Speed ratios v/\ensuremath{\Delta}v greater than 1000 and temperatures less than 1 mK have been achieved for the first time in a gas expansion into vacuum. The pulsed-beam source used in these experiments had an open time of 150 \ensuremath{\mu}sec, a nozzle diameter of 0.125 mm, and was operated with pure He gas at 294 K and pressures between 34 and 136 atm. Speed distributions for the collimated He beam were measured by time of flight. It is possible that such temperatures may be low enough to probe the low-energy limiting behavior predicted by Wigner in 1948 for the single-collision quantum dynamics of atoms and molecules.
Speed ratios v/\ensuremath{\Delta}v greater than 1000 and temperatures less than 1 mK have been achieved for the first time in a gas expansion into vacuum. The pulsed-beam source used in these experiments had an open time of 150 \ensuremath{\mu}sec, a nozzle diameter of 0.125 mm, and was operated with pure He gas at 294 K and pressures between 34 and 136 atm. Speed distributions for the collimated He beam were measured by time of flight. It is possible that such temperatures may be low enough to probe the low-energy limiting behavior predicted by Wigner in 1948 for the single-collision quantum dynamics of atoms and molecules.
Micrometer-sized nozzles and skimmers made from drawn glass tubes are described and tested for the production of highly monoenergetic He atom beams. Glass nozzles with diameters between 1 μm and 4 μm when operated at He source stagnation pressures of up to 1000 atm provide intense beams with measured speed ratios of S=50–100, in good agreement with the predicted behavior scaled from nozzles with larger openings. Miniature glass skimmers with diameters as small as 3 μm were also successfully tested with conventional 10 μm diameter nozzles. These miniature nozzle-beam sources can be used to greatly reduce the size of present-day He-atom surface-scattering time-of-flight spectrometers and to reduce the number of vacuum stages and the size of vacuum pumps. They also open up new experimental possibilities as illustrated by measurements of the spatial profiles of seeded nozzle beams.
Intense alkali beams containing up to 30% mole fraction of K2, Rb2, or Cs2 have been generated from a supersonic nozzle expansion, with oven temperatures up to ∼ 900°K (corresponding to alkali vapor pressures up to ∼ 350 torr) and nozzle temperatures ∼ 50–300°K higher. The total beam intensity obtained reached ∼ 5 × 1017 particles sec−1·sr−1. An inhomogeneous deflecting magnet was used to separate the dimer and atom components of the beam, and the variation of dimer concentration with oven pressure and nozzle temperature were studied for a variety of nozzle sizes (throat diameters 0.04–0.15 mm). The velocity distributions of the atom and dimer components were also measured and found to be quite similar; the peaks occur at velocities up to 70% higher than the most probable atom velocity in the oven, and the widths are very narrow, corresponding to random translational temperatures as low as ∼ 30°K, and Mach numbers up to ∼ 15. A simple one-dimensional theory of dimerization which takes into account vibrational relaxation and heat of condensation is found to be consistent with the data. It is shown that the observed velocity distributions and the variation of the dimer yield with pressure and nozzle diameter imply that most of the heat of condensation does not contribute to the beam acceleration but goes into vibrational excitation of the dimer molecules. A nominal value of ∼ 3 × 10−30 cm6 sec−1 at ∼ 600°K is obtained for the recombination rate constant for Cs+Cs+Cs→Cs2+Cs. A reduced variable plot is provided which enables the theory to be applied to nozzle expansions of any substance for which the chemical recombination is governed by three-body collisions.
The formation of dimers, trimers, and tetramers in a free jet cryogenic expansion of He4 atoms has been studied by diffraction from a nanostructure transmission grating. The final average velocities, speed ratios and ambient temperatures of the expansions for source temperatures of 30, 12, and 6 K and source pressures between 0.1 and 80 bar were determined from time-of-flight measurements of the He atoms. The final mole fractions of the He2, He3, and He4 clusters in the beam were determined from the intensities of the corresponding first-order diffraction peaks for the same range of source conditions. For each source temperature, the final mole fractions of these small clusters first rise, pass through a maximum and then decrease with increasing source pressure. The processes leading to the formation of these clusters are simulated with a kinetic model that allows for density and temperature changes in the expanding beam. The best-fit three-body recombination rate constant for dimer formation increases by over three orders of magnitude as the thermal energy decreases from 1 K to 1 mK, in qualitative agreement with recent theories.
We have produced a pulsed supersonic expansion of helium with an effective pre-expansion temperature below 100 K. This low initial gas temperature is achieved by cooling a pulsed gas valve and gas channel with liquid nitrogen. The resulting supersonic helium beam has a translational velocity that is approximately 55% that of the room-temperature source and an internal temperature of 270 mK. The increased Mach number for a cold supersonic expansion agrees with theoretical predictions and reflects the increased cross section of helium at low collision energies.
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.
Cryogenically cooled buffer gas beam sources of the molecule thorium monoxide (ThO) are optimized and characterized. Both helium and neon buffer gas sources are shown to produce ThO beams with high flux, low divergence, low forward velocity, and cold internal temperature for a variety of stagnation densities and nozzle diameters. The beam operates with a buffer gas stagnation density of \\(\\sim 10^{15}-10^{16}\\) cm\\(^{-3}\\) (Reynolds number \\(\\sim 1-100\\)), resulting in expansion cooling of the internal temperature of the ThO to as low as 2 K. For the neon (helium) based source, this represents cooling by a factor of about 10 (2) from the initial nozzle temperature of about 20 K (4 K). These sources deliver \\(\\sim 10^{11}\\) ThO molecules in a single quantum state within a 1-3 ms long pulse at 10 Hz repetition rate. Under conditions optimized for a future precision spectroscopy application [A C Vutha et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 074007], the neon-based beam has the following characteristics: forward velocity of 170 m/s, internal temperature of 3.4 K, and brightness of \\(3 \\times 10^{11}\\) ground state molecules per steradian per pulse. Compared to typical supersonic sources, the relatively low stagnation density of this source, and the fact that the cooling mechanism relies only on collisions with an inert buffer gas, make it widely applicable to many atomic and molecular species, including those which are chemically reactive, such as ThO.