Search

The emission spectrum of the AlD isotopologue has been experimentally studied using Fourier transform spectroscopy in the 17 , 700 − 28 , 500 cm−1 spectral region. AlD molecules were produced within an aluminum hollow-cathode lamp with two anodes in the presence of 2.7 Torr of Ne and 0.2 Torr of ND3. The observed bands have been assigned to the A 1 Π − X 1 Σ + ( 0 − 0 , 1 and 1 − 0 , 1 , 2 , 3 bands) and C 1 Σ + − A 1 Π ( 0 − 0 and 1 − 1 bands) transitions. In total, more than 450 rotational frequencies were measured with an absolute accuracy of about 0.002 cm−1. The current data have been combined with available measurements of the pure rotational lines by Halfen and Ziurys [Astrophys J 2010; 713: 520-523] and with the ro-vibrational bands by White et al. [J Chem Phys 1993;99:8371-8378] to provide improved spectroscopic constants of the X 1 Σ + , A 1 Π and C 1 Σ + states of AlD. These results match closely theoretical values obtained using a new set of ab initio potentials calculated with a spin-orbit Hamiltonian.

The spectrum of the AlD + isotopologue has been investigated at high resolution in the 27 , 000 − 29 , 000 cm − 1 region using a Fourier transform emission spectroscopy technique. The AlD + molecules were produced within a water-cooled aluminum hollow-cathode lamp in the presence of 1.5 Torr of Ne and 0.8 Torr of ND3. The (0, 0) and (1, 1) bands belonging to the A 2 Π − X 2 Σ + system were recorded with an instrumental resolution of 0.03 cm − 1 . In total, almost 500 rotational frequencies were measured with an absolute accuracy of about 0.005 cm − 1 . It improved the experimental accuracy of the determined frequencies by the factor 10 compared to the previous work [J Phys B: Mol Phys 1984;17:L861-L866]. The rotational analysis has shown irregularities in the Λ − doubling splitting of the A2Π v = 0 , 1 . Consequently, the A2Π state has been represented by the rotational term values, while the regular X 2 Σ + state by the molecular constants. The causes of the irregularities were identified in the interaction between the A2Π and the B 2 Σ + states, which lies about 3720 cm − 1 above. Supporting ab initio quantum chemical calculations, including spin-orbit effects, reproduce the observed spectroscopic constants including the small energy splittings due to spin-rotation interactions (for 2 Σ + states) and Λ − doubling.

We report on the UV photodissociation of specific vibrational states ( v = 2–45) of ClO + using velocity map ion imaging. The high vibrational states of ClO + are prepared via a double resonant scheme through the ClO (A 2 Π) state and ion-pair states followed by photoionization with a third photon. The absorption of a fourth photon results in photodissociation of the ClO + into two dominant asymptotic channels. The Cl + and O + fragment ion images reveal information on both the energetics of high-lying cation vibrational states and the low-lying dissociative electronic states that correlate to Cl + ( 3 P) + O( 3 P) and Cl( 2 P) + O + ( 4 S) asymptotic channels. We also report ab initio potentials for the bound ClO + and ion-pair states as well as calculations of the ClO + excited states relevant to the photodissociation process.

A realistic model of the dipole radiation forces in transverse Doppler cooling (with a σ+–σ− laser configuration) of an atomic beam of group 13 elements is studied within the quantum-kinetic equation framework. The full energy level sub-structure for such an atom with I = 0 (such as 66Ga) is analysed. Two cooling strategies are investigated; the first involving the 2P3/2 → 2D5/2 transition and the second a dual laser cooling experiment involving transitions 2P1/2 and 2P3/2 → 2S1/2. The latter scheme creates a velocity-independent dark-state resonance that inhibits a steady-state dipole cooling force. However, time-dependent calculations show that transient cooling forces are present that could be exploited for laser cooling purposes in pulsed laser fields.

