  Cold, velocity-controlled molecular beams consisting of a single quantum
state promise to be a powerful tool for exploring molecular scattering
interactions. In recent years, Stark deceleration has emerged as one of the
main methods for producing velocity-controlled molecular beams. However, Stark
deceleration is shown not to be effective at producing a molecular beam
consisting of a single quantum state in many circumstances. Therefore, quantum
state purity must be carefully considered when using Stark decelerated beams,
particularly in collision experiments where contributions from all quantum
states must be addressed.
