arXiv subjects

Gerard Meijer

Publications and source records attributed to Gerard Meijer.

At least 91 records · Page 5Linked to original sources

arXiv2008

Laser-induced alignment and orientation of quantum-state-selected large molecules

A strong inhomogeneous static electric field is used to spatially disperse a supersonic beam of polar molecules, according to their quantum state. We show that the molecules residing in the lowest-lying rotational states can be selected and used as targets for further experiments. As an illustration, we demonstrate an unprecedented degree of laser-induced 1D alignment $(<\cos^2θ_{2D}>=0.97)$ and strong orientation of state-selected iodobenzene molecules. This method should enable experiments on pure samples of polar molecules in their rotational ground state, offering new opportunities in molecular science.

physics.chem-ph↗

arXiv2008

Stark deceleration of OH radicals in low-field-seeking and high-field-seeking quantum states

The Stark deceleration of OH radicals in both low-field-seeking and high-field-seeking levels of the rovibronic ${}^2Π_{3/2},v=0,J=3/2$ ground state is demonstrated using a single experimental setup. Applying alternating-gradient focusing, OH radicals in their low-field-seeking ${}^2Π_{3/2},v=0,J=3/2,f$ state have been decelerated from 345 m/s to 239 m/s, removing 50 % of the kinetic energy using only 27 deceleration stages. The alternating-gradient decelerator allows to independently control longitudinal and transverse manipulation of the molecules. Optimized high-voltage switching sequences for the alternating-gradient deceleration are applied, in order to adjust the dynamic focusing strength in every deceleration stage to the changing velocity over the deceleration process. In addition we have also decelerated OH radicals in their high-field-seeking ${}^2Π_{3/2},v=0,J=3/2,e$ state from 355 m/s to 316 m/s. For the states involved, a real crossing of hyperfine levels occurs at 640 V/cm, which is examined by varying a bias voltage applied to the electrodes.

physics.chem-ph↗

arXiv2008

Operation of a Stark decelerator with optimum acceptance

With a Stark decelerator, beams of neutral polar molecules can be accelerated, guided at a constant velocity, or decelerated. The effectiveness of this process is determined by the 6D volume in phase space from which molecules are accepted by the Stark decelerator. Couplings between the longitudinal and transverse motion of the molecules in the decelerator can reduce this acceptance. These couplings are nearly absent when the decelerator operates such that only every third electric field stage is used for deceleration, while extra transverse focusing is provided by the intermediate stages. For many applications, the acceptance of a Stark decelerator in this so-called $s=3$ mode significantly exceeds that of a decelerator in the conventionally used ($s=1$) mode. This has been experimentally verified by passing a beam of OH radicals through a 2.6 meter long Stark decelerator. The experiments are in quantitative agreement with the results of trajectory calculations, and can qualitatively be explained with a simple model for the 6D acceptance. These results imply that the 6D acceptance of a Stark decelerator in the $s=3$ mode of operation approaches the optimum value, i.e. the value that is obtained when any couplings are neglected.

physics.atm-clus↗

arXiv2008

Reflection of OH molecules from magnetic mirrors

We have reflected a Stark-decelerated beam of OH molecules under normal incidence from mirrors consisting of permanent magnets. Two different types of magnetic mirrors have been demonstrated. A long-range flat mirror made from a large disc magnet has been used to spatially focus the reflected beam in the longitudinal direction ("bunching"). A short-range curved mirror composed of an array of small cube magnets allows for transverse focusing of the reflected beam.

physics.chem-ph↗

arXiv2008

Trapping molecules on a chip in traveling potential wells

A microstructured array of over 1200 electrodes on a substrate has been configured to generate an array of local minima of electric field strength with a periodicity of $120 μ$m about $25 μ$m above the substrate. By applying sinusoidally varying potentials to the electrodes, these minima can be made to move smoothly along the array. Polar molecules in low field seeking quantum states can be trapped in these traveling potential wells. This is experimentally demonstrated by transporting metastable CO molecules in 30 mK deep wells that move at constant velocities above the substrate.

physics.atom-ph↗

arXiv2008

AC electric trapping of neutral atoms

We study the dynamic behavior of ultracold neutral atoms in a macroscopic ac electric trap. Confinement in such a trap is achieved by switching between two saddle-point configurations of the electric field. The gradual formation of a stably trapped cloud is observed and the trap performance is studied versus the switching frequency and the symmetry of the switching cycle. Additionally, the electric field in the trap is mapped out by imaging the atom cloud while the fields are still on. Finally, the phase-space acceptance of the trap is probed by introducing a modified switching cycle. The experimental results are reproduced using full three-dimensional trajectory calculations.

