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1.
We describe s-wave collisions between ultracold 23Na and 87Rb atoms in the presence of a magnetic field. For any collision input channel knowledge of the singlet and triplet intermolecular potentials [1] enables us to predict the variation in scattering lengths as a function of the magnetic field. We employ the Born-Oppenheimer approximation to predict several Feshbach resonances for 23Na 87Rb. We also include a discussion of why these resonances are expected to be narrow as observed in the case of 87Rb 2.Received: 6 August 2004, Published online: 23 November 2004PACS:
34.90. + q Other topics in atomic collisions - 03.75.Hh Multicomponent condensates 相似文献
2.
The singlet and triplet potential energy surfaces for the reactions CF 3O 2 + I (1), CF 3O + OI (2) and CF 3 + OIO (3) are investigated using ab initio quantum mechanical methods. Four important isomeric energy minima were found, three on the singlet surface, CF 3OOI, CF 3OIO and CF 3IO 2 and one on the triplet surface 3CF 3OIO. CF 2O + FOI are shown to be the most probable products for all reactions, CF 3O +I and CF 3O + O( 3P) are possible for reactions (2) and (3) while the reaction pathway leading to CF 3O +OI is also possible for reaction (3). 相似文献
3.
More than 80 excited electronic states of the hydrohelium ion HeH + of 1, 3Σ +, 1, 3Π, 1, 3Δ, 1, 3Φ and 1, 3Γ symmetry have been calculated ab initio up to n = 6 for internuclear distances ranging from 0.5 to 100 bohr. The computations involve a configuration interaction (CI) treatment based on a home-made suite of programs that uses special basis sets designed for the representation of molecular Rydberg states. The results are compared with previous computations where these are available (up to n = 4), and it is found that except for the very lowest excited states, the present energies are consistently lower than those obtained previously, with an average lowering corresponding to several hundred cm ?1. It is shown that with the exception of its ground state, HeH + is an effective one-electron system having an overall electronic structure similar to H +2. The interaction of the excited electron with the He + 1 s core electron causes small singlet–triplet splittings to appear and ?-mixing interactions to occur, that are not present in H +2. 相似文献
4.
G2 ab initio molecular orbital calculations have been performed to study the potential energy surfaces (PESs) associated with the reactions of Cl + in its 3P ground state and in its 1D first excited state with hydrogen sulphide. [H 2, Cl, S] + singlet and triplet state cations present very different bonding characteristics. The latter are systematically ion-dipole or hydrogen-bonded weakly bound species, while the former are covalent molecular ions. As a consequence, although the Cl +( 3P) is 34.5 kcal mol ?1 more stable than Cl +( 1D), the global minimum of the singlet PES lies 37.3 kcal mol ?1 below the global minimum of the triplet PES. Both singlet and triplet potential energy surfaces show significant differences with respect to those associated with Cl + + H 2O reactions as well as with SH 2 reactions with F +. In both cases, the major product should be SH + 2; SH + and HCl + being the minor products, in agreement with the experimental evidence. The estimated heat of formation for the most stable H 2SCl + singlet state species is 198 ± 1 kcal mol ?1. 相似文献
5.
In a recent publication, Lobsiger et al. [Phys. Chem. Chem. Phys. 12, 5032 (2010)] presented infrared and electronic absorption spectra of supersonic jet-cooled 5-methyl-2-hydroxypyrimidine (5M2HP), the enol form of deoxythymine. In addition, they reported on the fast nonradiative decay of the S 1 population to a dark state. In the present paper, we have investigated the mechanism and rate constants of this nonradiative decay by means of quantum chemical multi-configuration methods. To this end, minima of the lowest excited singlet and triplet states as well as the minimum-energy crossing point of singlet and triplet potential energy hypersurfaces (PEHs) have been determined employing a numerical DFT/MRCI gradient where DFT/MRCI stands for a combination of density functional theory (DFT) and a semi-empirical multi-reference configuration interaction (MRCI) approach. Rate constants have been calculated in the Condon approximation using a time-dependent approach based on harmonic oscillator functions and electronic spin–orbit coupling matrix elements evaluated at the DFT/MRCI level. It is shown that the first excited triplet state possesses 3( n?→?π*) character in the gas phase. Fast intersystem crossing is mediated by the low-lying 3(π?→?π*) state whose PEH crosses both, the S 1 1( n?→?π*) and T 1 3( n?→?π*) PEHs. 相似文献
6.
