arXiv subjects

Gerard Meijer

Publications and source records attributed to Gerard Meijer.

At least 55 records · Page 3Linked to original sources

arXiv2012

Structure of Magnetic Lanthanide Clusters from Far-IR Spectroscopy: Tb (n = 5-9)

Small lanthanide clusters have interesting magnetic properties, but their structures are unknown. We have identified the structures of small terbium cluster cations Tb (n = 5-9) in the gas phase, by analysis of their vibrational spectra. The spectra have been measured via IR multiple photon dissociation of their complexes with Ar atoms in the 50-250 1/cm range with an infrared free electron laser. Density functional theory calculations using a 4f-in-core effective core potential (ECP) accurately reproduce the experimental far-IR spectra. The ECP corresponds to a 4f85d16s2 trivalent configuration of terbium. The assigned structures are similar to those observed in several other transition metal systems. From this, we conclude that the bonding in Tb clusters is through the interactions between the 5d and 6s electrons, and that the 4f electrons have only an indirect effect on the cluster structures.

physics.atm-clus↗

arXiv2012

Quantum-state resolved bimolecular collisions of velocity-controlled OH with NO radicals

Whereas atom-molecule collisions have been studied with complete quantum state resolution, interactions between two state-selected molecules have proven much harder to probe. Here, we report the measurement of state-resolved inelastic scattering cross sections for collisions between two open-shell molecules that are both prepared in a single quantum state. Stark-decelerated OH radicals were scattered with hexapole-focused NO radicals in a crossed beam configuration. Rotationally and spin-orbit inelastic scattering cross sections were measured on an absolute scale for collision energies between 70 and 300 cm$^{-1}$. These cross sections show fair agreement with quantum coupled-channels calculations using a set of coupled model potential energy surfaces based on ab initio calculations for the long-range non-adiabatic interactions and a simplistic short-range interaction. This comparison reveals the crucial role of electrostatic forces in complex molecular collision processes.

physics.chem-ph↗

arXiv2012

Magnetic dipole transitions in the OH $A\,^2Σ^+ \leftarrow X\,^2Π$ system

We report on the observation of magnetic dipole allowed transitions in the well-characterized $A\,^2Σ^+ - X\,^2Π$ band system of the OH radical. A Stark decelerator in combination with microwave Rabi spectroscopy is used to control the populations in selected hyperfine levels of both $Λ$-doublet components of the $X\,^2Π_{3/2},v=0,J=3/2$ ground state. Theoretical calculations presented in this paper predict that the magnetic dipole transitions in the $ν'=1 \leftarrow ν=0$ band are weaker than the electric dipole transitions by a factor of $2.58\times 10^3$ only, i.e., much less than commonly believed. Our experimental data confirm this prediction.

physics.chem-ph↗

arXiv2012

Alternating-Gradient Focusing of the Benzonitrile-Argon Van der Waals Complex

We report on the focusing and guiding of the van der Waals complex formed between benzonitrile molecules (C$_6$H$_5$CN) and argon atoms in a cold molecular beam using an ac electric quadrupole guide. The distribution of quantum states in the guided beam is non-thermal, because the transmission efficiency depends on the state-dependent effective dipole moment in the applied electric fields. At a specific ac frequency, however, the excitation spectrum can be described by a thermal distribution at a rotational temperature of 0.8 K. From the observed transmission characteristics and a combination of trajectory and Stark-energy calculations we conclude that the permanent electric dipole moment of benzonitrile remains unchanged upon the attachment of the argon atom to within \pm5%. By exploiting the different dipole-moment-to-mass (μ/m) ratios of the complex and the benzonitrile monomer, transmission can be selectively suppressed for or, in the limit of 0 K rotational temperature, restricted to the complex.

physics.atom-ph↗

arXiv2012

Traveling wave deceleration of heavy polar molecules in low-field seeking states

We demonstrate the deceleration of heavy polar molecules in low-field seeking states by combining a cryogenic source and a travelling-wave Stark decelerator. The cryogenic source provides a high intensity beam with low speed and temperature, and the travelling-wave decelerator provides large deceleration forces and high phase-space acceptance. We prove these techniques using YbF molecules and find the experimental data to be in excellent agreement with numerical simulations. These methods extend the scope of Stark deceleration to a very wide range of molecules.

physics.atom-ph↗

arXiv2012

Decelerating molecules with microwave fields

We here report on the experimental realization of a microwave decelerator for neutral polar molecules, suitable for decelerating and focusing molecules in high-field-seeking states. The multi-stage decelerator consists of a cylindrical microwave cavity oscillating on the TE 11n mode, with n=12 electric field maxima along the symmetry axis. By switching the microwave field on and off at the appropriate times, a beam of state-selected ammonia molecules with an incident mean velocity of 25 m/s is guided while being spatially focussed in the transverse direction and bunched in the forward direction. Deceleration from 20.0 m/s to 16.9 m/s and acceleration from 20.0 m/s to 22.7 m/s is demonstrated.

