Spatial deorientation of upper-Stark-state-selected supersonic beams of CH3F, CH3Cl, CH3Br, and CH3I

The spatial scrambling of upper-Stark-state (KMj<0)-selected beams of CH3F, CH3Cl, CH3Br, and CH3I in held-free space has been investigated. It has been proposed that the mechanism for spatial deorientation as the electric-field strength is reduced to zero is the change in precessional frequency and loss of spatial direction as the total angular momentum J decouples from the collapsing electric field and couples with the nuclear spin. Supersonic beams were quantum-state selected in a hexapole inhomogeneous electric field and directed between a pair of parallel field plates before being focused through a second hexapole field to a quadrupole mass spectrometer detector. Exposure of the beam to zero field in the parallel-plate region leads to an attenuation of the beam signal relative to the non-zero-field case due to defocusing of newly formed lower Stark states and KM=0 states in the second hexapole field. This phenomenon can be used to determine the effect of field strength on the orientation of upper Stark states within the beam. The beam signal at the detector was shown to remain constant for uniform field strengths greater than approximately 3 V cm(-1), with a signal attenuation of around 40% relative to this level at zero field. Attempts were made to measure the mean lifetime for spatial scrambling by pulsing the uniform field to ground potential for increasing intervals and observing the beam attenuation. However, these measurements were complicated by the effect of the beam velocity distribution on the signal and it was found that reproducible values could not be obtained, though the results of these experiments are consistent with lifetimes lying in the expected range from 100 to 300 mu s. [S1050-2947(99)00710-6].