Abstract The photodetachment of negative ions in a static electric field exhibits some new characteristic features and has beer considered in various theortical approaches. 1 Most of them, however, neglect the short-range interaction between the escaping electron and the atomic core, and must be modified to describe various resonant effects. Experiments 2 have shown very rich resonant structure in a dc-field, which can be attributed to the mixing of different excited states in the negative ion, to competition between elastic and inelastic decay channels, and to tunneling effects induced by the field. It is known that various resonant structures in Photoprocesses can be successfully described within standard multichannel quantum defect theory (MQDT). We present a modified MQDT frame transformation approach to extend the standard method to long-range potentials with nonspherical symmetry. In our treatment both the electron-field and electron-atom interactions are treated nonperturbatively and on an equal footing. The resulting theoretical calculations are compared with experimental data on field-modified H ? photodetachment in the vicinity of the n = 2 resonances. 相似文献
7.
Peculiar properties of ion-atom collision systems, in particular deviations from statistical populations of singlet and triplet
levels, can be studied by optical spectroscopy. We have extended earlier studies by VUV spectroscopy of a number of collision
systems at various collision energies in the 0.01-MeV/nucleon to 1-MeV/nucleon range, involving H 2
+, H +, He +, He 2+, Ne +, Ar +, and N 2
+ as projectiles and Ne, Ar, and N 2 as target gases. Statistically significant deviations of the relative intensities of singlet and triplet lines from simple
ratios are observed in the displaced terms of the valence shell of Ne III, corroborating and extending earlier work. For Ar
III, the energy dependences of singlet-to-triplet excitation ratios are very different for different projectiles. For N II,
in contrast, all observed line ratios are practically independent of the projectile energy.
Received 17 November 2000 and Received in final form 31 January 2001 相似文献
8.
The reasonable dissociation limit of the second excited singlet state
B 1∏ of 7LiH molecule is obtained. The accurate dissociation energy and
equilibrium geometry of the B 1\Pi state are calculated using a
symmetry-adapted-cluster configuration--interaction method in full active space. The
whole potential energy curve for the B 1∏ state is obtained over the
internuclear distance ranging from about 0.10nm to 0.54nm, and has a least-square
fit to the analytic Murrell--Sorbie function form. The vertical excitation energy is
calculated from the ground state to the
B 1∏ state and compared with previous theoretical results. The
equilibrium internuclear distance obtained by geometry optimization is found to be
quite different from that obtained by single-point energy scanning under the same
calculation condition. Based on the analytic potential energy function, the harmonic
frequency value of the B 1∏ state is estimated. A comparison of the
theoretical calculations of dissociation energies, equilibrium interatomic distances
and the analytic potential energy function with those obtained by previous
theoretical results clearly shows that the present work is more comprehensive and in
better agreement with experiments than previous theories, thus it is an improvement
on previous theories. 相似文献
9.
The lowest singlet and triplet excited states of the two paired electrons in a chemical bond are treated by the time-dependent Hartree-Fock (H.F.) method, with the π electrons of the double bond in ethylene as an example. The results depend on a dimensionless parameter g which describes the strength of the electron correlation effects, and they are compared with a simple configuration interaction calculation. When g is small the frequencies and amplitudes of the Hartree-Fock oscillations give an accurate estimate of the energies and intensities of the two lowest transitions, the correlation energy and the pair distribution function of the gound state. The correlation energy is related to the zero-point energy of the oscillations. As g increases the H.F. method overestimates the correlation corrections and breaks down completely if g = 1. At this point the triplet oscillation becomes unstable, because the molecular orbital wave-function with two paired electrons ceases to be the state of lowest energy. When g is large the H.F. results violate spin conservation and the exclusion principle. 相似文献
10.
