Bretislav Friedrich

Magnetic trapping of calcium monohydride molecules at millikelvin temperatures

Recent advances in the magnetic trapping and evaporative cooling of atoms to nanokelvin temperatures have opened important areas of research, such as Bose–Einstein condensation and ultracold atomic collisions. Similarly, the ability to trap and cool molecules should facilitate the study of ultracold molecular physics and collisions; improvements in molecular spectroscopy could be anticipated. Also, ultracold molecules could aid the search for electric dipole moments of elementary particles. But although laser cooling (in the case of alkali metals,,) and cryogenic surface thermalization (in the case of hydrogen,) are currently used to cool some atoms sufficiently to permit their loading into magnetic traps, such techniques are not applicable to molecules, because of the latters complex internal energy-level structure. (Indeed, most atoms have resisted trapping by these techniques.) We have reported a more general loading technique based on elastic collisions with a cold buffer gas, and have used it to trap atomic chromium and europium,. Here we apply this technique to magnetically trap a molecular species—calcium monohydride (CaH). We use Zeeman spectroscopy to determine the number of trapped molecules and their temperature, and set upper bounds on the cross-sectional areas of collisional relaxation processes. The technique should be applicable to many paramagnetic molecules and atoms.

Editorial Quo vadis, cold molecules?

We give a snapshot of the rapidly developing field of ultracold polar molecules abd walk the reader through the papers appearing in this topical issue.

Time evolution of pendular states created by the interaction of molecular polarizability with a pulsed nonresonant laser field

Previous investigations have shown that the instantaneous eigenstates of a molecule interacting via its polarizability with a strong electric field of a nonresonant laser pulse are pendular hybrids of field-free rotational states, aligned along the field direction. However, nonadiabatic effects during the time evolution of the initial field-free rotational state could cause the molecule to end up in a state described by a linear combination of pendular states (a rotational wavepacket) whose alignment properties are not a priori known. We report a computational study of the time evolution of these states. We solve the reduced time-dependent Schrodinger equation for an effective Hamiltonian acting within the vibronic ground state. Our numerical results show that the time evolution and the achievement of adiabatic behavior depend critically on the detailed characteristics of the laser pulse and the rotational constant of the molecule.

Enhanced orientation of polar molecules by combined electrostatic and nonresonant induced dipole forces

Recent experiments have demonstrated the efficacy of orienting low rotational states of a linear polar molecule in a static electric field, eS, or aligning a molecule (polar or not) in an intense nonresonant laser field, eL. We present theoretical results showing that the combined action of eS and eL can markedly sharpen orientation, particularly by introducing a pseudo-first-order Stark effect for tunneling doublets created by the polarizability interaction. Also, if eS and eL are not collinear, the molecular axis can be localized with respect to φ as well as θ, since M states as well as J states undergo hybridization. Another benefit is a means to eliminate “wrong way orientation” which otherwise occurs for “low-field seeking” states.

Cold molecules: theory, experiment, applications

COLD COLLISONS Theory of Cold Atomic and Molecular Collisions, J.M. Hutson Electric Dipoles at Ultralow Temperatures, J.L. Bohn Inelastic Collisions and Chemical Reactions of Molecules at Ultracold Temperatures, G. Quemener, N. Balakrishnan and A. Dalgarno Effects of External Electromagnetic Fields on Collisions of Molecules at Low Temperatures, T.V. Tscherbul and R.V. Krems PHOTOASSOCIATION Ultracold Molecule Formation by Photoassociation, W.C. Stwalley, P.L. Gould, and E.E. Eyler Molecular States Near a Collision Threshold, P.S. Julienne Prospects for Control of Ultracold Molecule Formation via Photoassociation with Chirped Laser Pulses, E. Luc-Koenig and F. Masnou-Seeuws Adiabatic Raman Photoassociation with Shaped Laser Pulses, E.A. Shapiro and M. Shapiro FEW- AND MANY-BODY PHYSICS Ultracold Feshbach Molecules, F. Ferlaino, S. Knoop, and R. Grimm Molecular Regimes in Ultracold Fermi Gases, D.S. Petrov, C. Salomon, and G.V. Shlyapnikov Theory of Ultracold Feshbach Molecules, T.M. Hanna, H. Martay and T. Kohler Condensed Matter Physics with Cold Polar Molecules, G. Pupillo, A. Micheli, H.P. Buchler, and P. Zoller COOLING AND TRAPPING Cooling, Trap Loading, and Beam Production Using a Cryogenic Helium Buffer Gas, W.C. Campbell and J.M. Doyle Slowing, Trapping, and Storing of Polar Molecules by Means of Electric Fields, S.Y.T. van de Meerakker, H.L. Bethlem, and G. Meijer TESTS OF FUNDAMENTAL LAWS Preparation and Manipulation of Molecules for Fundamental Physics Tests, M.R. Tarbutt, J.J. Hudson, B.E. Sauer, and E.A. Hinds Variation of the Fundamental Constants as Revealed by Molecules: Astrophysical Observations and Laboratory Experiments, V.V. Flambaum and M.G. Kozlov QUANTUM COMPUTING Quantum Information Processing with Ultracold Polar Molecules, S.F. Yelin, D. DeMille, and R. Cote COLD MOLECULAR IONS Sympathetically Cooled Molecular Ions: From Principles to First Applications, B. Roth and S. Schiller Index

