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It is argued that collision dynamics of atoms and molecules at ultracold temperatures (below 1 mK) are not readily predictable from knowledge of collision dynamics above 100 K. In the case of elastic collisions, it is well known that the collision cross section is constant as T [right arrow] 0 K but mass and symmetry effects are dramatic. The cases of inelastic and reactive collisions are less studied, but a [T.sup.-1/2] dependence of the cross section as T [right arrow] 0 K is expected. It seems that extrapolations of high-temperature inelastic and reactive behavior normally greatly underestimate ultracold-temperature rates. The prospects for experimental observation of ultracold collision dynamics are rapidly improving. Key words: ultracold molecules, collisions, reactions, hydrogen, scattering length.

Long range potential curves between a highly excited Cs (ns, 8 less than or equal to n less than or equal to 20) atom and a ground state Cs atom are calculated. Such curves would help to observe corresponding spectra.

By using a combination of molecular beam (MB) excitation spectra and two distinct ultracold molecule excitation spectra (UM+ and UM-), we have assigned high vibrational levels of the A and 3 1Σ+ states from absorption spectra of the mutually strongly perturbed A 1Σ+ - 3 1Σ+ - 1 1Π - 2 3Σ+ - b 3Π states of ultracold 39K85Rb molecules in the energy region between 15 116 and 16 225 cm-1 above the minimum of the ground X 1Σ+ state. The ultracold molecules (UM+ and UM-) are formed by radiative decay following photoassociation (PA) to a specific level of the 3(0+) state (UM+) or to a specific level of the 3(0-) state (UM-). We observe that the A and 3 1Σ+ states are observable in the UM+ spectra, but absent from the UM- spectra. This is explained by considering Hund's case (c) selection rules and transition dipole moments between the upper excited A 1Σ+ (2(0+)) state and the three Ω components (0+, 0-, and 1) at the ground-state dissociation limit. We propose further investigations of the extended potential wells of the A and 3 1Σ+ states by combining short-range MB excitation spectra in a narrow Franck-Condon (FC) window near Re of the X 1Σ+ state, and long-range UM (and PA) excitation spectra, which have much larger FC windows. [ABSTRACT FROM AUTHOR]

Electronic structure calculations have been carried out for all possible alkali tetramers that can be formed from X2 + X2 → X2X2, X2 + Y2 → X2Y2, and XY + XY → X2Y2 alkali dimer association reactions. Vibrationally stable rhombic (D2h) and planar (Cs) structures are found for all possible tetramers formed from the alkali metals, Li to Cs. All tetramer formation reactions (from ground state singlet homonuclear or heteronuclear dimers) are found to be exothermic with binding energies ranging from 6282 cm-1 for Li2Li2 to 1985 cm-1 for Cs2Cs2. Extensive calculations, carried out at long-range for several reactant pairs, indicate that there are barrier-less pathways for the formation of tetramers from dimer association reactions. At low temperatures, direct formation of tetramers is unlikely, owing to the large exothermicity associated with these association reactions, but atom exchange reactions (X2 + Y2 ↔ XY + XY) are possible for some species. [ABSTRACT FROM AUTHOR]

Long range interactions between the ground state alkali diatomics Na2–Na2, K2–K2, Na2–K2, and NaK-NaK are examined. Interaction energies are first determined from ab initio calculations at the coupled-cluster with singles, doubles, and perturbative triples [CCSD(T)] level of theory, including counterpoise corrections. Long range energies calculated from diatomic molecular properties (polarizabilities and dipole and quadrupole moments) are then compared with the ab initio energies. A simple asymptotic model potential ELR=Eelec+Edisp+Eind is shown to accurately represent the intermolecular interactions for these systems at long range. [ABSTRACT FROM AUTHOR]

The excited D 1Σ+ electronic state of 7LiH has been observed up to near its dissociation limit by a pulsed optical-optical double resonance fluorescence depletion spectroscopic technique. An extensive vibronic calculation has been performed with a diabatic approach with purely potential couplings involving a set of eight diabatic states of 1Σ+ symmetry, corresponding to seven neutral states and one ionic state. Twenty-six new vibrational levels have been observed. Both the derived vibrational energy spacings and the vibronic ones are similarly irregular. The observed spectral linewidths and vibronic resonance widths are found to vary similarly with increasing energy. Observed asymmetric spectral lineshapes may be attributed to the strong radial couplings between the discrete levels of the D 1Σ+ electronic state and the continuum states of the C 1Σ+ electronic state. The mutual agreement between the spectral results and the vibronic results demonstrates that the D 1Σ+ electronic state of 7LiH is better characterized by the vibronic approach. [ABSTRACT FROM AUTHOR]

