Spatial discrimination of Rydberg tagged molecular photofragments in an inhomogeneous electric field

A new approach to detecting the spatial and energetic distribution of photo dissociation products is demonstrated, in which an inhomogeneous electric f ield is used for spatially selective field ionization of Rydberg molecules. State-selected NO fragments from NO2 photodissociation in a supersonic bea m are excited to high-n Rydberg states and allowed to expand through the in homogeneous field provided by an octopole. The field in the octopole result s in the field ionization of those fragments that have moved to large radia l distances (similar to5 mm) from the beam axis, and therefore this device transmits only those Rydberg molecules with a low velocity component in the direction perpendicular to the beam. A detailed characterization of the pr operties of this "steradiancy analyzer" is carried out and factors limiting the energy resolution (currently approximately 100 cm(-1) at an excess ene rgy of 250 cm(-1)), along with possible improvements, are discussed in deta il. (C) 2001 American Institute of Physics.