We demonstrate an approach for producing a high-flux beam of
7Li4He dimers by performing postnozzle seeding of a lithium beam into a supersonic helium expansion. The molecular beam has a longitudinal temperature of 13(6) mK and a continuous flux on the order of
1011 dimers/s confined within a solid angle of 0.16 sr. Monte Carlo simulations of molecular formation based on
ab initio quantum scattering calculations are carried out and compared to the experimentally observed molecular flux. Extensions of this work could lead towards a more quantitative understanding of few-body collision processes of alkali and helium atoms.
We demonstrate an intense, continuous cold atom beam generated via post nozzle seeding of a supersonic helium jet with
7Li atoms. The nozzle is cooled to about 4.4 K to reduce the forward velocity of the atoms. The atomic beam is brought to a focus 175 cm from the nozzle by a 10 cm bore diameter magnetic hexapole lens. Absorption and fluorescence imaging of the focus show a flux of
2.3(4)×1012 atoms/s, brightness of
4.1(7)×1019 m−2s−1sr−1, forward velocity of 211(2) m/s, and longitudinal temperature of 7(3) mK. Results agree with a Monte Carlo simulation of the seeding dynamics and a particle tracing simulation of the atom lens. We project that 10 times higher flux would be possible with improved vacuum system design. Our method should provide a useful high-brightness source for atom-optical and other atomic and molecular physics applications.
We present three-dimensional Monte Carlo simulations of the capture of 1000 K
7Li or 500 K
87Rb atoms by a continuous supersonic
4He jet and show that intense, cold alkali-metal beams form. The simulations use differential cross sections obtained from quantum scattering calculations of
7Li or
87Rb atoms with
4He atoms for relative collision energies between $k\ifmmode\times\else\texttimes\fi{}1$ mK and $k\ifmmode\times\else\texttimes\fi{}3000$ K, where
k is the Boltzmann constant. For collision energies higher than approximately $k\ifmmode\times\else\texttimes\fi{}4$ K the collisions favor forward scattering, deflecting the
7Li or
87Rb atoms by no more than a few degrees. From the simulations, we find that about 1% of the lithium atoms are seeded into the
4He jet, resulting in a lithium beam with a most probable velocity of about
210m/s and number densities on the order of ${10}^{8}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Simulations predict narrow yet asymmetric velocity distributions which are verified by comparing to fluorescence measurements of the seeded
7Li atoms. We find agreement between simulated and experimentally measured seeded
7Li densities to be better than 50% across a range of
4He flow rates. We make predictions for seeding efficiency and cooling of
87Rb by a supersonic
4He jet. The seeding efficiency for
87Rb is expected to be similar to
7Li.
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.
We describe the generation and properties of a cryogenic beam of
7Li
2 dimers from sublimation of a neon matrix where lithium atoms have been implanted via laser ablation of solid precursors of metallic lithium or lithium hydride (LiH). Different sublimation regimes lead to pulsed molecular beams with different temperatures, densities and forward velocities. With laser absorption spectroscopy these parameters were measured using the molecular
7Li
2 (R) transitions A$^1Σ_u^+(v'=4,J'=J''+1)\leftarrow
X^1Σ_g^+ (v''=0,J''=0,1,3)
.Inatypicalregime,sublimatingamatrixat16K,translationaltemperaturesof6−−8Kwithadriftvelocityof130m\,
s^{-1}$ in a free expanding pulsed beam with molecular density of 10
9 cm
−3, averaged along the laser axis, were observed. Rotational temperatures around 5--7 K were obtained. In recent experiments we were able to monitor the atomic Li signal -- in the D2 line -- concomitantly with the molecular signal in order to compare them as a function of the number of ablation pulses. Based on the data and a simple model, we discuss the possibility that a fraction of these molecules are being formed in the matrix, by mating atoms from different ablation pulses, which would open up the way to formation of other more interesting and difficult molecules to be studied at low temperatures. Such a source of cryogenic molecules have possible applications encompassing fundamental physics tests, quantum information studies, cold collisions, chemistry, and trapping.