Analysis of the impacts of reactant molecular beam velocity spreads on product velocity resolution in crossed beam experiments

Owing to its convenience and accuracy in providing detailed information about quantum state populations and angular distributions of reaction products, the velocity map imaging technique has been extensively employed in crossed beam experiments for product detection. In crossed beam experiments, the two reactant beams always exhibit certain spreads in both magnitude and direction of the velocity, which will blur the scattered product images and lower the resolution of product detection. To systematically investigate the influence of reactant velocity spreads on the resolution of product detection in crossed beam experiments, a general model based on velocity vector analysis is constructed and presented here. The impacts of spreads of the magnitude and direction of the reactant velocities on the image resolution under three typical experimental conditions are analyzed, and ways for improving the resolution are briefly discussed. Guided by the above analysis, a fine adjustment of the Ar+ ion beam was performed, and the product image resolution for the charge transfer reaction Ar+(2P3/2) + N2 is effectively improved, particularly in the backward scattering region, which provides more accurate information on the reaction dynamics.