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The Journal of Chemical Physics

Stationary flow conditions in pulsed supersonic beams

Wolfgang Christen

Published Oct 17, 2013• 13 citations

Abstract

We describe a generally applicable method for the experimental determination of stationary flow conditions in pulsed supersonic beams, utilizing time-resolved electron induced fluorescence measurements of high pressure jet expansions of helium. The detection of ultraviolet photons from electronically excited helium emitted very close to the nozzle exit images the valve opening behavior-with the decided advantage that a photon signal is not affected by beam-skimmer and beam-residual gas interactions; it thus allows to conclusively determine those operation parameters of a pulsed valve that yield complete opening. The studies reveal that a "flat-top" signal, indicating constant density and commonly considered as experimental criterion for continuous flow, is insufficient. Moreover, translational temperature and mean terminal flow velocity turn out to be significantly more sensitive in testing for the equivalent behavior of a continuous nozzle source. Based on the widely distributed Even-Lavie valve we demonstrate that, in principle, it is possible to achieve quasi-continuous flow conditions even with fast-acting valves; however, the two prerequisites are a minimum pulse duration that is much longer than standard practice and previous estimates, and a suitable tagging of the appropriate beam segment.

DOI: 10.1063/1.4824155

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We describe a generally applicable method for the experimental determination of stationary flow conditions in pulsed supersonic beams, utilizing time-resolved electron induced fluorescence measurements of high pressure jet expansions of helium. The detection of ultraviolet photons from electronically excited helium emitted very close to the nozzle exit images the valve opening behavior-with the decided advantage that a photon signal is not affected by beam-skimmer and beam-residual gas interactions; it thus allows to conclusively determine those operation parameters of a pulsed valve that yield complete opening. The studies reveal that a "flat-top" signal, indicating constant density and commonly considered as experimental criterion for continuous flow, is insufficient. Moreover, translational temperature and mean terminal flow velocity turn out to be significantly more sensitive in testing for the equivalent behavior of a continuous nozzle source. Based on the widely distributed Even-Lavie valve we demonstrate that, in principle, it is possible to achieve quasi-continuous flow conditions even with fast-acting valves; however, the two prerequisites are a minimum pulse duration that is much longer than standard practice and previous estimates, and a suitable tagging of the appropriate beam segment.

A synchronous time-of-flight (TOF) technique is employed to make velocity distribution measurements in pulsed supersonic free-jet expansions. For helium expansions, the flow attains a steady–state condition with a terminal Mach number of 80, on a 50-μs time scale. The transient behavior is primarily due to the mechanical action of the valve. For a 650-μs-wide pulse, greater than 95% of the atoms issuing from the pulsed nozzle can be described by the steady-state parameters. However, the velocity dispersion increases at both the leading and trailing edges of the pulse. This dispersion can be quantitatively described by incorporating a time-dependent Mach number into the standard steady-state flux velocity distribution function.

Supersonic molecular beams are used in several scientific fields ranging from surface science to the spectroscopy of molecules, nanoparticles, and even viruses. It is therefore of interest to understand the fundamental properties of the expansion and resulting beam. Here, we present a study of the change in the most probable velocity with reservoir pressure, using a so-called Even-Lavie pulsed source with a 200-$\textmu{}\text{m-diam}$ nozzle. Experiments were carried out in the throughput range

{p}_{0}d=400--2000\phantom{\rule{0.28em}{0ex}}\mathrm{mbar}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}

, where

{p}_{0}

is the reservoir stagnation pressure and

d

is the nozzle diameter. Experiments were done on both neutral and metastable helium beams. A two-detector measurement setup was used to account for the valve opening delay. The measurements are modeled with theory which has hitherto only been applied to a

{p}_{0}d

regime up to 190

\text{mbar}\phantom{\rule{0.16em}{0ex}}\text{cm}

, showing good agreement.

The supersonic short-pulse helium atomic beam and its metastables were studied. A pulsed high-density helium atomic beam was ejected from a nozzle, collimated by a skimmer and then flowed through a penning-type discharge device. After discharging, the metastable density of the pulsed helium atomic beam was up to 1.24 × 10 11 cm −3 . Excellent symmetry of atomic beam profile was obtained. The deviation of angle of the atomic beam with respect to the axis was less than 0.14°. The mean diffusion angle of the pulsed beam was 4°, and a mean velocity of atomic beam was up to 1350 ms −1 . Laser induced fluorescence of metastable atoms has been studied.

Recent merged neutral beam experiments have introduced the possibility of measuring reactive collisions in the cold regime down to 10 mK. The lowest temperature attained in these experiments cannot be explained using the standard formalism developed for crossed molecular beam scattering. These low temperatures become accessible because pulsed supersonic beams develop a correlation in velocity-position space during free propagation such that the local velocity standard deviation decreases. This effect is responsible for a reduction in the attainable collision energy by more than 2 orders of magnitude along with an order of magnitude improvement in the resolution. We show that supersonic nozzles with short pulsed opening durations compared to the time-of-flight, such as the Even-Lavie valve, have a clear advantage in achieving low collision energies with improved resolution. We discuss possible improvements in the energy resolution by varying the detection time duration.

We describe a pulsed rotating supersonic beam source, evolved from an ancestral device [M. Gupta and D. Herschbach, J. Phys. Chem. A 105, 1626 (2001)]. The beam emerges from a nozzle near the tip of a hollow rotor which can be spun at high-speed to shift the molecular velocity distribution downward or upward over a wide range. Here we consider mostly the slowing mode. Introducing a pulsed gas inlet system, cryocooling, and a shutter gate eliminated the main handicap of the original device in which continuous gas flow imposed high background pressure. The new version provides intense pulses, of duration 0.1-0.6 ms (depending on rotor speed) and containing ∼10(12) molecules at lab speeds as low as 35 m/s and ∼10(15) molecules at 400 m∕s. Beams of any molecule available as a gas can be slowed (or speeded); e.g., we have produced slow and fast beams of rare gases, O(2), Cl(2), NO(2), NH(3), and SF(6). For collision experiments, the ability to scan the beam speed by merely adjusting the rotor is especially advantageous when using two merged beams. By closely matching the beam speeds, very low relative collision energies can be attained without making either beam very slow.