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

Abstract

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: 12 C, 27 Al, 48 Ti, 59 Ni, 96 Mo and 181 Ta. The plasma was produced by obliquely incident Q-switched pulses (τ = 5 ns and λ = 1.06 µm) 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 a cos n fit function, the distribution of ions with q 2 can already be well fitted by a cos n 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 A 3/4 law.

The fundamental determining factor of angular emission of multiple charged ions ejected by laser ablation of different metals and their binary alloys

“…This observation is in complete agreement with the investigations carried out by the Thum-Jager and Rohr [4] with Ti, Ta, Ni, and C. The observed increase in the value of exponent n with the increase in ionization state of LPP ions emitted from a given target means that the angular distribution of LPP ions becomes narrower as the ionization state increase from þ1 to þ4. It is because ions with higher ionization state possess more energy than lower ionization state since the acceleration of ions is proportional to their charge.…”

“…They found that the value of exponent n increases with increase in atomic mass M following the relation n ¼ a þ bM 1/2 , where a and b are numerical constants. The angular emission profile for six mono-atomic targets (C, Al, Ti, Ni, Mo, and Ta) studied by Thum-Jager and Rohr [4] lends support to the atomic mass school of thought. However, they found that the mass dependence of exponent n approximately follows M 3/4 e law.…”

Ion acceleration from intense laser-generated plasma: methods, diagnostics and possible applications

“…Due to the low thickness of the target the ion emission yield is low. Moreover the ion emission is not isotropic and the angular distribution of emitted ions is centred on the normal to the target surface with an aperture decreasing with the ion atomic mass and with the ion charge state, as reported in literature [16]. In thin fi lms the maximum ion acceleration is obtained for an optimal thickness at which the plasma electron density has a maximum value to maximize the electric fi eld accelerating ions.…”

Generation of intense streams of metallic ions with a charge state up to 10+ in a laser ion source

“…In addition, the increase of the exponent p with ion mass A approximately follows A 1/2 law, as published elsewhere. 15,16 A wider distribution of neutral atom emission was recorded for Ni and Cu and a narrower one for W and Pb. 3 The distribution of neutrals was even broader than that for the lightest ions.…”