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CHIMIA International Journal for Chemistry

Preparation of Translationally Cold Neutral Molecules

Giulia Di Domenicantonio, Benjamin Bertsche, Andreas Osterwalder

Published Sep 30, 2011• 3 citations
Publisher

Abstract

Efforts at EPFL to obtain translationally cold neutral molecules are described. Active deceleration of polar molecules is performed by confining the molecules in moving three-dimensional electrostatic traps, and by appropriately choosing the velocity of those traps. Alternatively, cold molecules can be obtained by velocity filtering. Here, the velocity of the molecules is not changed, but instead the cold molecules are extracted from a thermal sample by using the competition between the electrostatic force and the centrifugal force inside a bent electrostatic guide for polar molecules.

DOI
10.2533/chimia.2011.725

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Efforts at EPFL to obtain translationally cold neutral molecules are described. Active deceleration of polar molecules is performed by confining the molecules in moving three-dimensional electrostatic traps, and by appropriately choosing the velocity of those traps. Alternatively, cold molecules can be obtained by velocity filtering. Here, the velocity of the molecules is not changed, but instead the cold molecules are extracted from a thermal sample by using the competition between the electrostatic force and the centrifugal force inside a bent electrostatic guide for polar molecules.
The interaction of polar molecules with an inhomogeneous, electrostatic field to filter molecules from an effusive room-temperature source is exploited to create a slow molecule beam. Whether a dipolar molecule is trapped in or expelled from the electric field minimum depends on the orientation of the molecular dipole with respect to the electric field. These molecules are called low and high-field seekers, respectively. Thus, an electric field configuration with a field minimum on axis acts as a 2-dimensional trap for low-field seeking molecules with small transverse velocity components. A linear quadrupolar is opted, which is one of the simplest suitable geometries. The longitudinal velocity is limited by a bent section, where the centripetal force due to the radius of curvature and electric field gradient guide only the slowest molecules around the bend.
Recently, a decelerator for neutral polar molecules has been presented that operates on the basis of macroscopic, three-dimensional, traveling electrostatic traps [A. Osterwalder, S. A. Meek, G. Hammer, H. Haak, and G. Meijer, Phys. Rev. A 81, 051401 (2010)]. In the present paper, a complete description of this decelerator is given, with emphasis on the electronics and the mechanical design. Experimental results showing the transverse velocity distributions of guided molecules are shown and compared to trajectory simulations. An assessment of non-adiabatic losses is made by comparing the deceleration signals from (13)CO with those from (12)CO and with simulated signals.
Inspired by the spectacular successes in the field of cold atoms, there is currently great interest in cold molecules. Cooling molecules is useful for various fundamental physics studies and gives access to an exotic regime in chemistry where the wave property of the molecules becomes important. Although cooling molecules has turned out to be considerably more difficult than cooling atoms, a number of methods to produce samples of cold molecules have been demonstrated over the last few years. This paper aims to review the application of cold molecules and the methods to produce them. Emphasis is put on the deceleration of polar molecules using time-varying electric fields. The operation principle of the array of electrodes that is used to decelerate polar molecules is described in analogy with, and using terminology from, charged-particle accelerators. It is shown that, by applying an appropriately timed high voltage burst, molecules can be decelerated while the phase-space density, i.e. the number of molecules per position-velocity interval, remains constant. In this way the high density and low temperature in the moving frame of a pulsed molecular beam can be transferred to the laboratory frame. Experiments on metastable CO in states that are either repelled by or attracted to high electric fields are presented. Loading of slow molecules into traps and storage rings is discussed.
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.
Producing large samples of slow molecules from thermal-velocity ensembles is a formidable challenge. Here we employ a centrifugal force to produce a continuous molecular beam with a high flux at near-zero velocities. We demonstrate deceleration of three electrically guided molecular species, CH3F{\mathrm{CH}}_{3}\mathrm{F}CH3​F, CF3H{\mathrm{CF}}_{3}\mathrm{H}CF3​H, and CF3CCH{\mathrm{CF}}_{3}\mathrm{CCH}CF3​CCH, with input velocities of up to $200\text{ }\text{ }{\mathrm{m}\text{ }\mathrm{s}}^{\ensuremath{-}1}$ to obtain beams with velocities below $15\text{ }\text{ }{\mathrm{m}\text{ }\mathrm{s}}^{\ensuremath{-}1}$ and intensities of several ${10}^{9}\text{ }\text{ }{\mathrm{mm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$. The centrifuge decelerator is easy to operate and can, in principle, slow down any guidable particle. It has the potential to become a standard technique for continuous deceleration of molecules.
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.