physics.atom-ph↗

arXiv2008

Alternating gradient focusing and deceleration of large molecules

We have focused and decelerated benzonitrile (C$_7$H$_5$N) molecules from a molecular beam, using an array of time-varying inhomogeneous electric fields in alternating gradient configuration. Benzonitrile is prototypical for large asymmetric top molecules that exhibit rich rotational structure and a high density of states. At the rotational temperature of 3.5 K in the pulsed molecular beam, many rotational states are populated. Benzonitrile molecules in their absolute ground state are decelerated from 320 m/s to 289 m/s, and similar changes in velocity are obtained for excited rotational states. All measurements agree well with the outcome of trajectory calculations. These experiments demonstrate that such large polyatomic molecules are amenable to the powerful method of Stark deceleration.

physics.atom-ph↗

arXiv2008

A selector for structural isomers of neutral molecules

We have selected and spatially separated the two conformers of 3-aminophenol (C$_6$H$_7$NO) present in a molecular beam. Analogous to the separation of ions based on their mass-to-charge ratios in a quadrupole mass filter, the neutral conformers are separated based on their different mass-to-dipole-moment ratios in an ac electric quadrupole selector. For a given ac frequency, the individual conformers experience different focusing forces, resulting in different transmissions through the selector. These experiments demonstrate that conformer-selected samples of large molecules can be prepared, offering new possibilities for the study of gas-phase biomolecules.

physics.chem-ph↗

arXiv2008

The radiative lifetime of metastable CO ($a^3Π, v=0$)

We present a combined experimental and theoretical study on the radiative lifetime of CO in the $a^3Π_{1,2}, v=0$ state. CO molecules in a beam are prepared in selected rotational levels of this metastable state, Stark-decelerated and electrostatically trapped. From the phosphorescence decay in the trap, the radiative lifetime is measured to be $2.63\pm0.03$ ms for the $a^3Π_1, v=0, J=1$ level. From spin-orbit coupling between the $a^3Π$ and the $A^1Π$ state a 20% longer radiative lifetime of 3.16 ms is calculated for this level. It is concluded that coupling to other $^1Π$ states contributes to the observed phosphorescence rate of metastable CO.

physics.atom-ph↗

arXiv2008

Electrostatic trapping of metastable NH molecules

We report on the Stark deceleration and electrostatic trapping of $^{14}$NH ($a ^1Δ$) radicals. In the trap, the molecules are excited on the spin-forbidden $A ^3Π\leftarrow a ^1Δ$ transition and detected via their subsequent fluorescence to the $X ^3Σ^-$ ground state. The 1/e trapping time is 1.4 $\pm$ 0.1 s, from which a lower limit of 2.7 s for the radiative lifetime of the $a ^1Δ, v=0,J=2$ state is deduced. The spectral profile of the molecules in the trapping field is measured to probe their spatial distribution. Electrostatic trapping of metastable NH followed by optical pumping of the trapped molecules to the electronic ground state is an important step towards accumulation of these radicals in a magnetic trap.

physics.atom-ph↗

arXiv2007

Trapping of Rb atoms by ac electric fields

We demonstrate trapping of an ultracold gas of neutral atoms in a macroscopic ac electric trap. Three-dimensional confinement is obtained by switching between two saddle-point configurations of the electric field. Stable trapping is observed in a narrow range of switching frequencies around 60 Hz. The dynamic confinement of the atoms is directly visualized at different phases of the ac switching cycle. We observe about 10^5 Rb atoms in the 1 mm^3 large and several microkelvins deep trap with a lifetime of approximately 5 s.

physics.atom-ph↗

arXiv2007

Optical pumping of trapped neutral molecules by blackbody radiation

Optical pumping by blackbody radiation is a feature shared by all polar molecules and fundamentally limits the time that these molecules can be kept in a single quantum state in a trap. To demonstrate and quantify this, we have monitored the optical pumping of electrostatically trapped OH and OD radicals by room-temperature blackbody radiation. Transfer of these molecules to rotationally excited states by blackbody radiation at 295 K limits the $1/e$ trapping time for OH and OD in the $X^{2}Π_{3/2},v''=0,J''=3/2(f)$ state to 2.8 s and 7.1 s, respectively.

