André J. A. van Roij

Bio: André J. A. van Roij is an academic researcher from Radboud University Nijmegen. The author has contributed to research in topics: Excited state & Photodissociation. The author has an hindex of 13, co-authored 16 publications receiving 1055 citations.

Electrostatic trapping of ammonia molecules

Abstract: The ability to cool and slow atoms with light for subsequent trapping allows investigations of the properties and interactions of the trapped atoms in unprecedented detail. By contrast, the complex structure of molecules prohibits this type of manipulation, but magnetic trapping of calcium hydride molecules thermalized in ultra-cold buffer gas and optical trapping of caesium dimers generated from ultra-cold caesium atoms have been reported. However, these methods depend on the target molecules being paramagnetic or able to form through the association of atoms amenable to laser cooling, respectively, thus restricting the range of species that can be studied. Here we describe the slowing of an adiabatically cooled beam of deuterated ammonia molecules by time-varying inhomogeneous electric fields and subsequent loading into an electrostatic trap. We are able to trap state-selected ammonia molecules with a density of 10(6) cm(-3) in a volume of 0.25 cm3 at temperatures below 0.35 K. We observe pronounced density oscillations caused by the rapid switching of the electric fields during loading of the trap. Our findings illustrate that polar molecules can be efficiently cooled and trapped, thus providing an opportunity to study collisions and collective quantum effects in a wide range of ultra-cold molecular systems.

Gas-phase infrared photodissociation spectroscopy of cationic polyaromatic hydrocarbons

Abstract: Infrared spectra of gas-phase cationic naphthalene, phenanthrene, anthracene, and pyrene are recorded in the 500-1600 cm-1 range using multiphoton dissociation in an ion trap. Gas-phase polyaromatic hydrocarbons are photoionized by an excimer laser and stored in a quadrupole ion trap. Subsequent interaction with the intense infrared radiation of a free electron laser that is tuned in resonance with an infrared-allowed transition of the ion leads to sequential multiphoton absorption facilitated by rapid intramolecular vibrational redistribution. Absorption of more than 50-100 infrared photons raises the internal energy to above the dissociation threshold, leading eventually to fragmentation of the ion. Mass selective detection of the cationic species stored in the trap yields the infrared absorption spectrum of the parent ion.

Alternate gradient focusing and deceleration of a molecular beam.

Abstract: Neutral dipolar molecules can be decelerated and trapped using time-varying inhomogeneous electric fields. This has been demonstrated only for molecules in low-field seeking states, but can, in principle, be performed on molecules in high-field seeking states as well. Transverse stability is then much more difficult to obtain, however, since molecules in high-field seeking states always experience a force towards the electrodes. Here we demonstrate that an array of dipole lenses in alternate gradient configuration can be used to maintain transverse stability. A pulsed beam of metastable CO in high-field seeking states is accelerated from 275 to 289 m/s as well as decelerated from 275 to 260 m/s.

Vibrational Spectroscopy of Gas-Phase Metal-Carbide Clusters and Nanocrystals

Abstract: Neutral $\mathrm{Ti}{}_{8}\mathrm{C}{}_{12}$ and $\mathrm{Ti}{}_{14}\mathrm{C}{}_{13}$ clusters are produced in the gas phase with pulsed-nozzle laser vaporization. Infrared multiphoton excitation with a pulsed free-electron laser results in thermionic electron emission for these clusters, and the parent molecular ions are detected. Multiphoton ionization is strongly enhanced on vibrational resonances of these clusters, making it possible to observe infrared resonance-enhanced multiphoton ionization spectra. These spectra indicate C-C bonding for the ``met-cars'' while $\mathrm{Ti}{}_{14}\mathrm{C}{}_{13}$ has remarkable similarities to bulk TiC.

Longitudinal Focusing and Cooling of a Molecular Beam

Abstract: A neutral polar molecule experiences a force in an inhomogeneous electric field. This electric field can be designed such that a beam of polar molecules is exposed to a harmonic potential in the forward direction. In this potential the longitudinal phase-space distribution of the ensemble of molecules is rotated uniformly. This property is used to longitudinally focus a pulsed beam of ammonia molecules and to produce a beam with a longitudinal velocity spread of $0.76\text{ }\text{ }\mathrm{m}/\mathrm{s}$, corresponding to a temperature of $250\text{ }\ensuremath{\mu}\mathrm{K}$.

Interstellar Polycyclic Aromatic Hydrocarbon Molecules

Abstract: Large polycyclic aromatic hydrocarbon (PAH) molecules carry the infrared (IR) emission features that dominate the spectra of most galactic and extragalactic sources. This review surveys the observed mid-IR characteristics of these emission features and summarizes laboratory and theoretical studies of the spectral characteristics of PAHs and the derived intrinsic properties of emitting interstellar PAHs. Dedicated experimental studies have provided critical input for detailed astronomical models that probe the origin and evolution of interstellar PAHs and their role in the universe. The physics and chemistry of PAHs are discussed, emphasizing the contribution of these species to the photoelectric heating and the ionization balance of the interstellar gas and to the formation of small hydrocarbon radicals and carbon chains. Together, these studies demonstrate that PAHs are abundant, ubiquitous, and a dominant force in the interstellar medium of galaxies.

