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THE SIGNIFICANCE OF COMPRESSION SHOCKS IN THE PRODUCTION OF INTENSE MOLECULAR BEAMS WITH NOZZLES

K. Bier, O. F. Hagena

Published Nov 1, 1962• 0 citations
OpenAlex

Abstract

S>In the production of intensive molecular beams using nozzles, compression shocks must be avoided as much as possible since they cause a broadening of the velocity distribution and a reduction of the particle current density of the molecular beam. The conditions for the occurrence and the effect of such compression shocks was investigated by measurement of the velocity distribution and the beam intensity for the case when a convergent nozzle is used for the beam production. It was shown that under given conditions a strong compression shock connected with the collision system of the freely expanding gas beam occurs. This compression reduces the beam intensity in single-molecule beams by an order of magnitude and in condensed beams by two orders of magnitude. When this strong compression shock released from the first beam stop is avoided by a suitable selection of the research conditions, weak compression shocks emanating from the peeler can still occur. Possible reduction of this weak compression shock is discussed. (tr-auth)

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The Journal of Physical Chemistry A•Nov 30, 1999•108 citations
S>In the production of intensive molecular beams using nozzles, compression shocks must be avoided as much as possible since they cause a broadening of the velocity distribution and a reduction of the particle current density of the molecular beam. The conditions for the occurrence and the effect of such compression shocks was investigated by measurement of the velocity distribution and the beam intensity for the case when a convergent nozzle is used for the beam production. It was shown that under given conditions a strong compression shock connected with the collision system of the freely expanding gas beam occurs. This compression reduces the beam intensity in single-molecule beams by an order of magnitude and in condensed beams by two orders of magnitude. When this strong compression shock released from the first beam stop is avoided by a suitable selection of the research conditions, weak compression shocks emanating from the peeler can still occur. Possible reduction of this weak compression shock is discussed. (tr-auth)
Supersonic beams are a prevalent source of cold molecules used in the study of chemical reactions, atom interferometry, gas-surface interactions, precision spectroscopy, molecular cooling, and more. The triumph of this method emanates from the high densities produced in relation to other methods; however, beam density remains fundamentally limited by interference with shock waves reflected from collimating surfaces. We show experimentally that this shock interaction can be reduced or even eliminated by cryocooling the interacting surface. An increase of nearly an order of magnitude in beam density was measured at the lowest surface temperature, with no further fundamental limitation reached. Visualization of the shock waves by plasma discharge and reproduction with direct simulation Monte Carlo calculations both indicate that the suppression of the shock structure is partially caused by lowering the momentum flux of reflected particles and significantly enhanced by the adsorption of particles to the surface. We observe that the scaling of beam density with source pressure is recovered, paving the way to order-of-magnitude brighter, cold molecular beams.
Intense beams of cold and slow molecules are produced by supersonic expansion out of a rapidly rotating nozzle, as first demonstrated by M. Gupta and D. Herschbach [J. Phys. Chem. A 103, 10670 (1999); 105, 1626 (2001)]. An improved setup is presented that is able to accelerate or decelerate cold atomic and molecular beams by up to 500 m/s. Technical improvements are discussed and beam parameters are characterized by detailed analysis of time-of-flight density distributions. The possibility of combining this beam source with electrostatic fields for guiding polar molecules is demonstrated.
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.
This paper outlines new ways for separating the different channels of optical dephasing of molecules in beams. It is shown that both the population loss and the optical phase relaxation rates can be obtained under collisionless conditions (molecular beams). These dephasing rates, which measure directly the homogeneous width of the prepared resonance, can be obtained with an energy resolution of better than one part in 108. Under these conditions the solution of the density matrix equations of motion is given for effusive and nozzle beams which represent a statistically open ensemble. The results, which we applied to our recent measurements of optical T1 and T2, are discussed in different limits of power broadening, beam geometry, detector characteristics, temperature of the oven (or material container), and the transit time the molecules of different velocities spend in the laser beam. Furthermore, we indicate that the treatment of coherent transients in beams utilizing conventional Bloch equations is not valid since there is a loss of optically excited molecules. Finally, we discuss possible differences between small and large molecules when they undergo radiationless (or reactive) processes following the selective laser excitation (∼10 kHz–10 MHz).
The flow velocity of gas emerging from a supersonic nozzle mounted on a high-speed rotor can be largely canceled by the rotor velocity, thereby producing an intense beam of molecules traveling in a vacuum with translational speeds slowed to a few tens of meters per second. Centrifugal action significantly enhances the supersonic character of the gas flow from the rotating nozzle, further narrowing the spread of velocities in the emerging beam. These features are demonstrated by model calculations and experimental results for beams of Xe and for O 2 and CH 3 F seeded in Xe.