physics.atom-ph↗

arXiv2007

Magnetic trapping of buffer-gas cooled chromium atoms and prospects for the extension to paramagnetic molecules

We report the successful buffer-gas cooling and magnetic trapping of chromium atoms with densities exceeding $10^{12}$ atoms per cm$^{3}$ at a temperature of 350 mK for the trapped sample. The possibilities to extend the method to buffer-gas cool and magnetically trap molecules are discussed. To minimize the most important loss mechanism in magnetic trapping, molecules with a small spin-spin interaction and a large rotational constant are preferred. Both the CrH ($^6Σ^+$ ground state) and MnH ($^7Σ^+$) radicals appear to be suitable systems for future experiments.

physics.atom-ph↗

arXiv2006

Production and deceleration of a pulsed beam of metastable NH ($a ^1Δ$) radicals

We report on the production of a pulsed molecular beam of metastable NH ($a ^1Δ$) radicals and present first results on the Stark deceleration of the NH ($a ^1Δ, J=2, MΩ=-4$) radicals from 550 m/s to 330 m/s. The decelerated molecules are excited on the spin-forbidden $A ^3Π\leftarrow a ^1Δ$ transition, and detected via their subsequent spontaneous fluorescence to the $X ^3Σ^{-}, v"=0$ ground-state. These experiments demonstrate the feasibility of our recently proposed scheme [Phys. Rev. A 64 (2001) 041401] to accumulate ground-state NH radicals in a magnetic trap.

physics.atom-ph↗

arXiv2006

Optimizing the Stark-decelerator beamline for the trapping of cold molecules using evolutionary strategies

We demonstrate feedback control optimization for the Stark deceleration and trapping of neutral polar molecules using evolutionary strategies. In a Stark-decelerator beamline pulsed electric fields are used to decelerate OH radicals and subsequently store them in an electrostatic trap. The efficiency of the deceleration and trapping process is determined by the exact timings of the applied electric field pulses. Automated optimization of these timings yields an increase of 40 % of the number of trapped OH radicals.

physics.ins-det↗

arXiv2006

A versatile electrostatic trap

A four electrode electrostatic trap geometry is demonstrated that can be used to combine a dipole, quadrupole and hexapole field. A cold packet of 15ND3 molecules is confined in both a purely quadrupolar and hexapolar trapping field and additionally, a dipole field is added to a hexapole field to create either a double-well or a donut-shaped trapping field. The profile of the 15ND3 packet in each of these four trapping potentials is measured, and the dependence of the well-separation and barrier height of the double-well and donut potential on the hexapole and dipole term are discussed.

physics.atm-clus↗

arXiv2006

Compact in-place gate valve for molecular beam experiments

A high vacuum gate valve for skimmed molecular beam experiments is described. It is designed with a very short extent of only 10 mm along the molecular beam axis to minimize the distance between the molecular beam source and the experiment to provide the maximum molecular flux to the experiment. At the same time it provides free space on both sides of the skimmer to not disturb the supersonic expansion in front of the skimmer, to give optical access to the full distance between beam source and skimmer, and to allow for placing electrostatic devices very close behind the skimmer. The gate valve allows to maintain high vacuum conditions (10^{-8} mbar) in the experimental chamber while the source chamber is brought up to atmospheric pressure for modifications or maintenance. The valve can be operated from outside the vacuum chamber while maintaining vacuum conditions in all chambers.

physics.ins-det↗

arXiv2005

Direct measurement of the radiative lifetime of vibrationally excited OH radicals

Neutral molecules, isolated in the gas-phase, can be prepared in a long-lived excited state and stored in a trap. The long observation time afforded by the trap can then be exploited to measure the radiative lifetime of this state by monitoring the temporal decay of the population in the trap. This method is demonstrated here and used to benchmark the Einstein $A$-coefficients in the Meinel system of OH. A pulsed beam of vibrationally excited OH radicals is Stark decelerated and loaded into an electrostatic quadrupole trap. The radiative lifetime of the upper $Λ$-doublet component of the $X ^2Π_{3/2}, v=1, J=3/2$ level is determined as $59.0 \pm 2.0$ ms, in good agreement with the calculated value of $57.7 \pm 1.0$ ms.

physics.chem-ph↗