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

Quantum theory of open systems

The tunneling current from a scanning tunneling microscope was used to image and dissociate single O{sub 2} molecules on the Pt(111) surface in the temperature range of 40 to 150K. After dissociation, the two oxygen atoms are found one to three lattice constants apart. The dissociation rate as a function of current was found to vary as I{sup 0.8{plus_minus}0.2} , I{sup 1.8{plus_minus}0.2} , and I{sup 2.9{plus_minus}0.3} for sample biases of 0.4, 0.3, and 0.2V, respectively. These rates are explained using a general model for dissociation induced by intramolecular vibrational excitations via resonant inelastic electron tunneling. {copyright} {ital 1997} {ital The American Physical Society}

Single Molecule Dissociation by Tunneling Electrons

Helium nanodroplets have emerged as a new and exciting medium for studying the structure and dynamics of both this quantum solvent and impurities that can be doped into (onto) and grown inside (on the surface) of the droplets. Spectroscopic studies of these molecular impurities can provide detailed information on helium as a solvent and its interaction with the solute. This is particularly important given that helium is completely transparent to photons below 20 eV, making the direct spectroscopic study of liquid helium problematic. Since liquid helium is an extremely weak solvent, the corresponding perturbations to the spectrum of the solute molecules are often minor; really only evident because of the high resolution that is often achieved in such studies. As a result, helium nanodroplet spectra often resemble the corresponding gas-phase results. Indeed, for the case of rotational spectroscopy, the gas-phase Hamiltonian is often sufficient to describe the system, with the effects of the solvent being to...

Infrared spectroscopy of helium nanodroplets: novel methods for physics and chemistry

The observation of coherent dynamics ensuing from the excitation of molecular systems by femtosecond laser pulses is at the heart of femtochemistry. The time-dependent evolution of coherent superpositions of quantum states, the physical basis for the observation of coherent dynamics and their manipulation are of central importance to coherent control of physicochemical processes. In the early days of femtochemistry, there was significant skepticism regarding the type of information that could be learned from spectroscopic experiments using very short pulses. There were arguments that one could infer the dynamics from frequency-resolved experiments and that the femtosecond experiments did not offer new information. It was also assumed that experiments with very short pulses would smear the available spectroscopic information because of their broad bandwidths. Approximately two decades after the initial experiments, it has become clear that femtosecond experiments have opened an extremely valuable window into the dynamic behavior of atomic and molecular systems that is influencing how we think about physics, chemistry, and biology. In particular, we highlight the fact that some of the highest resolution spectroscopic measurements being carried out employ femtosecond laser pulses. These experiments, specifically those taking advantage of rotational coherence, provide resolution that rivals microwave spectroscopy. The reason spectroscopic information is not lost in femtochemistry experiments is coherence, a property that can be manipulated to control physicochemical processes by a number of different approaches, which will be reviewed here. This review presents a summary of some of the most salient contributions to the field of coherent laser control from an experimentalist’s perspective. While a number of theoretical papers have made key contributions to the field, it is from the experimental successes, as well as failures, that we can best learn how to implement new strategies and develop future applications. Coherent laser control, in the context of this review, encompasses experiments in which the coherent properties of the laser and/or the molecule are required for controlling a particular physicochemical process. We distinguish for each of the experiments between coherence in the laser field(s), * To whom correspondence should be addressed. Phone (517) 3559715 (ext-315). Fax (517) 353-1793. E-mail dantus@msu.edu. 1813 Chem. Rev. 2004, 104, 1813−1859

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Experimental coherent laser control of physicochemical processes.

A central problem in the theory of the dynamics of open quantum systems is the derivation of a rigorous and computationally tractable master equation for the reduced system density matrix. Most generally, the evolution of an open quantum system is described by a completely positive linear map. We show how to derive a completely positive Markovian master equation (the Lindblad equation) from such a map by a coarse-graining procedure. We provide a novel and explicit recipe for calculating the coefficients of the master equation, using perturbation theory in the weak-coupling limit. The only parameter external to our theory is the coarse-graining time-scale. We illustrate the method by explicitly deriving the master equation for the spin-boson model. The results are evaluated for the exactly solvable case of pure dephasing, and an excellent agreement is found within the time-scale where the Markovian approximation is expected to be valid. The method can be extended in principle to include non-Markovian effects.

