Section view of the pulsed Laval nozzle apparatus with quadrupole mass spectrometry and airfoil sampling geometry.
Comparison between (a) airfoil sampling in this work and (b) skimmer sampling implemented by Lee and co-workers.13 The pump-out ports in the source and detection regions are indicated: TP for turbomolecular pump, MP for mechanical pump, and DP for diffusion pump.
Orthographic view of the airfoil used for sampling in the apparatus described here. All dimensions are in inches.
Comparison of pulsed gas profiles obtained with airfoil and skimmer sampling. (a) C4H6 ion signal (1% 1,3-butadiene in N2 carrier gas) obtained by VUV synchrotron photoionization (10.6 eV) at a distance <1 mm after the pinhole of the airfoil. (b) Pulsed gas profile obtained with laser photoionization at a distance of 130 mm with 30° entrance angle and 6 cm long skimmer by Lee and co-workers.13 The ideal signals (black square pulse) are also indicated, which correspond to the duration of the voltage applied to the pulsed valves, i.e., 6.5 ms in this work and 5 ms by Lee and co-workers. It is clear that no “tail” is observed in the gas profile in (a) as opposed to (b). This residual effect is attributed to perturbation in the Laval expansion by thermalized gases that accumulate around the skimmer. No such effect is observed with the symmetric airfoil sampling device.
Predicted maximum k obs (s−1) that will be underestimated by less than 10% plotted as a function of the ionization point from the pinhole x (mm). The values are computed using Eq. (1) (see text) with T = 70 K and u = 675 m/s, which are typically observed with the M4 nozzle.
Time-resolved ion signal of m/z = 50 (C4H2 +) in the reaction of ethynyl radicals with acetylene. See text for explanation.
Beam velocity determination using the position of the maximum of photolysis product produced at the throat of the nozzle (see text). (a) Methyl radical transit time as a function of the distance between the exit of the Laval nozzle and the pinhole. (b) Distance from pinhole versus arrival time of methyl radicals formed at the throat, the slope of which yields the beam velocity. Error bars are given as ±0.01 ms for arrival times and ±0.005 m for distances.
Plot of k obs from exponential fits of C4H2 concentration profiles as a function of acetylene concentration.
Photoionization spectrum of m/z = 50 from the reaction of C2H with C2H2 (open circles). The black curve is a diacetylene PIE curve recorded by Goulay and co-workers.22
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