A counterintuitive scheme to produce ultracold hydrogen via fragmentation of laser cooled diatomic hydrides is presented where the final atomic H temperature is inversely proportional to the mass of the molecular parent. In addition, the critical density for formation of a Bose-Einstein condensate (BEC) at a fixed temperature is reduced by a factor $({m}_{H}/\phantom{\rule{0.0pt}{0ex}}{m}_{MH}{)}^{3/2}$ over directly cooled hydrogen atoms. The narrow Feshbach resonances between a $^{1}S_{0}$ atom and hydrogen are well suited to a tiny center of mass energy release necessary during fragmentation. With the support of ab initio quantum chemistry, it is demonstrated that BaH is an ideal diatomic precursor that can be laser cooled to a Doppler temperature of $\ensuremath{\sim}26\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$ with just two rovibronic transitions, the simplest molecular cooling scheme identified to date. Preparation of a hydrogen atom gas below the critical BEC temperature ${T}_{c}$ is feasible with present cooling technology, with optical pulse control of the condensation process.

Naturally occurring metal surfaces possess planes of mirror symmetry on the nanometer-length scale. This mirror symmetry can be lifted and chirality “physically” conveyed onto a surface by adsorbin...

Cavity ring-down spectra of the FCO radical, recorded over the wave number range 29 500–31 600 cm−1 reveal rotational structure of the electronically excited state for the first time. The spectra demonstrate the need for a complete re-assignment of the vibronic features: The rotationally resolved bands are successfully simulated as arising from c-type transitions from the ground X 2A′ state to the linear 2A″ component of the A 2Π state. The bands are attributed to two overlapping vibrational progressions: one progression involves excitation of the F–C–O bending mode (v3′), the other consists of a combination of v3′ and one quantum of the C–F stretch (v2′). Sharp rotational structure is only observed for sub-bands with K′=0; bands with K′>0 are diffuse, indicating rapid, rotation induced predissociation. Band origins, rotational constants for the excited state, and spectral linewidths have been derived from the K′=0–K″=1 sub-bands. All rotational lines are somewhat broadened and there is evidence of linew...

An electronic absorption spectrum of FO has yet to be observed experimentally but as a guide for future spectroscopic studies of this radical, CASSCF/MRCI ab initio calculations have been performed to locate and identify the lowest energy valence and Rydberg states. No bound valence excited potentials were found save the a4Σ- state. The calculations provide estimates of the transition energies Te to the lowest Rydberg states and the wavelengths required for spectroscopic detection. Results for both the doublet and quartet states are presented, revealing the 3sσ, 3pπ, 3pσ, 3dδ, 4sσ, 3dπ, and 4dδ Rydberg orbitals. It is likely that the repulsive valence states 12Σ- and 12Δ predissociate the lowest Rydberg states of the same symmetry. Consequently, the 2Π Rydberg states, three of which are found here, are the most promising candidates for optical detection.

Ab-initio calculations at the CASSCF and MRCI level have been performed on the valence doublet and quartet electronic states of the ClO radical in order to explore the predissociation dynamics of the A2ΠΩ state (Ω=1/2 or 3/2). The ab-initio calculations neglect spin–orbit coupling. Large basis sets with diffuse orbitals were employed to ensure the calculated X2Π and A2Π potentials match closely the available experimental data. In total, the potential energies of fifteen valence states were computed at a series of internuclear separations in order to map the potential energy curves. The X2Π, A2Π and a4Σ- potentials are bound; though the latter has yet to be observed experimentally. All the other states are repulsive, and the lowest ten cross the A2Π3/2 component at energies below the v′=6 vibrational level. Using arguments based on electronic configurations, candidate states for the predissociation of the A2ΠΩ state can be selected. Simulations of the predissociation were performed by Fermi golden rule calculations for each vibrational level, using the ab-initio repulsive potentials and RKR curves for the A2Π3/2 and A2Π1/2 states. The vibrational dependence of the predissociation rate in the A2Π3/2 component was used to derive the magnitude of the individual couplings with the repulsive potentials. Comparison between the lifetimes of the vibrational levels in the A2Π3/2 and A2Π1/2 components restricts the principal players in the predissociation to three repulsive states: 14ΣΩ+, 24ΣΩ- and 32ΠΩ. The possibility of coupling between the A2Π3/2 component and the 12Δ state is also suspected. A marked similarity is revealed for the variation of predissociation rates with v′ for the A2Π3/2 and A2Π1/2 components despite a spin–orbit state dependence of the absolute rates for individual v′ levels. A repulsive 22Σ+ state is identified which is likely to be involved in the UV photodissociation of ClO. Similar ab-initio calculations on FO have revealed no bound excited valence 2Π state correlating diabatically to the Cl(2P)+O(1D) dissociation limit.

