A detector for state-resolved molecular beam experiments using optical fibres

A detector suitable for state-resolved molecular beam analysis and for the determination of state-to-state differential cross-sections in crossed-beam experiments is described. It detects molecules via laser-induced fluorescence. A single optical fibre and a fibre bundle provide a flexible link between the laser and a photomultiplier outside a vacuum tank. Internal state resolution is achieved by tuning the laser to an appropriate transition frequency. The angular resolution is limited by the smaller of the probe laser diameter and the width of the collimating slit in the detector. The overall detection sensitivity D for molecules in a specific rovibronic state depends on the optical transition probability and the power of the probe laser. A typical value for alkali molecules is D >- 10-2. The typical background signal is 1-10 counts s-1. This allows the detection of a flux of particles in a specific level of 50-500 particles mm-2 s-1 with a signal-to-background ratio of unity since fluorescence is collected for an area of 2 mm2. Representative measurements involving a sodium molecular beam are shown. (author)