A new pulsed Laval nozzle apparatus with vacuum ultraviolet (VUV) synchrotron
photoionization quadrupole mass spectrometry is constructed to study low-temperature radicalneutral
chemical reactions of importance for modeling the atmosphere of Titan and the outer
planets. A design for the sampling geometry of a pulsed Laval nozzle expansion has been
developed that operates successfully for the determination of rate coefficients by time-resolved
mass spectrometry. The new concept employs airfoil sampling of the collimated expansion with
excellent sampling throughput. Time-resolved profiles of the high Mach number gas flow
obtained by photoionization signals show that perturbation of the collimated expansion by the
airfoil is negligible. The reaction of C2H with C2H2 is studied at 70 K as a proof-of-principle
result for both low-temperature rate coefficient measurements and product identification based
on the photoionization spectrum of the reaction product versus VUV photon energy. This
approach can be used to provide new insights into reaction mechanisms occurring at kinetic rates
close to the collision-determined limit.