Hydrogen molecule in a strong ultrashort magnetic field is investigated through a current-density functional theory (CDFT)
and quantum fluid dynamics (QFD) based approach employing current-density dependent vector exchange-correlation potential
and energy density functional derived with a vorticity variable. The numerical computations through the CDFT based approach
are performed for the H 2 molecule, starting initially from its field-free ground state, in a parallel internuclear axis and magnetic field-axis configuration
with the internuclear separation R ranging from 0.1 a.u. to 14.0 a.u., and the strength of the time-dependent (TD) magnetic field varying between 0−10 11 G over a few femtoseconds. The numerical results are compared with that obtained using an approach based on the current-density
independent approximation under similar computational constraints but employing only scalar exchange-correlation potential
dependent on the electronic charge-density alone. The current-density based approach yields exchange- and correlation energy
as well as electronic charge-density of the H 2 molecule drastically different from that obtained using current-independent approach, in particular, at TD magnetic field-strengths
>10 9 G during a typical time-period of the field when the magnetic-field had attained maximum applied field-strength and is switched
to a decreasing ramp function. This nonadiabatic behavior of the TD electronic charge-density is traced to the TD vorticity-dependent
vector exchange-correlation potential of the CDFT based approach. The interesting electron dynamics of the H 2 molecule in strong TD magnetic field is further elucidated by treating electronic charge-density as an ‘electron-fluid’.
The present work also reveals interesting real-time dynamics on the attosecond time-scale in the electronic charge-density
distribution of the hydrogen molecule. 相似文献
11.
Cyclonona‐3,5,7‐trienylidene ( 1 ) changes from being a transition state (TS) to minimum states when substituted by α‐methyl groups and ?‐X, where X = CMe 2, NMe, PMe, O, S, cyclopropyl, and SiMe 2 ( 2 , 3 , 4 , 5 , 6 , 7 , 8 , respectively) at density functional theory. Specifically, the parent carbene 1 exhibits a negative vibrational force constant and proves to be an unreachable electrophilic TS while shows Cs symmetry with an NBO atomic charge of +0.70 on its carbenic center. It has a triplet ground state with a rather small singlet‐triplet energy gap (Δ Es–t = ?4.1 kcal/mol). In contrast, all of its seven scrutinized derivatives enjoy reachable global minima, with C1 symmetry, desired nucleophilicity, and singlet closed shell (S cs) ground states (for all but 8 which remains triplet). Stability is indicated by relative Δ Es–t values: 2 > 3 > 4 > 5 > 6 > 7 > 1 > 8 . The highest Δ Es–t as well as NBO carbenic atomic negative charge (?0.74) are displayed by 2 . Our carbenes ( 2 , 3 , 4 , 5 , 6 , 7 ) appear more nucleophilic than the synthesized N‐heterocyclic carbenes (imidazol‐2‐ylidenes). Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
12.
The lower excited states of 2-benzoylthiophene have been studied using ab initio quantum chemical methods based on multiconfigurational wave functions. Six singlet and six triplet excited states have been characterized. The geometry has been optimized for the two lowest triplet states, which are responsible for the photoreactivity of the chromophore in the photosensitizing drug tiaprofenic acid. The T 1(π → π?) and T 2( n → π?) states have been found to be close in energy with the π → π? state slightly lower. The excited states have been characterized using density difference and spin density plots. The different photochemical behaviour of the two triplet states can be rationalized from the theoretical data. 相似文献
13.
A number of 1,3,1′,3′‐tetrasubstituted bisadamantylidenes are examined with 4 different density functional methods (BP86‐D3, ωB97X‐D, B3LYP, and B3LYP‐D3) and the 6‐311G(d) basis set. With increasing steric bulk, these substituted bisadamantylidenes become ever more twisted about the central carbon‐carbon double bond. This manifests in significantly reduced singlet‐triplet gaps from that of typical alkenes, with the tetra‐ t‐butylbisadamantylidene 17 , twisted to almost 90°, having a gap of only 2.2 kcal mol ?1. While its large strain energy may preclude the synthesis of 17 , other less‐strained tetrasubstituted analogues are more feasible synthetic targets and still possess small singlet‐triplet gaps, which should be testable by variable temperature NMR and EPR spectroscopy. The twisted alkenes also result in a low rotational energy barrier, and 4 examples with a low barrier to cis‐trans isomerization are presented. These too should be testable by experiment. 相似文献
14.
