Download Physics and Chemistry with Cold Molecules

Editorial
DOI: 10.1002/cphc.201601131
Physics and Chemistry with Cold Molecules
John M. Doyle,*[a] Bretislav Friedrich,*[b] and Edvardas Narevicius*[c]
The previous century saw the emergence of techniques that
made it possible to extract microscopic information—and understanding—from the chaos of macroscopic gaseous and
other systems. Among the first was the molecular beam technique that demonstrated the principal virtues of controlled
atomic and molecular motion for elucidating the burning
questions of the day, including those concerning the validity of
quantum mechanics. As emphasized by Otto Stern in 1946,[1]
“the most distinctive characteristic property of the molecular
ray method is its simplicity and directness. It enables us to
make measurements on isolated neutral atoms or molecules
with macroscopic tools. For this reason it is especially valuable
for testing and demonstrating directly fundamental assumptions of the theory.” In his 1988 article[2] “Molecular Beams: our
legacy from Otto Stern,” Norman Ramsey compiled a list of
what he called “major contributions to physics from the field
of molecular beams.” There were thirty two items on that list,
ranging from space quantization to molecular scattering to
tests of time reversal symmetry. In the 1960s, the molecular
beam technique made inroads into chemistry as well, by fulfilling the pipe dream of disentangling from gaseous chaos elementary chemical reactions as single binary collisions of chemically well-defined reagents.[3] Chemical reaction dynamics that
came about as a result has remained one of the chief preoccupations of chemical/molecular physics to date.
In the 1990s, yet another pipe dream came about that was
triggered by the renaissance then taking place in atomic physics.[4] This renaissance was nurtured by the development of
techniques to cool and slow down atoms. Based on a combination of molecular beams with laser cooling, these techniques
enabled to reach a regime where interatomic interactions were
governed by s-wave scattering, and ultimately led to the realization of quantum degeneracy in atomic gases. No sooner had
cold atoms become the talk of physicists worldwide than cold
molecules appeared as a pipe dream of molecular physicists
and physical chemists. For unlike atoms, molecules possess vi[a] Prof. J. M. Doyle
Harvard University, Department of Physics
17 Oxford Street, Cambridge, MA 02138 (USA)
E-mail: [email protected]
[b] Prof. B. Friedrich
Fritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4–6, D-14195 Berlin (Germany)
E-mail: [email protected]
[c] Prof. E. Narevicius
Chemical Physics Department
Perlman Chemical Sciences Building, Room 119A
Weizmann Institute of Science, 76100 Rehovot (Israel)
E-mail: [email protected]
ChemPhysChem 2016, 17, 3581 – 3582
brational and rotational degrees of freedom and can be polar,
that is, carry a nonzero body-fixed electric dipole moment.
Ever since the late 1990s, this has been widely regarded in the
atomic, molecular and optical physics community as well by
physical chemists as a non-trivial enrichment of the repertoire
offered by cold atoms and therefore worthy of exploring.
This Special Issue of ChemPhysChem, dedicated to cold mole-
cules, is a testimonial to where these explorations have led us
so far.
The invited papers—there are twenty eight of them—fall into
several, partly overlapping categories. The thickest batch of
contributions addresses the recalcitrant problem of producing
translationally cold molecules. Most of the techniques developed make use of molecular beams—increasingly of the
buffer-gas beam variety—and of spatially and temporally
shaped static electric and magnetic as well as radio frequency,
microwave and optical fields. The techniques offer either
pulses or continuous streams of slow molecules whose velocities are in the 10 m s@1 range (buffer-gas beams alone yield
speeds of about 50–100 m s@1). Also covered in the Special
Issue is the production of translationally ultracold molecules
from ultracold atoms via optical and Raman association and
population transfer techniques as well as direct laser cooling
of molecular beams.
Excitation dynamics, including to and with Rydberg states, as
revealed in collisional studies is another major theme of the
Special Issue. State-of-the-art experiments prove capable of
providing a high-definition image of inelastic scattering channels due to precise control of internal and translational degrees of freedom of the electrically neutral collision partners.
Ion–molecule scattering studies of reactive channels at colli-
sion energies down to the Kelvin range are showcased in another batch of invited papers. Like the work mentioned above
on the neutrals, the ion–molecule studies make use of the
merged-beam technique that is capable of greatly reducing
the collision energy. The requisite cooling of the ions’ internal
degrees of freedom as well as of their translation is achieved
sympathetically, either by thermalizing the ions in question
with a cloud of ultracold atoms or with cryogenic helium
buffer gas. Apart from insights into the reaction dynamics, also
the elucidation of the properties of stabilized ion-molecule reaction intermediates is reported. These “ion papers” are complemented by a state-of-the-art vibrational spectroscopy study
of trapped ion clusters.
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Editorial
The progress on the subject of controlling molecular rotation, Finally, a couple of papers analyze the behavior of polar moleincluding the production of directional (aligned/oriented)
states, is described in a quintet of papers whose emphasis
ranges from super-rotors to Efimov states to the separation of
species and states via optical deflection. Some of the studies
identify a “gyroscopic effect” that ensures the preservation of
the alignment of rotational angular momentum throughout
a sequence of many collisions.
cules cold enough to be trapped in an optical lattice and exploited for simulations of double-well behavior or for universal
quantum computing. Whether realizable in the near future or
not, systems of trapped molecules offer themselves as subjects
for archetypal studies that often lead to analytic results.
Two invited papers deal with molecules embedded in super-
Chem “Physics and Chemistry with Cold Molecules” will partake
in the enthusiasm for the field of the authors and guest editors
alike as well as share their view that much of the work done in
the gas phase to answer the “burning questions” of our day is
ever more relying on cold molecules.
fluid helium nanodroplets. A recent description of the embedded molecules as quasi-particles made it possible to understand quantitatively the renormalization of the molecular moments of inertia in the droplets and provided further insights
into this intriguing many-body problem. One of the papers
generalizes the quasiparticle approach to the case when the
embedded molecule is polar and the whole system subject to
an electrostatic field. The second paper appearing in this Special Issue deals with systematic effects observed for the case of
molecular clusters self-assembled within a helium nanodroplet.
ChemPhysChem 2016, 17, 3581 – 3582
www.chemphyschem.org
We hope that the readers of the Special Issue of ChemPhys-
[1] Otto Stern, Nobel Lectures, Physics 1942–1962, 1964, Amsterdam: Elsevier
Publishing Company.
[2] N. Ramsey, Z. Phys. D 1988, 10, 121.
[3] D. Herschbach, Adv. Chem. Phys. 1966, 10, 319 – 393.
[4] D. Kleppner, Rev. Mod. Phys. 1999, 71, S78 – S84.
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