High precision photoassociation spectroscopy is performed in ultracold cesium gas, with detunings as large as 51 cm-1 below the Cs(6S1/2)+Cs(6P3/2) asymptote. Trap-loss fluorescence detection is used for detecting the photoassociation to excited state ultracold molecules. Long vibrational progressions are assigned to electronic states of 0g-, 0u+, and 1g symmetry. The spectral data are fitted to a LeRoy-Bernstein equation, in order to obtain the effective coefficients of the leading long-range interaction term (C3/R3) and the relative vibrational quantum numbers measured down from dissociation. Additionally we present evidence for perturbations between the 0g- state and the dark 2u state. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The weakly bound long-range potential curves between a highly excited alkali atom M*(nes) and a ground state alkali atom M(ngs) are calculated using simple but reasonably accurate models for long-range dispersion and exchange interactions for all homonuclear and heteronuclear combinations. For K2, where experimental results are available, the agreement is quite good (binding energies of observed vibrational levels within ∼10%). We find that at least a zero-point vibrational level occurs for ne-ng≤12. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using a potential energy curve (based primarily on the RKR potential of Amiot and Vergès [J. Chem. Phys. 112, 7068 (2000)]) and a dipole moment function (based primarily on ab initio calculations of Park et al. [Chem. Phys. 257, 135 (2000)]), we have calculated radiative transition probabilities (Einstein A coefficients), radiative lifetimes, and dipole moment expectation values involving all vibrational levels (for several rotational quantum numbers) of the X [sup 1]Σ[sup +] ground state of [sup 39]K[sup 85]Rb. We observe that the radiative lifetimes of vibrationally excited levels, in particular, are ∼10[sup 3]–10[sup 6] seconds, far too long to be significant in most ultracold experiments involving [sup 39]K[sup 85]Rb or its isotopomers. Comparison with other molecules (LiH and HF) suggests that simple scaling (A∼μ[sup 2]ν[sup 3]∼τ[sup -1]) will predict similarly long lifetimes for many other heteronuclear molecules, e.g., RbCs. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The paper presents the study of the fully mixed A ¹Σ[sup +] -b ³Π complex of the NaRb molecule based on high-resolution sub-Doppler spectroscopy and intensity measurements, ab initio relativistic calculations of energies, transition moments and spin-orbit interactions, as well as an inverted channel-coupling approach (ICCA) deperturbation analysis. A two-laser V-type pump-probe excitation scheme was employed to obtain A←X transition frequencies to 16 A-state vibrational levels from v = 6 to v = 21 with J from 8 to 23. Additionally, relative intensities in laser-induced A→X fluorescence spectra have been recorded, including progressions with all observable transitions to the ground state vibronic levels, the latter yielding unambiguous v assignment of the A-state levels observed. All experimental rovibronic term values and all measured intensity distributions were embedded in a direct simultaneous weighted nonlinear fitting in the framework of an elaborated ICCA allowing us to obtain deperturbed relativistic diabatic potentials of the interacting A ¹Σ[sup +] and b ³Π states. To make this possible, ab initio structure calculations of the spin-orbit singlet-triplet coupling parameter, the spin-orbit splitting of the b ³Π state, the transition dipole moments, and the electronic energy differences for internuclear distance 3.0-7.0 Å have been performed using second order many-body multipartitioning perturbation theory. The developed ICCA is proved to be appropriate for deperturbation analysis of strongly coupled electronic states provided that accurate nonadiabatic matrix elements are known. This allows unambiguous assignment of the vibrational levels of the b ³Π[sub 0] state, which is not directly observed. [ABSTRACT FROM AUTHOR]

Examines the exchange energy for the two lowest electronic states, correlating with two ground state atoms, for several alkali diatomic molecules as a function of large internuclear distance. Expression for the exchange energy at long range where the atomic overlap is small; Dependence of the perturbation theory on the properties of separated atoms.

Focuses on spectroscopic data for the two lowest states of NaK that dissociate to ground state sodium and potassium atoms. Determination of experimental values of the separate Coulombic and exchange contributions to long-range interaction potentials; Dissociation energy of both states.

Reports on the theory and analysis of sodium dimmer Rydberg states observed by all-optical triple resonance (AOTR) spectroscopy. Long-range potential theory (LRPT) in Hund's case; Analysis of the AOR data; Calculation of theoretical quantum defects for the series in Hund's case.