physics.chem-ph↗

arXiv2011

Producing translationally cold, ground-state CO molecules

Carbon monoxide molecules in their electronic, vibrational, and rotational ground state are highly attractive for trapping experiments. The optical or ac electric traps that can be envisioned for these molecules will be very shallow, however, with depths in the sub-milliKelvin range. Here we outline that the required samples of translationally cold CO (X$^1Σ^+$, $v"$=0, $N"$=0) molecules can be produced after Stark deceleration of a beam of laser-prepared metastable CO (a$^3Π_1$) molecules followed by optical transfer of the metastable species to the ground state \emph{via} perturbed levels in the A$^1Π$ state. The optical transfer scheme is experimentally demonstrated and the radiative lifetimes and the electric dipole moments of the intermediate levels are determined.

physics.atom-ph↗

arXiv2011

A traveling wave decelerator for neutral polar molecules

Recently, a decelerator for neutral polar molecules has been presented that operates on the basis of macroscopic, three-dimensional, traveling electrostatic traps (Osterwalder et al., 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.

physics.atom-ph↗

arXiv2011

Stark-selected beam of ground-state OCS molecules characterized by revivals of impulsive alignment

We make use of an inhomogeneous electrostatic dipole field to impart a quantum-state-dependent deflection to a pulsed beam of OCS molecules, and show that those molecules residing in the absolute ground state, $X ^1Σ^+$, $\ket{00^00}$, J=0, can be separated out by selecting the most deflected part of the molecular beam. Past the deflector, we irradiate the molecular beam by a linearly polarized pulsed nonresonant laser beam that impulsively aligns the OCS molecules. Their alignment, monitored via velocity-map imaging, is measured as a function of time, and the time dependence of the alignment is used to determine the quantum state composition of the beam. We find significant enhancements of the alignment (\costhetasqtd $= 0.84$) and of state purity ($> 92$%) for a state-selected, deflected beam compared with an undeflected beam.

physics.chem-ph↗

arXiv2011

An electrostatic elliptical mirror for neutral polar molecules

Focusing optics for neutral molecules finds application in shaping and steering molecular beams. Here we present an electrostatic elliptical mirror for polar molecules consisting of an array of microstructured gold electrodes deposited on a glass substrate. Alternating positive and negative voltages applied to the electrodes create a repulsive potential for molecules in low-field-seeking states. The equipotential lines are parallel to the substrate surface, which is bent in an elliptical shape. The mirror is characterized by focusing a beam of metastable CO molecules and the results are compared to the outcome of trajectory simulations.

physics.atom-ph↗

arXiv2011

Rotational-state-specific guiding of large molecules

A beam of polar molecules can be focused and transported through an ac electric quadrupole guide. At a given ac frequency, the transmission of the guide depends on the mass-to-dipole-moment (m/\textmu) ratio of the molecular quantum state. Here we present a detailed characterization of the m/\textmu selector, using a pulsed beam of benzonitrile (C$_6$H$_5$CN) molecules in combination with rotational quantum state resolved detection. The arrival time distribution as well as the transverse velocity distribution of the molecules exiting the selector are measured as a function of ac frequency. The \textmu/$Δ$\textmu resolution of the selector can be controlled by the applied ac waveforms and a value of up to 20 can be obtained with the present setup. This is sufficient to exclusively transmit molecules in the absolute ground state of benzonitrile, or rather in quantum states that have the same m/\textmu value as the ground state. The operation characteristics of the m/\textmu selector are in quantitative agreement with the outcome of trajectory simulations.

physics.atm-clus↗

arXiv2011

State- and conformer-selected beams of aligned and oriented molecules for ultrafast diffraction studies

The manipulation of the motion of neutral molecules with electric or magnetic fields has seen tremendous progress over the last decade. Recently, these techniques have been extended to the manipulation of large and complex molecules. In this article we introduce experimental approaches to the manipulation of large molecules, i.e., the deflection, focusing and deceleration using electric fields. We detail how these methods can be exploited to spatially separate quantum states and how to select individual conformers of complex molecules. We briefly describe mixed-field orientation experiments made possible by the quantum-state selection. Moreover, we provide an outlook on ultrafast diffraction experiments using these highly controlled samples.

physics.chem-ph↗

arXiv2011

Ionization of oriented carbonyl-sulfide molecules by intense circularly polarized laser pulses