The UV absorption of ClO at wavelengths between 285 and 320 nm has been investigated using cavity ring-down spectroscopy. This wavelength region spans the (0,0) to (7,0) bands of the A2Π3/2–X2Π3/2 and A2Π1/2–X2Π1/2 transitions. The previously unobserved A2Π3/2–X2Π3/2 (0,0) and (1,0) absorption bands have been recorded with rotational resolution, and spectra of the (2,0) to (6,0) bands of the A2Π1/2–X2Π1/2 transition are shown for the first time. Analysis of the spectra gives refined band origins and rotational constants for the A2Π v′ levels and reveals a strong v′ dependence in the linewidths of rotational features. The lifetimes of the A2Π3/2v′=0–2 and A2Π1/2v′=2–4 levels are revised from previous estimates, and the lifetimes of A2Π1/2v′=5 and 6 levels have been determined. The deduced predissociation rates for A2Π3/2v′=3–7 confirm earlier studies. The lifetimes of vibrational levels of the two spin–orbit components of the A2ΠΩ state are markedly different. No evidence of J′-dependence in the predissociation is found, in contrast to the corresponding A2Π3/2 state of the IO radical.

The IO A2Π3/2–X2Π3/2 (v′,0) bands with v′ = 0–5 and the (1,1), (2,1) and (3,1) hot bands have been investigated in absorption using cavity ring-down spectroscopy. Analysis of the spectra gives refined band origins and rotational constants for the v′ levels and reveals strongly v′-dependent predissociation rates for the A2Π3/2 state. Fitting of spectral lineshapes for the rotationally resolved (2,0) band shows that v′ = 2 undergoes a rotation-induced predissociation, most probably via coupling to a 2Σ− state, with lifetimes for the rotational levels that range from ca. 1 ns at J′ = 1.5 to 15 ps at J′ = 50.5. In contrast, however, the (0,0) and (3,0) bands, which are also rotationally structured, exhibit apparently J′-independent predissociation rates. The (1,0), (4,0) and (5,0) bands are sufficiently lifetime-broadened that no rotational structure is evident. Fits to the band contours give average homogeneous (FWHM) linewidths for the various vibrational bands of 0.30 ± 0.03 cm−1 for v′ = 0, 6 ± 1 cm−1 for v′ = 1, 0.80 ± 0.05 cm−1 for v′ = 3, 9 ± 2 cm−1 for v′ = 4 and 60 ± 10 cm−1 for v′ = 5. The dominant predissociation mechanism for v′ = 0,1,3,4, and 5 is attributed to spin–orbit coupling between the A2Π3/2 state and one or more Ω = 3/2 repulsive states.

Ultracold hybrid ion-atom traps offer the possibility of microscopic manipulation of quantum coherences in the gas using the ion as a probe. However, inelastic processes, particularly charge transfer can be a significant process of ion loss and has been measured experimentally for the Yb$^{+}$ ion immersed in a Rb vapour. We use first-principles quantum chemistry codes to obtain the potential energy curves and dipole moments for the lowest-lying energy states of this complex. Calculations for the radiative decay processes cross sections and rate coefficients are presented for the total decay processes. Comparing the semi-classical Langevin approximation with the quantum approach, we find it provides a very good estimate of the background at higher energies. The results demonstrate that radiative decay mechanisms are important over the energy and temperature region considered. In fact, the Langevin process of ion-atom collisions dominates cold ion-atom collisions. For spin dependent processes \cite{kohl13} the anisotropic magnetic dipole-dipole interaction and the second-order spin-orbit coupling can play important roles, inducing couplingbetween the spin and the orbital motion. They measured the spin-relaxing collision rate to be approximately 5 orders of magnitude higher than the charge-exchange collision rate \cite{kohl13}. Regarding the measured radiative charge transfer collision rate, we find that our calculation is in very good agreement with experiment and with previous calculations. Nonetheless, we find no broad resonances features that might underly a strong isotope effect. In conclusion, we find, in agreement with previous theory that the isotope anomaly observed in experiment remains an open question., Comment: 7 figures, 1 table accepted for publication in J. Phys. B: At. Mol. Opt. Phys. arXiv admin note: text overlap with arXiv:1107.1141