Examines the ultracold photoassociative spectroscopy of heteronuclear alkali-metal diatomic molecules. Measurements of the heteronuclear cold collisions by the photoassociative spectroscopy; Significance of the elastic spectroscopy for mixed atomic gases; Comparison between the potassium-rubidium system and the heteronuclear alkali diatomic molecule.

This paper critically surveys the best available data for the two lowest states (X 1Σ+g and a 3Σ+u) of Na2. Since both states are known up to near dissociation, they can be used to determine the separate Coulomb and exchange contributions to the intermediate and long range interaction potentials. The multipolar expansion representation of the Coulomb (dispersion) energy at long range (-∑nCn/Rn) and the exponential representation of the exchange energy at long range (Ae-aR) are tested. [ABSTRACT FROM AUTHOR]

In order to make an unambiguous assignment of the atomic asymptotic limits for the previously observed Rydberg states of K2, two more low-lying Rydberg states (one 1Δg state and one 1Πg state), were observed with sub-Doppler resolution using cw optical–optical double resonance spectroscopy. The correlation of the molecular Rydberg states with the diabatic and adiabatic atomic limits, the quantum defects, and the diabatic and adiabatic dissociation energies are discussed and the principal quantum number assignment of Broyer et al. reinforced. [ABSTRACT FROM AUTHOR]

Laser induced fluorescence studies of the lithium diffuse band at 458 nm were performed using single-and double-photon excitations. Single-photon excitation was performed using UV lines of Ar++ and Kr++ lasers, and the diffuse band fluorescence was induced by collisions in 7Li2 and 6Li2. The double-photon excitation process populated relevant upper states through several accidental coincidences for two-step excitation in 7Li2. In two cases, in addition to structured continuum emission, an adjacent line spectrum was observed. In five other cases, two-photon accidental coincidences produced collision induced fluorescence of the 7Li2 diffuse band. We also found (in a few cases) that the (two-photon) excitation energy of the 2 3Πg state (with respect to the 1 3Σ+u state) was collisionally transferred to simultaneously excited lithium 2p atoms, thus creating a lithium atom in the 4d state. [ABSTRACT FROM AUTHOR]

The 1 3Δg state of Na2 has been studied extensively by both filtered fluorescence and ionization detection and analyzed by both Dunham-type expansion and near-dissociation expansion (NDE) models in the analysis. Our observations have covered 99.998% of the potential well depth with the outermost Rydberg–Klein–Rees (RKR) turning point at 28.02 Å. NDE analysis gives Te=28 032.468 (±0.021) cm-1, De=7162.436 (±0.021) cm-1, and Re=3.463 81 (±0.000 28) Å. Significant long-range behavior in the near dissociation levels has been observed. Fitting of the RKR turning points gives the long-range coefficients C5=1.388 (±0.031)×106 cm-1 Å5 and C6=0.4008 (±0.0046)×108 cm-1 Å6. These newly observed results show reasonable agreement with recent theoretical calculations. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Polarized emission resulting from the quantum state-selected photodissociation of K2 through quasibound levels of the B 1Πu state has been studied experimentally by high-resolution all-optical triple resonance techniques, and theoretically by generalized quantum defect theory using the adiabatic/recoil model for the dissociation dynamics. Observation of alignment in the initial state (from the photopreparation steps) and in the photofragment (from the photodissociation step) provides detailed insight into the dynamical evolution. Excellent agreement between the theoretical calculations and the experimental results is shown. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper critically surveys the best available spectroscopic data for the two lowest electronic states (X 1Σ+g and a 3Σ+u) of K2. Since both states are known up to dissociation, they can be used to determine Coulomb and exchange contributions to the intermediate and long range interaction potentials. The multipolar expansion representation of the Coulomb (dispersion) energy at long range (-ΣnCnR-n) and the exponential representation of the exchange energy (Ae-αR) as well as a variety of theoretical calculations are compared with these empirical results. Finally, dissociation energy values are discussed and improved dissociation energies for the X 1Σ+g (De=4449.1±1.0 cm-1) and the a 3Σ+u state (De=252.9±1.1 cm-1) proposed. © 1994 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The 4 1Σ+g ‘‘shelf’’ state of Na2 was extensively investigated using optical–optical double resonance spectroscopy and an ultrasensitive ionization detector. A total of 179 vibrational levels-(0≤v≤127 are from previous work by using filter-photomultiplier tube fluorescence detection) were observed in this state, which covered more than 99.8% of the well depth. The Rydberg–Klein–Rees (RKR) potential energy curve was constructed with maximum outer turning point R178+ = 17.60 Å. The comparison of the long range portion of the RKR potential energy curve and the potential curve calculated from V(R)=ΣnCn/Rn is discussed. Avoided crossings around the shelf with the 3 1Σ+g state and near dissociation with the 5 1Σ+g state are also discussed. [ABSTRACT FROM AUTHOR]

