Vibrational energy transfer in ultracold molecule - molecule collisions

We present a rigorous study of vibrational relaxation in p-H2 + p-H2 collisions at cold and ultracold temperatures and identify an efficient mechanism of ro-vibrational energy transfer. If the colliding molecules are in different rotational and vibrational levels, the internal energy may be transferred between the molecules through an extremely state-selective process involving simultaneous conservation of internal energy and total rotational angular momentum. The same transition in collisions of distinguishable molecules corresponds to the rotational energy transfer from one vibrational state of the colliding molecules to another.Comment: 4 pages, 4 figure

Two-probe theory of scanning tunneling microscopy of single molecules: Zn(II)-etioporphyrin on alumina

We explore theoretically the scanning tunneling microscopy of single molecules on substrates using a framework of two local probes. This framework is appropriate for studying electron flow in tip/molecule/substrate systems where a thin insulating layer between the molecule and a conducting substrate transmits electrons non-uniformly and thus confines electron transmission between the molecule and substrate laterally to a nanoscale region significantly smaller in size than the molecule. The tip-molecule coupling and molecule-substrate coupling are treated on the same footing, as local probes to the molecule, with electron flow modelled using the Lippmann-Schwinger Green function scattering technique. STM images are simulated for various positions of the stationary (substrate) probe below a Zn(II)-etioporphyrin I molecule. We find that these images have a strong dependence on the substrate probe position, indicating that electron flow can depend strongly on both tip position and the location of the dominant molecule-substrate coupling. Differences in the STM images are explained in terms of the molecular orbitals that mediate electron flow in each case. Recent experimental results, showing STM topographs of Zn(II)-etioporphyrin I on alumina/NiAl(110) to be strongly dependent on which individual molecule on the substrate is being probed, are explained using this model. A further experimental test of the model is also proposed.Comment: Physical Review B, in pres

Dissociation and Decay of Ultra-cold Sodium Molecules

The dissociation of ultracold molecules is studied by ramping an external magnetic field through a Feshbach resonance. The observed dissociation energy shows non-linear dependence on the ramp speed and directly yields the strength of the atom-molecule coupling. In addition, inelastic molecule-molecule and molecule-atom collisions are characterized

Collisional decay of 87Rb Feshbach molecules at 1005.8 G

We present measurements of the loss-rate coefficients K_am and K_mm caused by inelastic atom-molecule and molecule-molecule collisions. A thermal cloud of atomic 87Rb is prepared in an optical dipole trap. A magnetic field is ramped across the Feshbach resonance at 1007.4 G. This associates atom pairs to molecules. A measurement of the molecule loss at 1005.8 G yields K_am=2 10^-10 cm^3/s. Additionally, the atoms can be removed with blast light. In this case, the measured molecule loss yields K_mm=3 10^-10 cm^3/s

Single-molecule stochastic resonance

Stochastic resonance (SR) is a well known phenomenon in dynamical systems. It consists of the amplification and optimization of the response of a system assisted by stochastic noise. Here we carry out the first experimental study of SR in single DNA hairpins which exhibit cooperatively folding/unfolding transitions under the action of an applied oscillating mechanical force with optical tweezers. By varying the frequency of the force oscillation, we investigated the folding/unfolding kinetics of DNA hairpins in a periodically driven bistable free-energy potential. We measured several SR quantifiers under varied conditions of the experimental setup such as trap stiffness and length of the molecular handles used for single-molecule manipulation. We find that the signal-to-noise ratio (SNR) of the spectral density of measured fluctuations in molecular extension of the DNA hairpins is a good quantifier of the SR. The frequency dependence of the SNR exhibits a peak at a frequency value given by the resonance matching condition. Finally, we carried out experiments in short hairpins that show how SR might be useful to enhance the detection of conformational molecular transitions of low SNR.Comment: 11 pages, 7 figures, supplementary material (http://prx.aps.org/epaps/PRX/v2/i3/e031012/prx-supp.pdf

Semifluxon molecule under control

Josephson junctions with a phase drop pi in the ground state allow to create vortices of supercurrent carrying only half of the magnetic flux quantum Phi_0~2.07*10^-15 Wb. Such semifluxons have two-fold degenerate ground states denoted up (with flux +Phi_0/2 and supercurrent circulating clockwise) and down (with flux -Phi_0/2 and supercurrent circulating counterclockwise). We investigate a molecule consisting of two coupled semifluxons in a 0-pi-0 long Josephson junction. The fluxes (polarities) of semifluxons are measured by two on-chip SQUIDs. By varying the dc bias current applied to the 0-pi-0 junction, we demonstrate controllable manipulation and switching between two states, up-down and down-up, of a semifluxon molecule. These results provide a major step towards employing semifluxons as bits or qubits for classical and quantum digital electronics

Molecular formations in ultracold mixtures of interacting and noninteracting atomic gases

Atom-molecule equilibrium for molecular formation processes is discussed for boson-fermion, fermion-fermion, and boson-boson mixtures of ultracold atomic gases in the framework of quasichemical equilibrium theory. After presentation of the general formulation, zero-temperature phase diagrams of the atom-molecule equilibrium states are calculated analytically; molecular, mixed, and dissociated phases are shown to appear for the change of the binding energy of the molecules. The temperature dependences of the atom or molecule densities are calculated numerically, and finite-temperature phase structures are obtained of the atom-molecule equilibrium in the mixtures. The transition temperatures of the atom or molecule Bose-Einstein condensations are also evaluated from these results. Quantum-statistical deviations of the law of mass action in atom-molecule equilibrium, which should be satisfied in mixtures of classical Maxwell-Boltzmann gases, are calculated, and the difference in the different types of quantum-statistical effects is clarified. Mean-field calculations with interparticle interactions (atom-atom, atom-molecule, and molecule-molecule) are formulated, where interaction effects are found to give the linear density-dependent term in the effective molecular binding energies. This method is applied to calculations of zero-temperature phase diagrams, where new phases with coexisting local-equilibrium states are shown to appear in the case of strongly repulsive interactions.Comment: 35 pages, 14 figure