We present combined experimental and theoretical results on strong-field ionization of oriented carbonyl-sulphide molecules by circularly-polarized laser pulses. The obtained molecular frame photoelectron angular distributions show pronounced asymmetries perpendicular to the direction of the molecular electric dipole moment. These findings are explained by a tunneling model invoking the laser-induced Stark shifts associated with the dipoles and polarizabilities of the molecule and its unrelaxed cation. The focus of the present article is to understand the strong-field ionization of one-dimensionally-oriented polar molecules, in particular asymmetries in the emission direction of the photoelectrons. In the following article (Phys. Rev. A 83, 023406 (2011)) the focus is to understand strong-field ionization from three-dimensionally-oriented asymmetric top molecules, in particular the suppression of electron emission in nodal planes of molecular orbitals.

physics.chem-ph↗

arXiv2011

Ionization of 1D and 3D oriented asymmetric top molecules by intense circularly polarized femtosecond laser pulses

We present a combined experimental and theoretical study on strong-field ionization of a three-dimensionally oriented asymmetric top molecule, benzonitrile (C$_7$H$_5$N), by circularly polarized, nonresonant femtosecond laser pulses. Prior to the interaction with the strong field, the molecules are quantum-state selected using a deflector, and 3-dimensionally (3D) aligned and oriented adiabatically using an elliptically polarized laser pulse in combination with a static electric field. A characteristic splitting in the molecular frame photoelectron momentum distribution reveals the position of the nodal planes of the molecular orbitals from which ionization occurs. The experimental results are supported by a theoretical tunneling model that includes and quantifies the splitting in the momentum distribution. The focus of the present article is to understand strong-field ionization from 3D-oriented asymmetric top molecules, in particular the suppression of electron emission in nodal planes of molecular orbitals. In the preceding article [Dimitrovski et al., Phys. Rev. A 83, 023405 (2011)] the focus is to understand the strong-field ionization of one-dimensionally-oriented polar molecules, in particular asymmetries in the emission direction of the photoelectrons.

physics.chem-ph↗

arXiv2011

Suppression of non-adiabatic losses of molecules from chip-based microtraps

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 non-adiabatic transitions between levels in which the molecules are trapped and levels in which the molecules are not trapped can be suppressed. We use 12-CO and 13-CO (a 3-Pi(1), v=0) molecules, prepared in the upper Lambda-doublet component of the J=1 rotational level, and study the trap loss as a function of an offset magnetic field. The experimentally observed suppression (enhancement) of the non-adiabatic transitions for 12-CO (13-CO) with increasing magnetic field is quantitatively explained.

physics.atom-ph↗

arXiv2011

Scattering of Stark-decelerated OH radicals with rare-gas atoms

We present a combined experimental and theoretical study on the rotationally inelastic scattering of OH ($X\,^2Π_{3/2}, J=3/2, f$) radicals with the collision partners He, Ne, Ar, Kr, Xe, and D$_2$ as a function of the collision energy between $\sim 70$ cm$^{-1}$ and 400~cm$^{-1}$. The OH radicals are state selected and velocity tuned prior to the collision using a Stark decelerator, and field-free parity-resolved state-to-state inelastic relative scattering cross sections are measured in a crossed molecular beam configuration. For all OH-rare gas atom systems excellent agreement is obtained with the cross sections predicted by close-coupling scattering calculations based on accurate \emph{ab initio} potential energy surfaces. This series of experiments complements recent studies on the scattering of OH radicals with Xe [Gilijamse \emph{et al.}, Science {\bf 313}, 1617 (2006)], Ar [Scharfenberg \emph{et al.}, Phys. Chem. Chem. Phys. {\bf 12}, 10660 (2010)], He, and D$_2$ [Kirste \emph{et al.}, Phys. Rev. A {\bf 82}, 042717 (2010)]. A comparison of the relative scattering cross sections for this set of collision partners reveals interesting trends in the scattering behavior.

physics.atm-clus↗

arXiv2010

Driving rotational transitions in molecules on a chip

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 transitions between two of these quantum states can be induced while the molecules are on the chip. We use CO (a 3-Pi(1), v=0) molecules, prepared in the J=1 rotational level, and induce the J=2 <-- J=1 rotational transition with narrow-band sub-THz (mm-wave) radiation. First, the mm-wave source is characterized using CO molecules in a freely propagating molecular beam, and both Rabi cycling and rapid adiabatic passage are examined. Then, we demonstrate that the mm-wave radiation can be coupled to CO molecules that are less than 50 micron above the chip. Finally, CO molecules are guided in the J=1 level to the center of the chip where they are pumped to the J=2 level, recaptured, and guided off the chip.

physics.atom-ph↗

arXiv2010

Multiple packets of neutral molecules revolving for over a mile

The level of control that one has over neutral molecules in beams dictates their possible applications. Here we experimentally demonstrate that state-selected, neutral molecules can be kept together in a few mm long packet for a distance of over one mile. This is accomplished in a circular arrangement of 40 straight electrostatic hexapoles through which the molecules propagate over 1000 times. Up to 19 packets of molecules have simultaneously been stored in this ring structure. This brings the realization of a molecular low-energy collider within reach.

physics.chem-ph↗