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.

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Cold and ultracold molecules: science, technology and applications

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.

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Ultracold photoassociation spectroscopy: Long-range molecules and atomic scattering

The ITER neutral beam (NB) injectors are the first injectors that will have to operate under conditions and constraints similar to those that will be encountered in a fusion reactor. These injectors will have to operate in a hostile radiation environment and they will become highly radioactive due to the neutron flux from ITER. The injectors will use a single large ion source and accelerator that will produce 40?A 1?MeV D? beams for pulse lengths of up to 3600?s.Significant design changes have been made to the ITER heating NB (HNB) injector over the past 4 years. The main changes are: Modifications to allow installation and maintenance of the beamline components with an overhead crane. The beam source vessel shape has been changed and the beam source moved to allow more space for the connections between the 1?MV bushing and the beam source. The RF driven negative ion source has replaced the filamented ion source as the reference design. The ion source and extractor power supplies will be located in an air insulated high voltage (?1?MV) deck located outside the tokamak building instead of inside an SF6 insulated HV deck located above the injector. Introduction of an all metal absolute valve to prevent any tritium in the machine to escape into the NB cell during maintenance. This paper describes the status of the design as of December 2008 including the above mentioned changes.The very important power supply system of the neutral beam injectors is not described in any detail as that merits a paper beyond the competence of the present authors.The R&D required to realize the injectors described in this paper must be carried out on a dedicated neutral beam test facility, which is not described here.

http://iopscience.iop.org/article/10.1088/0029-5515/49/4/045006/pdf

Status of the ITER heating neutral beam system

We demonstrate the production of ultracold polar RbCs molecules in their vibronic ground state, via photoassociation of laser-cooled atoms followed by a laser-stimulated state transfer process. The resulting sample of X1Sigma+ (nu = 0) molecules has a translational temperature of approximately 100 microK and a narrow distribution of rotational states. With the method described here it should be possible to produce samples even colder in all degrees of freedom, as well as other bialkali species.

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Optical production of ultracold polar molecules.

We give a snapshot of the rapidly developing field of ultracold polar molecules and walk the reader through the papers appearing in this Topical Issue.

https://link.springer.com/content/pdf/10.1140%2Fepjd%2Fe2004-00151-x.pdf

Editorial: Quo vadis, cold molecules?

Photodetachment microscopy has been performed on a beam of 32S- ions. Analysing the electron images obtained, we find that the electron affinity measurements performed with the photodetachment microscope contain a small bias, due to the difference between the actual and assumed values of the applied electric field. Having a measure of this bias, we can reanalyse older data recorded on the negative ions O- and Si- along similar lines. As a consequence, the values of the electron affinities of Oxygen, Silicon and Sulfur can be given with an improved accuracy. The recommended values (with expanded uncertainties) are now 11784.676(7)cm-1 for 16O, 11207.246(8)cm-1 for 28Si, and 16752.974(5)cm-1 for 32S, i.e. 1.461 113 5(12), 1.389 521 3(13) and 2.077 104 0(9) eV, respectively.

The electron affinities of O, Si, and S revisited with the photodetachment microscope

We demonstrate the existence of a new mechanism for the formation of ultracold molecules via photoassociation of cold cesium atoms. The experimental results, interpreted with numerical calculations, suggest that a resonant coupling between vibrational levels of the ${0}_{u}^{+}$ $({6s+6p}_{1/2})$ and $({6s+6p}_{3/2})$ states enables formation of ultracold molecules in vibrational levels of the ground state well below the $6s+6s$ dissociation limit. Such a scheme should be observable with many other electronic states and atomic species.

Resonant coupling in the formation of ultracold ground state molecules via photoassociation.

The photoassociative spectroscopy of the Cs2
0
g

 -
 long-range molecular state dissociating into the asymptote is performed, using cold cesium atoms in a vapor-cell magneto-optical trap (MOT). Vibrational levels from v
=0 to v
=132 are identified, and their rotational structure is well resolved up J
=8 for levels below v
=74. These data are analyzed in terms of the Rydberg-Klein-Rees (RKR) procedure, and correspond to 99.6% of an effective potential curve with a minimum at A and a cm-1 depth. Both ab initio calculations and simple model estimations predict a double-well structure for this potential curve, which cannot be reproduced presently by the RKR approach but which is confirmed by the presence of giant structures in the spectrum. The 1u(
 )long-range state is also observed for the first time in Cs2.

Photoassociative spectroscopy of the  long-range state

The effect of an external static magnetic field of arbitrary orientation with respect to the electric field, on the electron interference ring patterns observed by the photodetachment microscope is studied both experimentally and theoretically. The design of the interaction chamber has been modified to superimpose a controlled uniform magnetic field on the whole volume accessible to the interfering electron. Contrary to a previous study in weaker fields, where the overall dimension of the interferogram was not modified, the effect of the magnetic field here encompasses a regime of magnetic refocusing. A quantitative analysis is carried out using a closed-orbit perturbative calculation of the interference phase at the centre of the ring pattern. The essential result of this work is still the invariance of the extreme interference phase whatever the direction and magnitude of the applied magnetic field, up to values 100 times larger than in the previous experimental study. This property can be applied to revise former electron affinity measurements. Partly due to the previously unsuspected robustness of the electron interferograms vs. magnetic fields, partly thanks to the 2006 CODATA revision of the energy conversion factors, one can update the values of the electron affinities of 16O, 28Si and 32S to 1.4611134(9), 1.3895210(7) and 2.0771040(6) eV respectively.

Effect of a magnetic field in photodetachment microscopy

Photodetachment microscopy is carried out on a beam of 127I− ions with a nanosecond pulsed laser. The photoelectron interferograms are recorded by means of a digital camera that images the light spots produced by the amplified photoelectrons on a phosphor screen. This is the first implementation of such an optical imaging technique in photodetachment microscopy. Due to their sensitivity to the photoelectron energy, the recorded electron interferograms can be quantitatively analysed to produce a measure of the electron affinity of iodine eA(127I) with an accuracy improved by more than a factor of 2 with respect to the best previous measurement. The result is 2467 287.4(29) m−1 or 3.059 0463(38) eV.

https://iopscience.iop.org/article/10.1088/0953-4075/42/12/125001/pdf

Cyril Drag

Papers

The electron affinities of O, Si, and S revisited with the photodetachment microscope

Resonant coupling in the formation of ultracold ground state molecules via photoassociation.

Photoassociative spectroscopy of the \(\) long-range state

Effect of a magnetic field in photodetachment microscopy

Pulsed photodetachment microscopy and the electron affinity of iodine