  Molecular collisions can be studied at very low relative kinetic energies, in
the milliKelvin range, by merging codirectional beams with much higher
translational energies, extending even to the kiloKelvin range, provided that
the beam speeds can be closely matched. This technique provides far more
intensity and wider chemical scope than methods that require slowing both
collision partners. Previously, at far higher energies, merged beams have been
widely used with ions and/or neutrals formed by charge transfer. Here we assess
for neutral, thermal molecular beams the range and resolution of collision
energy that now appears attainable, determined chiefly by velocity spreads
within the merged beams. Our treatment deals both with velocity distributions
familiar for molecular beams formed by effusion or supersonic expansion, and an
unorthodox variant produced by a rotating supersonic source capable of scanning
the lab beam velocity over a wide range.
