Efficient production of ground-state potassium molecules at sub-mK temperatures by two-step photoassociation

Time-dependent quantum simulation of coronene photoemission spectra

Photoelectron spectroscopy is usually described by a simple equation that relates the binding energy of the photoemitted electron, Ebinding, its kinetic energy, Ekinetic, the energy of the ionizing photon, Ephoton, and the work function of the spectrometer, ϕ, Ebinding = Ephoton - Ekinetic - ϕ. Behind this equation there is an extremely rich physics, which we describe here using as an example a relatively simple conjugated molecule, namely coronene. The theoretical analysis of valence band and C1s core level photoemission spectra showed that multiple excitations play an important role in determining the intensities of the final spectrum. An explicit, time-evolving model is applied, which is able to count all possible photo-excitations occurring during the photoemission process, showing that they evolve on a short time-scale, of about 10 fs. The method reveals itself to be a valid approach to reproduce photoemission spectra of polycyclic aromatic hydrocarbons (PAHs).

The X (X = F, Cl, I) effect on the photoassociation of H+X→HX

This work explores the vibrational state-selective photoassociation (PA) in the ground state of the HX (X = F, Cl, I) molecule by solving the time-dependent Schrödinger equation. For the three systems, the vibrational level of [Formula: see text] is set to be the target state and the PA probability of the target state is calculated and compared by considering different initial collision momentums. It is found that the PA probabilities are in accordance with Franck–Condon overlap integral for the HI and HCl systems, but it is not the case for the HF system. Moreover, for the HF system, it is shown that the PA probability of the target state is largest and the multiphoton transition is more likely to occur.

Formation of ultracold polar molecules in the rovibrational ground state

Ultracold LiCs molecules in the absolute ground state X1Sigma+, v'' = 0, J'' = 0 are formed via a single photoassociation step starting from laser-cooled atoms. The selective production of v'' = 0, J'' = 2 molecules with a 50-fold higher rate is also demonstrated. The rotational and vibrational state of the ground state molecules is determined in a setup combining depletion spectroscopy with resonant-enhanced multiphoton ionization time-of-flight spectroscopy. Using the determined production rate of up to 5 x 10(3) molecules/s, we describe a simple scheme which can provide large samples of externally and internally cold dipolar molecules.

Control of Branching Ratios in the Dissociative Ionization of Deuterium Chloride

The dissociative ionization of deuterium chloride (DCl) has been investigated by employing femtosecond laser pulses at 805 nm. The product branching ratio D(+)/Cl(+) of the fragments D(+) and Cl(+) is strongly affected by the chirp alpha of the laser pulses. The ratio can be controlled by a factor of 3 ranging from D(+)/Cl(+) = 0.7 at alpha = -800 fs(2) to D(+)/Cl(+) = 1.9 at alpha = +150 fs(2). The observation can be rationalized by a model where negative chirp favors intra-electronic state excitation, and positive chirp favors inter-electronic state excitation in the dissociation of the molecular ion. Complementary experiments on hydrogen chloride (HCl) are discussed.

Ab initio relativistic calculation of the RbCs molecule

We apply the relativistic configuration-interaction valence-bond method to calculate various characteristics of the alkali-metal RbCs dimer. These include the electronic potentials and transition dipole moments between the ground and first excited states and permanent dipole moments of the X 1sigma+ and a 3sigma+ states of the ground configuration. In addition, we estimate the lifetime of the rovibrational levels of the X state due to blackbody radiation. These data can help experimentalists to optimize photoassociative formation of ultracold RbCs molecules and their longevity in a trap or in an optical lattice. Extended basis sets, constructed from Dirac-Fock and Sturm's orbitals, have been used to ensure convergence of our calculations. We compare our data with other theoretical and experimental results when they were available.

Reactivity enhancement of ultracold O(P3)+H2 collisions by van der Waals interactions

The role of van der Waals forces in O((3)P)+H(2)(upsilon=1,j=0) collisions is investigated theoretically at low and ultralow temperatures. Quantum scattering calculations have been performed for zero total angular momentum using the lowest London-Eyring-Polanyi-Sato double-polynomial (3)A(") potential-energy surface reported by [Rogers et al., J. Phys. Chem. A 104, 2308 (2000)] and its recent BMS1 and BMS2 extensions developed by [Brandao et al., J. Chem. Phys. 121, 8861 (2004)] which provide a more accurate treatment of the van der Waals interaction. Our calculations show that van der Waals forces strongly influence chemical reactivity at ultracold translational energies. The presence of a zero-energy resonance for the BMS1 surface is found to enhance reactivity in the ultracold regime and shift the Wigner threshold to lower temperatures.

