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arXiv:0806.2523 · Physical Review Letters

Electrostatic Extraction of Cold Molecules from a Cryogenic Reservoir

L. D. van Buuren, Christian Sommer, M. Motsch, Sebastian Pohle, Markus Schenk, Josef Bayerl, Pepijn W. H. Pinkse, Gerhard Rempe

Published Jan 21, 2009• 93 citations
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Abstract

We present a method which delivers a continuous, high-density beam of slow and internally cold polar molecules. In our source, warm molecules are first cooled by collisions with a cryogenic helium buffer gas. Cold molecules are then extracted by means of an electrostatic quadrupole guide. For ${\mathrm{ND}}_{3}$ the source produces fluxes up to $(7{\ifmmode\pm\else\textpm\fi{}}_{4}^{7})\ifmmode\times\else\texttimes\fi{}{10}^{10}\text{ }\text{ }\mathrm{\text{molecules}}/\mathrm{s}$ with peak densities up to $(1.0{\ifmmode\pm\else\textpm\fi{}}_{0.6}^{1.0})\ifmmode\times\else\texttimes\fi{}{10}^{9}\text{ }\text{ }\mathrm{\text{molecules}}/{\mathrm{cm}}^{3}$. For ${\mathrm{H}}_{2}\mathrm{CO}$ the population of rovibrational states is monitored by depletion spectroscopy, resulting in single-state populations up to $(82\ifmmode\pm\else\textpm\fi{}10)%$.

DOI
10.1103/physrevlett.102.033001

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We present a method which delivers a continuous, high-density beam of slow and internally cold polar molecules. In our source, warm molecules are first cooled by collisions with a cryogenic helium buffer gas. Cold molecules are then extracted by means of an electrostatic quadrupole guide. For ND3{\mathrm{ND}}_{3}ND3​ the source produces fluxes up to (7\ifmmode±\else\textpm\fi47)\ifmmode×\else\texttimes\fi1010  molecules/s(7{\ifmmode\pm\else\textpm\fi{}}_{4}^{7})\ifmmode\times\else\texttimes\fi{}{10}^{10}\text{ }\text{ }\mathrm{\text{molecules}}/\mathrm{s}(7\ifmmode±\else\textpm\fi47​)\ifmmode×\else\texttimes\fi1010  molecules/s with peak densities up to (1.0\ifmmode±\else\textpm\fi0.61.0)\ifmmode×\else\texttimes\fi109  molecules/cm3(1.0{\ifmmode\pm\else\textpm\fi{}}_{0.6}^{1.0})\ifmmode\times\else\texttimes\fi{}{10}^{9}\text{ }\text{ }\mathrm{\text{molecules}}/{\mathrm{cm}}^{3}(1.0\ifmmode±\else\textpm\fi0.61.0​)\ifmmode×\else\texttimes\fi109  molecules/cm3. For H2CO{\mathrm{H}}_{2}\mathrm{CO}H2​CO the population of rovibrational states is monitored by depletion spectroscopy, resulting in single-state populations up to (82\ifmmode±\else\textpm\fi10)(82\ifmmode\pm\else\textpm\fi{}10)%(82\ifmmode±\else\textpm\fi10).
This thesis reports on the combination of buffer-gas cooling with electric velocity filtering to produce a high-density guided beam of slow and internally cold polar molecules. In the continuous source, warm molecules are first cooled by collisions with a cryogenic helium buffer gas. Cold molecules are then extracted by means of an electric quadrupole guide. For formaldehyde the population of individual rotational states in the guide is monitored by depletion spectroscopy, resulting in single-state populations up to 82 %. This thesis also reports on a technique to produce pulses of slow guided molecules. The pulses exhibit a narrow velocity distribution around a tunable velocity.
We describe the combination of buffer-gas cooling with electrostatic velocity filtering to produce a high-flux continuous guided beam of internally cold and slow polar molecules. In a previous paper (L.D. van Buuren et al., Phys. Rev. Lett., 2009, 102, 033001) we presented results on density and state purity for guided beams of ammonia and formaldehyde using an optimized set-up. Here we describe in more detail the technical aspects of the cryogenic source, its operation, and the optimization experiments that we performed to obtain the best performance. The versatility of the source is demonstrated by the production of guided beams of different molecular species.
The measurement of the rotational state distribution of a velocity-selected, buffer-gas-cooled beam of ND3 is described. In an apparatus recently constructed to study cold ion-molecule collisions, the ND3 beam is extracted from a cryogenically cooled buffer-gas cell using a 2.15 m long electrostatic quadrupole guide with three 90° bends. (2+1) resonance enhanced multiphoton ionization spectra of molecules exiting the guide show that beams of ND3 can be produced with rotational state populations corresponding to approximately T(rot) = 9-18 K, achieved through manipulation of the temperature of the buffer-gas cell (operated at 6 K or 17 K), the identity of the buffer gas (He or Ne), or the relative densities of the buffer gas and ND3. The translational temperature of the guided ND3 is found to be similar in a 6 K helium and 17 K neon buffer-gas cell (peak kinetic energies of 6.92(0.13) K and 5.90(0.01) K, respectively). The characterization of this cold-molecule source provides an opportunity for the first experimental investigations into the rotational dependence of reaction cross sections in low temperature collisions.
Slow molecules are efficiently extracted from a thermal reservoir by exploiting the interaction between polar molecules and the field provided by an electrostatic quadrupole guide. Electrostatic velocity filtering is widely applicable, which is demonstrated by producing guided beams of ammonia ND3, formaldehyde H2CO, and the water isotopologs H2O, D2O, and HDO. Ultraviolet depletion spectroscopy in a cold guided beam of formaldehyde makes the population in individual rotational states visible and testifies their key role in velocity filtering. Enhanced Rayleigh scattering into an optical cavity is investigated as an alternative non-destructive detection method for cold molecules. For various room-temperature gases, the observed enhancement is well described by interference of electromagnetic fields scattered into the resonator by a classical driven dipole oscillator.
A comprehensive characterisation of cold molecular beams from a cryogenic buffer-gas cell, providing insight into the physics of buffer-gas cooling, is presented. Cold molecular beams are extracted from a cryogenic cell by electrostatic guiding, which is also used to measure their velocity distribution. The rotational-state distribution of the molecules is probed by radio-frequency resonant depletion spectroscopy. With the help of complete trajectory simulations, yielding the guiding efficiency for all of the thermally populated states, it is possible to determine both the rotational and the translational temperature of the molecules at the output of the buffer-gas cell. This thermometry method is demonstrated for various regimes of buffer-gas cooling and beam formation as well as for molecular species of different sizes (CH3 F and CF3 CCH). Comparison of the rotational and translational temperatures provides evidence of faster rotational thermalisation for the CH3 F/He system in the limit of low He density. In addition, the relaxation rates for different rotational states appear to be different.
The Stark interaction of polar molecules with an inhomogeneous electric field is exploited to select slow molecules from a room-temperature reservoir and guide them into an ultrahigh vacuum chamber. A linear electrostatic quadrupole with a curved section selects molecules with small transverse and longitudinal velocities. The source is tested with formaldehyde (H2CO)({\mathrm{H}}_{2}\mathrm{CO})(H2​CO) and deuterated ammonia (ND3).({\mathrm{ND}}_{3}).(ND3​). With H2CO{\mathrm{H}}_{2}\mathrm{CO}H2​CO a continuous flux is measured of $\ensuremath{\approx}{10}^{9}/\mathrm{s}$ and a longitudinal temperature of a few kelvin. The data are compared with the result of a Monte Carlo simulation.