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arXiv:0812.2850 · New Journal of Physics

Collisional effects in the formation of cold guided beams of polar molecules

M. Motsch, Christian Sommer, Martin Zeppenfeld, L. D. van Buuren, Pepijn W. H. Pinkse, Gerhard Rempe

Published May 14, 2009• 19 citations
arXivHTMLPDF

Abstract

High fluxes of cold polar molecules are efficiently produced by electric guiding and velocity filtering. Here, we investigate different aspects of the beam formation. Variations of the source parameters such as density and temperature result in characteristic changes in the guided beam. These are observed in the velocity distribution of the guided molecules as well as in the dependence of the signal of guided molecules on the trapping electric field. A model taking into account velocity-dependent collisional losses of cold molecules in the region close to the nozzle accurately reproduces this behaviour. This clarifies an open question on the parameter dependence of the detected signal and gives a more detailed understanding of the velocity-filtering and guiding process.

DOI
10.1088/1367-2630/11/5/055030

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High fluxes of cold polar molecules are efficiently produced by electric guiding and velocity filtering. Here, we investigate different aspects of the beam formation. Variations of the source parameters such as density and temperature result in characteristic changes in the guided beam. These are observed in the velocity distribution of the guided molecules as well as in the dependence of the signal of guided molecules on the trapping electric field. A model taking into account velocity-dependent collisional losses of cold molecules in the region close to the nozzle accurately reproduces this behaviour. This clarifies an open question on the parameter dependence of the detected signal and gives a more detailed understanding of the velocity-filtering and guiding process.
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.
Electrostatic velocity filtering and guiding is an established technique to produce high fluxes of cold polar molecules. In this paper we clarify different aspects of this technique by comparing experiments to detailed calculations. In the experiment, we produce cold guided beams of the three water isotopologs H2O{\mathrm{H}}_{2}\mathrm{O}H2​O, D2O{\mathrm{D}}_{2}\mathrm{O}D2​O, and HDO. Their different rotational constants and orientations of electric dipole moments lead to remarkably different Stark shift properties, despite the molecules being very similar in a chemical sense. Therefore, the signals of the guided water isotopologs differ on an absolute scale and also exhibit characteristic electrode voltage dependencies. We find excellent agreement between the relative guided fractions and voltage dependencies of the investigated isotopologs and predictions made by our theoretical model of electrostatic velocity filtering.
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.
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.
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.
We propose a scheme of a surface electrostatic velocity filter capable of preparing cold polar molecules on the surface of a substrate by selecting a low-velocity component of an effusive beam from a thermal gas reservoir. Using ND3 as a molecular sample, the dependence of the performance of the filter on the parameters of both the filter setup and the incident molecular beam is investigated by using a theoretical model and Monte Carlo simulations. A detailed study of the guiding process of molecules, including the evolution of phase space density of the packet in the filter, is carried out and shows that the beam selection process is mainly completed in the front part of the filter.