Laser cooling of vibrational degrees of freedom of a molecular system

We consider the cooling of vibrational degrees of freedom in a photoinduced excited electronic state of a model molecular system. For the various parameters of the potential surfaces of the ground and excited electronic states and depending on the excitation frequency of a single-mode laser light, the average energy or average vibrational temperature of the excited state passes through a minimum. The amount of cooling is quantified in terms of the overlap integral between the ground and excited electronic states of the molecule. We have given an approach to calculate the Franck-Condon factor for a multimode displaced-distorted-rotated oscillator surface of the molecular system. This is subsequently used to study the effect of displacement, distortion, and Duschinsky rotation on the vibrational cooling in the excited state. The absorption spectra and also the average energy or the effective temperature of the excited electronic state are studied for the above model molecular system. Considering the non-Condon effect for the symmetry-forbidden transitions, we have discussed the absorption spectra and average temperature in the excited-state vibrational manifold.

Quantum dynamics of the Li+HF→H+LiF reaction at ultralow temperatures

Quantum-mechanical calculations are reported for the Li+HF(v=0,1,j=0)-->H+LiF(v',j') bimolecular scattering process at low and ultralow temperatures. Calculations have been performed for zero total angular momentum using a recent high-accuracy potential-energy surface for the X2A' electronic ground state. For Li+HF(v=0,j=0), the reaction is dominated by resonances due to the decay of metastable states of the Li cdots,...F-H van der Waals complex. Assignment of these resonances has been carried out by calculating the eigenenergies of the quasibound states. We also find that while chemical reactivity is greatly enhanced by vibrational excitation, the resonances get mostly washed out in the reaction of vibrationally excited HF with Li atoms. In addition, we find that at low energies, the reaction is significantly suppressed due to the less-efficient tunneling of the relatively heavy fluorine atom.

Quantum mechanical investigation of rovibrational relaxation of H2 and D2 by collisions with Ar atoms

We report quantum mechanical calculations of cross sections and rate coefficients for rovibrational relaxation of H2 and D2 by collisions with Ar atoms over a wide range of temperatures including the ultracold limit. Limiting values of the rate coefficients for vibrational and rotational quenching at zero temperature were computed and sensitivity of the results to the choice of the interaction potential is investigated. We also demonstrate dramatic change in the behavior of the rate coefficients at low temperatures when the van der Waals potential supports a quasibound level very close to the dissociation threshold.

On the role of van der Waals interaction in chemical reactions at low temperatures

It is shown that van der Waals interaction potential plays a crucial role in chemical reactions at low temperatures. By taking the Cl+HD reaction as an illustrative example, we demonstrate that quasibound states of the van der Waals potential preferentially undergo chemical reaction rather than vibrational predissociation. Prereaction occurs even when the wave functions of the quasibound states peak far out into the entrance channel, outside the region of the van der Waals well. It is found that chemical reaction dominates over nonreactive vibrational quenching in collisions of vibrationally excited HD molecules with ground state chlorine atoms at ultracold temperatures.

Production and state-selective detection of ultracold RbCs molecules

Using resonance-enhanced two-photon ionization, we detect ultracold, metastable RbCs molecules formed in their lowest triplet state a (3)Sigma(+) via photoassociation in a laser-cooled mixture of 85Rb and 133Cs atoms. We obtain extensive bound-bound excitation spectra of these molecules, which provide detailed information about their vibrational distribution, as well as spectroscopic data on several RbCs molecular states including a (3)Sigma(+), (2) (3)Sigma(+), and (1) (1)Pi. Analysis of this data allows us to predict strong transitions from observed levels to the absolute vibronic ground state of RbCs, potentially allowing the production of stable, ultracold polar molecules at rates in excess of 10(6) s(-1).