From completely positive maps to the quantum Markovian semigroup master equation

A new krypton-containing compound, HKrF, has been prepared in a low-temperature Kr matrix via VUV photolysis of the HF precursor and posterior thermal mobilization of H and F atoms. All three fundamental vibrations have been observed in the FTIR spectra at ∼1950 cm−1 (H–Kr stretch), ∼650 cm−1 (bending), and ∼415 cm−1 (Kr–F stretch). Two distinct sites of HKrF have been identified. The energy difference between the H–Kr stretching vibrations for the two sites is remarkably large (26 cm−1), indicating a strong influence of the environment. In annealing after the photolysis of the precursor, HKrF is formed in two different stages: at 13–16 K from closely trapped H+F pairs and at T>24 K due to more extensive mobility of H and F atoms in the matrix. HKrF in a less stable site decreases at temperatures above 32 K, the other site being stable up to the sublimation temperature of the matrix. The photodecomposition cross section for HKrF has been measured between 193 and 350 nm and compared with the cross sections...

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HKrF in solid krypton

The vibrational spectroscopy and the matrix-site geometries of several novel rare-gas compounds in the matrix environment were computed theoretically, and compared with experiment. Ab initio calculations are used in the fitting of analytical potential surfaces for the HRgY molecules and for the interactions between HRgY and the matrix atoms Rg. With these potentials, matrix-site geometries for the molecule in the solid are computed. Finally, the vibrational spectroscopy of HRgY in the Rg matrix is computed using the vibrational self-consistent field (VSCF) method. The VSCF includes anharmonic effects, that are essential in this case. The version of VSCF used here includes coupling between HRgY and the vibrations of the solid atoms. The vibrations of 72 matrix atoms are treated. The main results are: (1) The matrix shifts are considerably greater than typically found for neutral, strongly bond molecules, but are much smaller than discrepancies between theory and experiment. This can be attributed to the insufficient accuracy of the potentials used for the HRgY molecules. This calls for better future description of the electronic structure of HRgY. (2) The matrix shifts and splitting effects are interpreted by the calculations in terms of the site geometries involved. These effects are very different for HArF, HKrF than for HXeCl, HXeI. (3) The computed matrix-site splittings are in semiquantitative accord with experiment. This supports the interaction potentials used between HRgY and the matrix. The results provide insights on the effects of the matrix on the rare-gas molecules.

Vibrational spectroscopy and matrix-site geometries of HArF, HKrF, HXeCl, and HXeI in rare-gas solids

We report the production in the gas phase of ionically bound HXeI molecules. The molecules are generated by the photodissociation of HI molecules in large Xen clusters and are identified from the asymmetry of the detected H atom fragments arising from the dissociation of oriented HXeI. The orientation, resulting from a synergistic action of a pulsed laser field with a weak electrostatic field, is quite pronounced, due to a large ratio of the polarizability anisotropy to the rotational constant of HXeI.

Photodissociation of oriented HXeI molecules generated from HI-Xen clusters

Pump–probe measurements of I2 in solid Ar are reported and analyzed to extract a description of cage response to impulsive excitation, from the gentle kick, up to the breaking point. The most informative data are obtained through wavepacket motion on cage-bound, but otherwise dissociative, potentials where the chromophore acts as a transducer to drive the cage and to report on the local dynamics. This general class of dynamics is identified and analyzed as a function of energy in Ar, Kr, and Xe. The overdriven cage rebounds with a characteristic period of 1.2 ps that shows little dependence on excitation amplitude in all hosts. After rebound, the cage rings as a local resonant mode in Ar, with a period of 1 ps and dephasing time of 3 ps. This mode remains at the Debye edge in Kr and Xe, with periods of 630 and 800 fs, and dephasing times of 8 and 6 ps, respectively. In the bound B-state, the cage fluctuates toward its dilated equilibrium structure on a time scale of 3 ps, which is extracted from the down-...

Zsolt Bihary

Papers

From completely positive maps to the quantum Markovian semigroup master equation

HKrF in solid krypton

Vibrational spectroscopy and matrix-site geometries of HArF, HKrF, HXeCl, and HXeI in rare-gas solids

Photodissociation of oriented HXeI molecules generated from HI-Xen clusters

Dynamics and the breaking of a driven cage: I2 in solid Ar.