Using optical–optical double resonance and an ultrasensitive ionization detector, we have been able to extensively study a wide range of internuclear distances for the 5 1Σ+g state of Na2. Using a combination of spectroscopic data and the ab initio calculations, we have constructed a hybrid potential which has an outer well at shelf region. For this hybrid potential, there are 54 vibrational levels in the inner well (which all have been observed in this work) and 12 vibrational levels in the outer well (of which only the last level has been observed in this work). Above the potential barrier, 78 additional vibrational levels have been observed in this work. These spectroscopic data represent about 99.8% of the potential well depth. [ABSTRACT FROM AUTHOR]

The 3 1Σ+g state of Na2 has been extensively studied using an optical–optical double resonance technique and a shielded cylindrical space-charge-limited ionization detector. A wide range of vibrational quantum numbers 0≤v≤141 was observed which covered over 99.9% of the potential well depth. A ‘‘shelf’’ on the potential energy curve was found near v=23. A hybrid potential was constructed based on the Rydberg–Klein–Rees (RKR) potential curve and a local inverse perturbation approach (IPA) for the shelf region. For this potential, there is a shallow second local minimum ∼5 cm-1 in depth at the shelf region. The long range energy also compares well with long range theory. [ABSTRACT FROM AUTHOR]

All quasibound vibrational energy levels v=27 and 28 of the Na2 3 1Πg state were accessed by using optical–optical double resonance and detected with a shielded ionization detector. The line broadening due to the tunneling effect was observed to be larger than 1 cm-1 at the rovibrational level v=28, J=31. The potential curve has been determined by the Rydberg–Klein–Rees method and modified using Jeung’s calculation for the large distance barrier and comparison of the calculated and the measured line widths. The barrier height was found to be ∼126 cm-1 above its asymptotic limit (3s+4p) at 5.98 Å. There also exists an unobserved shallow outer well with depth ∼35 cm-1 at 9.5 Å according to our modification of Jeung’s calculated potential. One way to possibly probe this outer well is also discussed. [ABSTRACT FROM AUTHOR]

Due to an avoided crossing, the 4 1Σ+g state of Na2 has an unusual potential energy curve with a ‘‘shelf’’ on its outer wall. By using optical–optical double resonance techniques, vibrational levels from v=0 to 130 have been observed and the shelf was found around v=48–60. From the experimental data we have successfully constructed a Rydberg–Klein–Rees (RKR) potential curve, which can reproduce measured data with a standard deviation of 0.064 cm-1. A set of Dunham coefficients was obtained for the lower part v=0–45 of that state. Several interesting effects related to the shelf, such as the unusual behavior of Bv and Dv, of the vibrational wave functions and the semiclassical vibrational periods, and of the rotational effective potential, are discussed. [ABSTRACT FROM AUTHOR]

We report the observation of bound–free emission on the 1 3Π→1 3Σ+ band of the NaK molecule. The spectra, which consist of oscillating continua in the near-infrared, have been analyzed to determine parameters describing the repulsive wall of the 1 3Σ+ state above the dissociation limit. Spectra calculated using a potential of the form Ae-BR +C for the 1 3Σ+ state were compared to experimental spectra to yield the following values: A=5.94×105 cm-1, B=1.605 Å-1, C=-220.520 cm-1. This potential, which is referenced to the bottom of the RKR 1 3Σ+ well (De =209.1 cm-1), is valid over the range R=3.4–4.5 Å (R=6.4–8.5 a.u.). The relative transition dipole moment of the 1 3Π→1 3Σ+ band has also been determined over a limited range in R (7.5

Direct dye laser excitations of the K2 yellow, Rb2 orange, and Cs2 near-infrared diffuse bands have been investigated. Experimental results are shown to be consistent with the assumed bound–free 2 3Πg–1 3∑+u excitations. It is found that for Rb2 and Cs2, spin–orbit interactions become so significant that the 2 3Πg state is strongly split into three quite independent component states. [ABSTRACT FROM AUTHOR]