BaH (and its isotopomers) is an attractive molecular candidate for laser cooling to ultracold temperatures and a potential precursor for the production of ultracold gases of hydrogen and deuterium. The theoretical challenge is to simulate the laser cooling cycle as reliably as possible and this paper addresses the generation of a highly accurate ab initio $^{2}\Sigma^+$ potential for such studies. The performance of various basis sets within the multi-reference configuration-interaction (MRCI) approximation with the Davidson correction (MRCI+Q) is tested and taken to the complete basis set limit. It is shown that the calculated molecular constants using a 46 electron Effective Core-Potential (ECP), the augmented polarized core-valence quintuplet basis set (aug-pCV5Z-PP) but only including three active electrons in the MRCI calculation are in close agreement with the available experimental values. The predicted dissociation energy D$_e$ for the X$^2\Sigma^+$ state (extrapolated to the complete basis set (CBS) limit) is 16895.12 cm$^{-1}$ (2.094 eV), which agrees within 0.1$\%$ of a revised experimental value of $, Comment: 14 pages, 9 figures: final accepted version

Large parts of the periodic table cannot be cooled by current laser-based methods. We investigate whether zero energy fragmentation of laser cooled fluorides is a potential source of ultracold fluorine atoms. We report new ab initio calculations on the lowest electronic states of the BeF diatomic molecule including spin-orbit coupling, the calculated minima for the valence electronic states being within 1 pm of the spectroscopic values. A four colour cooling scheme based on the A(2)Π← X(2)Σ(+) transition is shown to be feasible for this molecule. Multi-Reference Configuration Interaction (MRCI) potentials of the lowest energy Rydberg states are reported for the first time and found to be in good agreement with experimental data. A series of multi-pulse excitation schemes from a single rovibrational level of the cooled molecule are proposed to produce cold fluorine atoms.

In order to study ultracold charge-transfer processes in hybrid atom-ion traps, we have mapped out the potential energy curves and molecular parameters for several low lying states of the Rb, Yb$^+$ system. We employ both a multi-reference configuration interaction (MRCI) and a full configuration interaction (FCI) approach. Turning points, crossing points, potential minima and spectroscopic molecular constants are obtained for the lowest five molecular states. Long-range parameters, including the dispersion coefficients are estimated from our {\it ab initio} data. The separated-atom ionization potentials and atomic polarizability of the ytterbium atom ($\alpha_d=128.4$ atomic units) are in good agreement with experiment and previous calculations. We present some dynamical calculations for (adiabatic) scattering lengths for the two lowest (Yb,Rb$^+$) channels that were carried out in our work. However, we find that the pseudo potential approximation is rather limited in validity, and only applies to nK temperatures. The adiabatic scattering lengths for both the triplet and singlet channels indicate that both are large and negative in the FCI approximation., Comment: 8 pages, 3 figures, 5 tables

Fragmentation of the first and second electronically excited states of methyl chloride and bromide cations has been studied, the ground state of each molecular ion being stable to dissociation. Product branching ratios and kinetic energy release upon dissociation were investigated using He I photoelectron-photoion coincidence (PEPICO) measurements. The principal fragmentation of the excited states is a C-X bond cleavage. Both excited states of the ion CH 3 Br + follow essentially statistical routes in this channel, though small deviations from a fully randomized distribution are noted and these become more marked for vibrationally excited A-state parent

Ab initio calculations for the strongly exoergic ${\mathrm{Li}}_{2}+\mathrm{F}$ harpoon reaction are presented using density-functional theory, complete active space self-consistent field, and multireference configuration interaction methods to argue that this reaction would be an ideal candidate for investigation with ultracold molecules. The lowest six states are calculated with the aug-correlation-consistent polarized valence triple-zeta basis set and at least two can be accessed by a ground rovibronic ${\mathrm{Li}}_{2}$ molecule with zero collision energy at all reaction geometries. The large reactive cross section (characteristic of harpoon reactions) and chemiluminescent products are additional attractive features of these reactions.

