Time-of-Flight Velocity Analysis of Atomic and Molecular Beams

Abstract

A systematic analysis of the time‐of‐flight (TOF) method for measuring the distribution of speeds in molecular beams is presented. The conditions required for resolving a given beam speed distribution are evaluated by calculating the TOF signal for a finite open time τ of the beam shutter. The halfwidth of the TOF signal Δt exceeds the ideal half width Δt⁰(for τ→0) by not more than 2.5%, if the resolution R=Δt/τ is equal to or larger than 5. The peak signal amplitude Um is not less than 0.8 U₀/R, where U₀ is the signal corresponding to the unchopped beam (τ→∞). Design criteria for beam chopper and detection system are discussed in terms of the resolution, the range of speeds to be measured, and the optimum signal‐to‐noise ratio. A TOF system used for the detection of high‐intensity molecular beams with large speed ratio, as well as for low‐intensity scattered beams, is described and typical experimental results are presented.