对Ge原子采用6-311++G **基函数,Te和Se原子采用SDB-cc-pVTZ基函数,利用密度泛函理论的局域自旋密度近似方法优化得到了GeTe和GeSe分子的稳定构型,并计算了外电场作用下GeTe和GeSe基态分子的平衡核间距、总能量、最高已占据分子轨道能量E H、最低未占分子轨道能量E L、能隙、谐振频率和红外谱强度. 在上述计算的基础上利用单激发组态相互作用-局域自旋密度近似方法研究了GeTe和GeSe分子在外电场下的激发特性. 结果表明:随着正向电场强度的增大,分子核间距逐渐增大,分子总能量逐渐降低,谐振频率逐渐减小,红外谱强度则逐渐增大. 在0-2.0569×10 10 V·m -1的电场范围内,GeTe分子的E H 均高于GeSe分子的E H;随着正向电场的增大,GeTe与GeSe的E H差逐渐变大,GeTe的E L低于GeSe的E L,它们的E L均随正向电场的增大而增大. 无外场时,GeTe分子的能隙比GeSe分子的能隙要小;在外电场反向增大的过程中, GeTe和GeSe的分子能隙始终减小. 外电场的大小和方向对GeTe和GeSe分子的激发能、振子强度及跃迁的波长均有较大影响.
关键词:
GeTe
GeSe
外电场
激发态 相似文献
15.
α‐Cyclopropyl stability impacts on singlet and triplet heterocyclic carbenes with acyclic, cyclic, and cyclic‐unsaturated structures are compared and contrasted to di‐ t‐butyl as well as t‐butylcyclopropylcarbenes through appropriate isodesmic reactions at B3LYP/AUG‐cc‐pVTZ level. Substitution of one of the t‐butyl groups of di‐ t‐butylcarbene with a cyclopropyl alters the ground state multiplicity from triplet to singlet with a singlet–triplet energy separation (Δ Es–t) of 7.2 kcal/mol. Additional heteroatom substitution increases Δ Es–t values for the resulting α‐heteroatom cyclopropylcarbenes in the following order: amino > oxy > thio > phophino. α‐Cyclopropyl group stabilizes singlet states of all our carbenes two to three times more than their corresponding triplet states. The Δ Es–t values of all the carbenes are increased through cyclization, while the introduction of unsaturation in the rings causes small and rather random changes. To probe the kinetic stability of the species, we calculated the transition states for the opening of cyclopropyl through 1,2‐ C shift. Interestingly, the 4.1 kcal/mol energy barrier in cyclopropylcarbene is significantly increased in the presence of heteroatoms to 31.2 kcal/mol for aminocyclopropylcarbene. The reactivity of the species is discussed in terms of nucleophilicity and electrophilicity issues showing our carbenes, especially acyclic ones, more nucleophilic than the common N‐heterocyclic carbenes. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
16.
Quantum yields of pyrene fluorescence and bis[2-(2′-benzothienyl)-pyridinato-N,C 3′](acetylacetonate)iridium [Btp 2Ir(acac)] phosphorescence upon excitation via a matrix of poly-N-epoxypropylcarbazole (PEPC) or poly-N-epoxypropyl-3,6-dibromocarbazole (DBrPEPC), respectively, were found to be lower than those for the compounds directly excited in a polystyrene (PS) matrix. It was established that the energy in PEPC was transferred to an acceptor by both singlet excitons (by migration and long-range dipole–dipole interaction) and triplet excitons (through migration and short-range exchange electron interaction); however, only by triplet excitons in DBrPEPC, which did not show any fluorescence. The energy-transfer efficiency in PEPC by singlet excitons was higher than by triplet excitons. The observed effects were explained by the fact that energy transfer to the acceptor competed with such processes as localization of the excitons in the “tail” energy states, dissociation of singlet excitons into geminal electron–hole pairs (EHP), and capture of triplet excitons by polymer oxidation products. 相似文献
17.