Observation of ultracold ground-state heteronuclear molecules

We report the observation of translationally ultracold heteronuclear ground-state molecules in a two-species magneto-optical trap containing 39K and 85Rb atoms. The KRb molecules are produced via photoassociation and detected by multiphoton ionization. We had characterized their temperature and measured their formation rate constant. We believe that the two-species trap could be used as a reliable source of ultracold molecules to be captured by electrostatic, magnetic, or optical traps. This possibility will certainly motivate further investigation of quantum collective effects as well as high-resolution spectroscopy of the rovibrational level structure of cold heteronuclear molecular systems.

Production of ultracold, polar RbCs* molecules via photoassociation

We have produced ultracold, polar RbCs* molecules via photoassociation in a laser-cooled mixture of Rb and Cs atoms. Using a model of the RbCs* molecular interaction which reproduces the observed rovibrational structure, we infer decay rates in our experiments into deeply bound X(1)Sigma(+) ground-state RbCs vibrational levels as high as 5 x 10(5) s(-1) per level. Population in such deeply bound levels could be efficiently transferred to the vibrational ground state using a single stimulated Raman transition, opening the possibility to create large samples of stable, ultracold polar molecules.

Rotational quenching in ionic systems at ultracold temperatures

The behavior of the rotational quenching for a molecular ion in collision with closed-shell neutral gases is investigated. We confirm that Wigner's threshold law for inelastic scattering holds in the presence of a long-range interaction due to polarization forces decreasing as the inverse fourth power of the distance but find that, because of the contributions of the higher angular momenta, its range of applicability is markedly reduced when compared to the scattering by neutral species. The calculations of the quenching cross sections make evident the special features of ionic systems at ultralow collision energies and yield rate coefficients of the order of 10(-9) x cm(3) x s(-1), much larger than those found for the quenching of neutral molecules.

Blackbody-radiation-assisted laser cooling of molecular ions

The translational motion of molecular ions can be effectively cooled sympathetically to temperatures below 100 mK in ion traps through Coulomb interactions with laser-cooled atomic ions. The distribution of internal rovibrational states, however, gets in thermal equilibrium with the typically much higher temperature of the environment within tens of seconds. We consider a concept for rotational cooling of such internally hot, but translationally cold, heteronuclear diatomic molecular ions. The scheme relies on a combination of optical pumping from a few specific rotational levels into a "dark state" with redistribution of rotational populations mediated by blackbody radiation.

Accumulation of cold cesium molecules via photoassociation in a mixed atomic and molecular trap

We have realized a mixed atomic and molecular trap, constituted by a Cs vapor-cell magneto-optical trap and a quadrupolar magnetic C s(2) trap, using the same magnetic field gradient. We observed the trapping of 2x 10(5) molecules, formed and accumulated in the metastable a (3)Sigma(+ )(u) state at a temperature of 30+/-10 microK through a approximately 150 ms photoassociation process. The lifetime of the trapped molecular cloud limited by the Cs background gas pressure is on the order of 1 s.

Quantum computation with trapped polar molecules

We propose a novel physical realization of a quantum computer. The qubits are electric dipole moments of ultracold diatomic molecules, oriented along or against an external electric field. Individual molecules are held in a 1D trap array, with an electric field gradient allowing spectroscopic addressing of each site. Bits are coupled via the electric dipole-dipole interaction. Using technologies similar to those already demonstrated, this design can plausibly lead to a quantum computer with greater, approximately > or = 10(4) qubits, which can perform approximately 10(5) CNOT gates in the anticipated decoherence time of approximately 5 s.

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

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+6p1/2) and (6s+6p3/2) 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.

Multimode dynamics of a coupled ultracold atomic-molecular system

We analyze the coherent multimode dynamics of a system of coupled atomic and molecular Bose gases. Starting from an atomic Bose-Einstein condensate with a small thermal component, we observe a complete depletion of the atomic and molecular condensate modes on a short time scale due to a significant population of excited states. Giant coherent oscillations between the two condensates for typical parameters are almost completely suppressed. Our results cast serious doubts on the common use of the 2-mode model for the description of coupled ultracold atomic-molecular systems and should be considered when planning future experiments with ultracold molecules.

Forced molecular rotation in an optical centrifuge

Intense linearly polarized light induces a dipole force that aligns an anisotropic molecule to the direction of the field polarization. Rotating the polarization causes the molecule to rotate. Using femtosecond laser technology, we accelerate the rate of rotation from 0 to 6 THz in 50 ps, spinning chlorine molecules from near rest up to angular momentum states J approximately 420. At the highest spinning rate, the molecular bond is broken and the molecule dissociates.