Differential scattering experiments with the crossed-beam method have been performed to study vibrational excitation of HF and HCl by electron impact at very low collision energies. The excitation functions for nu =1 and nu =2 have been measured from threshold to some eV above. In each case, an unusually strong and sharp threshold peak is observed with a broad resonance region following thereafter. The absolute cross sections reach values of the order of 10-15 cm2 for the threshold peaks in the nu =1 channel. The angular distributions are isotropic in the whole resonance region. The threshold resonances are interpreted as quasi-bound 2 Sigma + states in the dipole field possibly correlated with H*+X-. The broad resonance region is attributed to a very short-lived resonance state which may be the HX- 2 Sigma + state correlated with ground-state fragments H+X-.

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.

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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 (τ = 5 ns and λ = 1.06 µm) of a Nd:YAG laser focused onto a relatively large area of about 5 × 10 −4 cm 2 at an energy density of 26 J cm −2 . Angular emission distributions of absolute particle numbers have been measured for 27 Al, 48 Ti, 59 Ni, 96 Mo, 108 Ag, 119 Sn and 181 Ta 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.