On the possibility of orienting rotationally cooled polar molecules in an electric field

It has long been thought that only symmetric top molecules (or equivalent) can be appreciably oriented in an electric field. This assessment is unduly pessimistic. In molecular beams produced by supersonic expansion, the rotational temperature can be made very low (∼ 1 K). For many diatomic, linear, or asymmetric top molecules, quite substantial orientation can thereby be attained for a large fraction of the beam at accessible field strengths (∼ 100 kV/cm or less). We present model calculations and an experimental design to evaluate the method by observing the fluorescence of photofragments from oriented molecules. Nomograms are provided from which the orientation can be estimated for linear molecules from the dipole moment, rotational constant, rotational temperature, and field strength.

Stern and Gerlach: How a Bad Cigar Helped Reorient Atomic Physics

The history of the Stern–Gerlach experiment reveals how persistence, accident, and luck can sometimes combine in just the right ways.

Charge transfer and structured vibrational distributions in H++CH4 low‐energy collisions

Inelastic and charge transfer collisions of protons with methane molecules have been investigated in a perpendicular‐plane crossed beam experiment via the detection of the scattered protons and H atoms, respectively. Time‐of‐flight analysis of the protons and H atoms at scattering angles 0°≤θ≤10° and collision energies 10≤E≤30 eV provided information on internal energy distributions of the CH4 and CH+4 products. Excitation of the n(ν1 ,ν3) +m (ν2 ,ν4) type vibrations, with n,m=0, 1, 2,⋅⋅⋅was found to be the most probable assignment of the observed structured energy distributions of CH4 (1 A1 ) at θ≤4°. At θ>4°, the energy transfer increases steeply up to the dissociation limit while the vibrational structure was no longer resolved. In the case of charge transfer, the observed narrow internal energy distributions corresponding to a most probable average internal energy of CH+4 of about 0.95 eV was centered at the recombination energy of the proton indicative of quasiresonant charge transfer. In addition, f...

The stochastic dance of circling sperm cells: sperm chemotaxis in the plane

Biological systems such as single cells must function in the presence of fluctuations. It has been shown in a two-dimensional experimental setup that sea urchin sperm cells move toward a source of chemoattractant along planar trochoidal swimming paths, i.e. drifting circles. In these experiments, a pronounced variability of the swimming paths is observed. We present a theoretical description of sperm chemotaxis in two dimensions which takes fluctuations into account. We derive a coarse-grained theory of stochastic sperm swimming paths in a concentration field of chemoattractant. Fluctuations enter as multiplicative noise in the equations for the sperm swimming path. We discuss the stochastic properties of sperm swimming and predict a concentration-dependence of the effective diffusion constant of sperm swimming which could be tested in experiments.

Molecules are cool

The development of methods for slowing and trapping gaseous species has led to a renaissance in atomic physics, which is now also progressing into molecular/chemical physics. The latest advances come from two groups who have devised techniques that, in principle, provide new approaches for trapping molecules and spectroscopically studying them.