The mass dependence of the jet formation in laser-produced particle beams

The jet formation in laser-produced particle beams has been investigated as a function of the atomic mass of the target. The plasma was produced by an obliquely incident Q-switched pulse (tau = 5 ns and lambda =1.06 mu m) of a Nd:YAG laser focused onto a relatively large area of about 5 x 10(-4) cm(2) at an energy density of 26 J cm(-2). Angular emission distributions of absolute particle numbers have been measured for Al-27, Ti-48, Ni-59, Mo-96, Ag-108, Sn-119 and Ta-181 targets for identical laser-target conditions. It is found that the measured angular emission profile and the number of particles ablated depend systematically on the atomic mass M. if the angular behaviour is described as usual in terms of a two-component structure by the superposition of a smooth cosine-like background distribution and an additional steep emission structure which is approximated by a cos(n) function, it is found that the exponent n gradually increases on going from Al to Ta, closely following a M-1/2 fit. Since the degree of ionization of the expanding plasma is relatively low, the observed angular behaviour is essentially that of the neutral particles.