Rare gas dimers in a nozzle beam

M. P. Sinha, A.J. Schultz, Richard N. Zare

By spectroscopically analyzing the white light and laser-induced fluorescence excited in a nozzle beam of Na2 molecules, we have measured the population distribution of the (v,J) levels of the ground state. The Na2 molecules are produced in nozzle beams with various stagnation pressures (50–240 torr) of alkali metal and with nozzles of different throat diameters (0.12–0.50 mm). We find at a stagnation pressure of 50 torr and a nozzle diameter of 0.5 mm a Boltzmann distribution characterized by a vibrational temperature of 153 ± 5°K and a rotational temperature of 55± 10°K. Beams under different stagnation conditions have essentially the same internal state distribution. Studies of K2 dimers produced in a nozzle beam with various stagnation pressures (20–300 torr) and a 0.25 mm nozzle throat diameter also show cooling in both vibrational and rotational modes. A search is made for atomic fluorescence arising from the photodissociation of dimers in high vibrational levels of the ground state. No evidence is found for the presence of vibrationally-excited dimers in the nozzle beam.

Fujio Shimizu

Robert J. Gordon, Y. T. Lee, Dudley R. Herschbach

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.

Donald Harris Levy

Bas van de Meerakker

Matthias Hillenkamp, Sharon Keinan, Uzi Even

Supersonic expansions of pure and seeded rare gases have been investigated experimentally, measuring the translational and rotational temperatures. The lowest achievable translational temperature in the jet depends on both gas properties as well as on experimental boundary conditions like nozzle shape and nozzle–skimmer distance. We show that there is a limit to the lowest temperature achieved, under practical conditions, set by condensation in the jet. A large cluster binding energy enhances the formation of clusters and they release their condensation energy into the beam. The spatial confinement of the jet extends to long distances, and is sensitive to the shape of the nozzle. The confined jet forms a narrow cone of high intensity, and results in increased collision probability and cluster formation.