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arXiv:0811.4123 · Physical Review A

Nonadiabatic transitions in electrostatically trapped ammonia molecules

Moritz Kirste, Boris G. Sartakov, Melanie Schnell, Gerard Meijer

Published May 27, 2009• 37 citations
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Abstract

Nonadiabatic transitions are known to be major loss channels for atoms in magnetic traps but have thus far not been experimentally reported upon for trapped molecules. We have observed and quantified losses due to nonadiabatic transitions for three isotopologues of ammonia in electrostatic traps by comparing the trapping times in traps with a zero and a nonzero electric field at the center. Nonadiabatic transitions are seen to dominate the overall loss rate even for the present samples that are at relatively high temperatures of 30 mK. It is anticipated that losses due to nonadiabatic transitions in electric fields are omnipresent in ongoing experiments on cold molecules.

DOI
10.1103/physreva.79.051401

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Nonadiabatic transitions are known to be major loss channels for atoms in magnetic traps but have thus far not been experimentally reported upon for trapped molecules. We have observed and quantified losses due to nonadiabatic transitions for three isotopologues of ammonia in electrostatic traps by comparing the trapping times in traps with a zero and a nonzero electric field at the center. Nonadiabatic transitions are seen to dominate the overall loss rate even for the present samples that are at relatively high temperatures of 30 mK. It is anticipated that losses due to nonadiabatic transitions in electric fields are omnipresent in ongoing experiments on cold molecules.
In AMO (Atomic, Molecular, and Optical) physics, an important part of making precise measurements is obtaining and keeping high phase-space densities of the atoms or molecules being trapped. When working with atoms, nonadiabatic transitions from trapped states to non-trapped states are a key component of trap loss. The theoretical consensus has been that electrostatically trapped molecules, unlike atoms have very low loss rates due to nonadiabatic transitions. However, [1] performed the first experimental study of loss due to nonadiabatic transitions in electrostatically trapped ammonia and claim that they cause significant trap loss. This thesis presents my work on developing a computer simulation to model ammonia in an electrostatic trap and a simple model of trap loss due to nonadiabatic transitions. This thesis will provide background information on the slowing and trapping of molecules and the nonadiabatic transition that causes trap loss.
We analyze the dynamics of a paramagnetic, dipolar molecule in a generic ``magneto-electrostatic'' trap where both magnetic and electric fields may be present. The potential energy that governs the dynamics of the molecules is found using a reduced molecular model that incorporates the main features of the system. We discuss the shape of the trapping potentials for different field geometries, as well as the possibility of nonadiabatic transitions to untrapped states, i.e., the analog of Majorana transitions in a quadrupole magnetic atomic trap. Maximizing the lifetime of molecules in a trap is of great concern in current experiments, and we assess the effect of nonadiabatic transitions on obtainable trap lifetimes.
Polar molecules in selected quantum states can be guided, decelerated, and trapped using electric fields created by microstructured electrodes on a chip. Here we explore how nonadiabatic transitions between levels in which the molecules are trapped and levels in which the molecules are not trapped can be suppressed. We use 12C^{12}\mathrm{C}12CO and 13C^{13}\mathrm{C}13CO ($a {}^{3}{\ensuremath{\Pi}}_{1},v=0$) molecules, prepared in the upper $\ensuremath{\Lambda}$-doublet component of the J=1J=1J=1 rotational level, and study the trap loss as a function of an offset magnetic field. The experimentally observed suppression (enhancement) of the nonadiabatic transitions for 12C^{12}\mathrm{C}12CO (13C^{13}\mathrm{C}13CO) with increasing magnetic field is quantitatively explained.
An electrostatic lattice is composed of many micro-scale traps. It is a powerful tool for manipulating cold polar molecules on a chip with the help of the interaction between the dipole moment and the inhomogeneous electric field. To the best of our knowledge, the existing electrostatic lattice still suffers the nonadiabatic transition that will lead to the escape of molecules from the electrostatic trap. In the paper, we skillfully design the electrode structure to avoid the zero-field zone at the center of every single microtrap, and the nonadiabatic transitions can be prevented. First, we numerically calculate the distribution of the spatial electrostatic field. Then, the influences of voltages on the electric field intensity and the height of the trap center are investigated. We simulate of the trajectory of molecules using the Monte Carlo method and figure out the factors impacting the loading efficiency of our electrostatic lattice. At last, we discuss how the voltages and the loading positions affect the positional distribution of trapped molecules.
We present an experimental and theoretical study of atom-molecule collisions in a mixture of cold, trapped N atoms and NH molecules at a temperature of $\ensuremath{\sim}600\text{ }\text{ }\mathrm{mK}$. We measure a small N+NH\mathrm{N}+\mathrm{NH}N+NH trap loss rate coefficient of ${k}_{\mathrm{loss}}^{(\mathrm{N}+\mathrm{NH})}=9(5)(3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$. Accurate quantum scattering calculations based on ab initio interaction potentials are in agreement with experiment and indicate the magnetic dipole interaction to be the dominant loss mechanism. Our theory further indicates the ratio of N+NH\mathrm{N}+\mathrm{NH}N+NH elastic-to-inelastic collisions remains large (>100>100>100) into the mK regime.
A polar molecule experiences a force in an inhomogeneous electric field. Using this force, neutral molecules can be decelerated and trapped. It is shown here that this can in principle be done without loss in phase-space density. Using a series of 64 pulsed inhomogeneous electric fields a supersonic beam of ammonia molecules (14NH3,{(}^{14}{\mathrm{NH}}_{3},(14NH3​, 14ND3,{}^{14}{\mathrm{ND}}_{3},14ND3​, 15ND3){}^{15}{\mathrm{ND}}_{3})15ND3​) is decelerated. Subsequently, the decelerated molecules are loaded into an electrostatic quadrupole trap. Densities on the order of 107molecules/cm3{10}^{7} {\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{u}\mathrm{l}\mathrm{e}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3}107molecules/cm3 at a temperature of 25 mK are obtained for 14ND3{}^{14}{\mathrm{ND}}_{3}14ND3​ and 15ND3{}^{15}{\mathrm{ND}}_{3}15ND3​ separately and simultaneously. This corresponds to a phase-space density in the trap of $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13},$ 50 times less than the initial phase-space density in the beam.