped lithium molecules. The interaction time for attachment is limited by free plasma expansion at the ion acoustic velocity. We have set an upper bound on the attachment rate constant to lithium molecules inu* = 11 andu* = 13 states of k < or approx. = 1 x 10-8 cm3 sec-1, corresponding to an attachment cross section sigma< or approx. = 3 x 10-16 cm2