This paper derives a general procedure for the numerical solution of the Lindblad equations that govern the coherences arising from multicoloured light interacting with a multilevel system. A systematic approach to finding the conservative and dissipative terms is derived and applied to the laser cooling of gallium. An improved numerical method is developed to solve the time-dependent master equation and results are presented for transient cooling processes. The method is significantly more robust, efficient and accurate than the standard method and can be applied to a broad range of atomic and molecular systems. Radiation pressure forces and the formation of dynamic dark-states are studied in the gallium isotope 66Ga., Comment: 15 pages, 8 figures

A scheme to produce ultracold hydrogen or deuterium atoms via photo-fragmentation, even allowing direct formation of a quantum gas

We investigate the ultracold, elastic-scattering of a trapped atom and a trapped ion. The low-energy potential surfaces of the [174Yb−87Rb]+ are estimated and the polarizability and higher-order multipole coefficients are obtained from ab initio calculations. The polarizability of the Ytterbium atom is well reproduced and the scattering length is estimated. The excited-state Yb+-Rb system is analysed in detail with a view to applications in hybrid ion-atom traps.

The feasibility of laser cooling AlH and AlF is investigated using ab initio quantum chemistry. All the electronic states corresponding to the ground and lowest two excited states of the Al atom are calculated using multi-reference configuration interaction (MRCI) and the large AV6Z basis set for AlH. The smaller AVQZ basis set is used to calculate the valence electronic states of AlF. Theoretical Franck-Condon factors are determined for the A(1)Π→ X(1)Σ(+) transitions in both radicals and found to agree with the highly diagonal factors found experimentally, suggesting computational chemistry is an effective method for screening suitable laser cooling candidates. AlH does not appear to have a transition quite as diagonal as that in SrF (which has been laser cooled) but the A(1)Π→ X(1)Σ(+) transition transition of AlF is a strong candidate for cooling with just a single laser, though the cooling frequency is deep in the UV. Furthermore, the a(3)Π→ X(1)Σ(+) transitions are also strongly diagonal and in AlF is a practical method for obtaining very low final temperatures around 3 μK.

Strategies to produce an ultracold sample of carbon atoms are explored and assessed with the help of quantum chemistry. After a brief discussion of the experimental difficulties using conventional methods, two strategies are investigated. The first attempts to exploit charge exchange reactions between ultracold metal atoms and sympathetically cooled C(+) ions. Ab initio calculations including electron correlation have been conducted on the molecular ions [LiC](+) and [BeC](+) to determine whether alkali or alkaline earth metals are a suitable buffer gas for the formation of C atoms but strong spontaneous radiative charge exchange ensure they are not ideal. The second technique involves the stimulated production of ultracold C atoms from a gas of laser cooled carbides. Calculations on LiC suggest that the alkali carbides are not suitable but the CH radical is a possible laser cooling candidate thanks to very favourable Frank-Condon factors. A scheme based on a four pulse STIRAP excitation pathway to a Feshbach resonance is outlined for the production of atomic fragments with near zero centre of mass velocity.

Published ab initio and pseudopotential calculations for the dialkali halide systems suggest that the preferred colinear geometry is for the metal to approach the metal end of the alkali halide. Here, ab initio calculations on the Li2F system reveal that the well depth on the halide side in this radical is much deeper and is a local saddle point associated with the ionic nonlinear global minima. Although many features of the pseudopotential surfaces are confirmed, significant differences are apparent including the existence of a linear excited A Σ2u+ state instead of a triangular one, a considerably deeper global minimum some 50% lower in energy and a close approach between the X A21 and the A Σ2u+ states, with the A Σ2u+ minimum 87 kJ mol−1 below the ground state asymptote. All the results can be rationalised as the avoided crossings between a long range, covalent potential dominant within the LiLiF geometry and an ionic state that forms the global minimum. Calculations on the third A2′ potential indicat...

Using a novel non-linear optical technique enantiomeric excess within a translationally disordered overlayer on a metal surface has been monitored for the first time.

The dissociation of the five electronic states of BF+3 lying below 21.2 eV has been investigated using a photoelectron–photoion coincidence technique. Above the dissociation threshold BF+2 is the sole ionic dissociation product. Earlier theoretical work suggests strong vibronic couplings for this manifold of electronic states and the dynamical implications of such interactions are considered in an examination of the energy redistribution in the fragmentation. The possible involvement of an electronically excited state of BF+2 is outlined and ab initio calculations to establish the excitation energy of such a state are reported.