Cold and ultracold molecules: science, technology and applications

Abstract: This paper presents a review of the current state of the art in the research field of cold and ultracold molecules. It serves as an introduction to the focus issue of New Journal of Physics on Cold and Ultracold Molecules and describes new prospects for fundamental research and technological development. Cold and ultracold molecules may revolutionize physical chemistry and few-body physics, provide techniques for probing new states of quantum matter, allow for precision measurements of both fundamental and applied interest, and enable quantum simulations of condensed-matter phenomena. Ultracold molecules offer promising applications such as new platforms for quantum computing, precise control of molecular dynamics, nanolithography and Bose-enhanced chemistry. The discussion is based on recent experimental and theoretical work and concludes with a summary of anticipated future directions and open questions in this rapidly expanding research field.

Ultracold photoassociation spectroscopy: Long-range molecules and atomic scattering

Abstract: Photoassociation is the process in which two colliding atoms absorb a photon to form an excited molecule. The development of laser-cooling techniques for producing gases at ultracold !!1 mK" temperatures allows photoassociation spectroscopy to be performed with very high spectral resolution. Of particular interest is the investigation of molecular states whose properties can be related, with high precision, to the properties of their constituent atoms with the “complications” of chemical binding accounted for by a few parameters. These include bound long-range or purely long-range vibrational states in which two atoms spend most or all of their time at large internuclear separations. Low-energy atomic scattering states also share this characteristic. Photoassociation techniques have made important contributions to the study of all of these. This review describes what is special about photoassociation spectroscopy at ultracold temperatures, how it is performed, and a sampling of results including the determination of scattering lengths, their control via optical Feshbach resonances, precision determinations of atomic lifetimes from molecular spectra, limits on photoassociation rates in a Bose-Einstein condensate, and briefly, production of cold molecules. Discussions are illustrated with examples on alkali-metal atoms as well as other species. Progress in the field is already past the point where this review can be exhaustive, but an introduction is provided on the capabilities of photoassociation spectroscopy and the techniques presently in use.

Laser cooling of a diatomic molecule

Abstract: The development of Doppler laser cooling techniques allowed unprecedented access to ultracold temperatures of less 1 millikelvin. The motion of particles effectively ceases at such temperatures, enabling physical phenomena to be studied and controlled in extraordinary detail. Although laser cooling of atoms was demonstrated about 30 years ago, these techniques had not previously been extended to molecules. Ultracold molecules may prove even more interesting than ultracold atoms, because their greater internal complexity can potentially be exploited to investigate and manipulate a wide variety of physical phenomena, ranging from quantum information processing to chemical reactions and particle physics. Currently the only technique for producing ultracold molecules is by binding together ultracold alkali atoms to produce bi-alkali molecules. A team from Yale University now presents an experimental demonstration of laser cooling of a diatomic molecule — the polar molecule strontium monofluoride (SrF). With further refinement, the technique should enable the production of large samples of molecules at ultracold temperatures for species that are chemically distinct from bi-alkalis. Laser cooling has not yet been extended to molecules because of their complex internal structure. At present, the only technique for producing ultracold molecules is to bind ultracold alkali atoms to produce bialkali molecules. These authors experimentally demonstrate laser cooling of the polar molecule strontium monofluoride, reaching temperatures of a few millikelvin or less. The technique should allow the production of molecules at microkelvin temperatures for species that are chemically distinct from bialkalis. It has been roughly three decades since laser cooling techniques produced ultracold atoms1,2,3, leading to rapid advances in a wide array of fields. Laser cooling has not yet been extended to molecules because of their complex internal structure. However, this complexity makes molecules potentially useful for a wide range of applications4. For example, heteronuclear molecules possess permanent electric dipole moments that lead to long-range, tunable, anisotropic dipole–dipole interactions. The combination of the dipole–dipole interaction and the precise control over molecular degrees of freedom possible at ultracold temperatures makes ultracold molecules attractive candidates for use in quantum simulations of condensed-matter systems5 and in quantum computation6. Also, ultracold molecules could provide unique opportunities for studying chemical dynamics7,8 and for tests of fundamental symmetries9,10,11. Here we experimentally demonstrate laser cooling of the polar molecule strontium monofluoride (SrF). Using an optical cycling scheme requiring only three lasers12, we have observed both Sisyphus and Doppler cooling forces that reduce the transverse temperature of a SrF molecular beam substantially, to a few millikelvin or less. At present, the only technique for producing ultracold molecules is to bind together ultracold alkali atoms through Feshbach resonance13 or photoassociation14. However, proposed applications for ultracold molecules require a variety of molecular energy-level structures (for example unpaired electronic spin5,9,11,15, Omega doublets16 and so on). Our method provides an alternative route to ultracold molecules. In particular, it bridges the gap between ultracold (submillikelvin) temperatures and the ∼1-K temperatures attainable with directly cooled molecules (for example with cryogenic buffer-gas cooling17 or decelerated supersonic beams18). Ultimately, our technique should allow the production of large samples of molecules at ultracold temperatures for species that are chemically distinct from bialkalis.