  A chopper wheel construct is used to shorten the duration of a molecular beam
to 13 microseconds. Molecular beams seeded with NO or with Br2 and an initial
pulse width of greater or equal to 200 microseconds were passed through a
spinning chopper wheel, which was driven by a brushless DC in vacuo motor at a
range of speeds, from 3,000 rpm to 80,000 rpm. The resulting duration of the
molecular-beam pulses measured at the laser detection volume ranged from 80
microseconds to 13 microseconds, and was the same for both NO and Br2. The
duration is consistent with a simple analytical model, and the minimum pulse
width measured is limited by the spreading of the beam between the chopper and
the detection point as a consequence of the longitudinal velocity distribution
of the beam. The setup adopted here effectively eliminates buildup of
background gas without the use of a differential pumping stage, and a clean
narrow pulse is obtained with low rotational temperature.
