Angular emission distributions of neutrals and ions in laser ablated particle beams

The present work represents investigations of angular emission distributions in laser-produced particle beams resolved for the different ion groups (up to q=4) and the neutral particle component. The measurements are for a spectrum of target masses: 12C, 27Al, 48Ti, 59Ni, 96Mo and 181Ta. The plasma was produced by obliquely incident Q-switched pulses (Τ=5 ∼ns and Λ=1.06∼MU m) of a Nd-YAG laser focused to energy densities ranging from about 20 to 180∼J∼cm-2. For the first time the results reveal in detail that the emission distributions systematically depend on the degree of ionization of the particles in the cloud. While for the neutral particles the angular emission is always dominated by a broad background with an additional, but less pronounced, peaked component, the background component rapidly but continuously diminishes with the increasing charge state of the ions. If, in the usual way, the emission distribution is approximated by the superposition of a cosine and and a cosn fit function, the distribution of ions with qgeq 2 can already be well fitted by a cosn function alone. It seems highly probable, that this behaviour essentially is a result of the recombination dynamics during the expansion. It was found that the effect holds for all atomic masses investigated, whereby the mass dependence of the exponent n for all species behaves alike, approximately following a A3/4 law. (author)