Amino-acyl-quinoxalinone yellow dyes are cyclised analogues of the yellow azomethine dyes developed for, and still used in, silver halide colour photography. Unlike image azomethine dyes, which are rapidly deactivated in their excited states by torsion about the azomethine bond, amino-acyl-quinoxalinone dyes have an interesting photophysics because torsion is not possible due to their cyclised structure. We report results from studies on singlet and triplet state properties, and singlet oxygen yields, of the yellow dye, 7-diethylamino-3-(2,2-dimethyl-propionyl)-5-methyl-1-phenyl-1H-quinoxalin-2-one, in polar and nonpolar solvents. The dye photophysics is characterised by a weak fluorescence, with a solvent dependent emission yield (ΦF?≈?0.002–0.004), and short singlet state lifetime (τexpt?≈?20–50 ps), both increasing by a factor of ≈2 in going from polar acetonitrile to non-polar dioxane as solvent. DFT ZINDO calculations show a transition involving significant electron transfer from the diethyl-amino group into the carbonyl region of the molecule. In solution, in the presence of oxygen, the triplet state decays almost exclusively by oxygen quenching, and singlet oxygen is produced in high yield (Φ??≈?0.5–0.55). The triplet state absorbs across the 450–750 nm region with maxima around 480 and 650 nm, and moderate molar absorption coefficients (ca. 6000–8000 M?1 cm?1). In a glass at 77 K, triplet decay gives a red phosphorescence, with λmax?≈?640–650 nm, and a ?≈?0.25 s lifetime. If singlet oxygen yields are a good indication of triplet yields, then internal conversion and intersystem crossing occur with roughly equal efficiency. 相似文献
18.
Excitation of lowest electronic states of the thymine molecules in the gas phase is studied by elec- tron energy loss spectroscopy. In addition to dipole-allowed transitions to singlet states, transitions to the lowest triplet states were observed. The low-energy features of the spectrum at 3.66 and 4.61 eV are identified with the excitation of the first triplet states 1 3 A′ (π → π*) and 1 3 A″ ( n → π*). The higher-lying features at 4.96, 5.75, 6.17, and 7.35 eV are assigned mainly to the excitation of the π → π* transitions to the singlet states of the molecule. The excitation dynamics of the lowest states is studied. It is found that the first triplet state 1 3 A′(π → π*) is most efficiently excited at a residual energy close to zero, while the singlet 2 1 A′(π → π*) state is excited with almost identical efficiency at different residual energies. 相似文献
19.
Stationary points on the lowest singlet and triplet In 2 + H 2O potential energy surfaces (PESs) have been explored using the coupled cluster method, including single and double excitations with perturbative triples (CCSD(T)), and the density functional theory (DFT), employing the effective core potential (ECP) for indium (In), which accounts for scalar relativistic effects, with the triple-zeta quality basis set. The CCSD(T) calculated binding energy and anharmonic ν 2-bending mode frequency for the triplet ground-state addition complex, In 2… OH 2( 3B 1), are consistent with the complex detected in the matrix isolation infrared (IR) spectroscopic study under the thermal conditions. The two minimum energy crossing points between the triplet and the singlet PESs that have been located between the structures of In 2…OH 2 and the transition state for the O–H bond breakage are not likely to be thermally accessible under the low-temperature matrix conditions. With the CCSD(T)-calculated In 2 + H 2O reaction profile and anharmonic vibrational frequencies for several In 2(H)(OH) insertion product isomers, we support the IR matrix isolation detection (by two experimental groups) of the lowest energy singlet double-bridged In(μ-H)(μ-OH)In isomer. For the proposed two-step mechanism of H 2 elimination from the In 2(H)(OH) species, the estimated energy barriers are also compatible with experiment. 相似文献
20.
Summary Monofilamentary Bi(2223) tapes with J
c(77 K, 0 tesla) up to 30 000 A/cm 2 have been prepared by cold rolling using the powder-in-tube method. An optimization of the precursor powders has led to a
higher phase purity after the reaction heat treatment. The deformation process has been optimized in order to increase the
oxide density and to reduce sausaging effects on the oxide thickness. The transport properties of these tapes have been studied
in a wide range of temperature (4.2K- T
c) and magnetic fields (up to 28 tesla). The critical-current values at 77 K fields of 0.5 T and 1 T parallel to the tape surface
are 10 000 A/cm 2 and 5400 A/cm 2, respectively. At 4.2 K the J
c value decreases from 1.6·10 5 A/cm 2 at 0 T to 6·10 4 A/cm 2 at 15 T. At fields higher than 15 tesla a very low field dependence of J
c has been found, regardless of the tape orientation. Transport properties have also been studied by cutting small sections
of the tape in order to investigate the local critical-current distribution. It has been found that, even in rolled tapes
of good quality ( J
c (77 K, 0 T)>20000 A/cm 2), the J
c distribution is homogeneous: the critical current density increases gradually from the centre of the tape to the sides, the
latter exhibiting much higher J
c (46000 A/cm 2) than in the centre (18000 A/cm 2).
Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994. 相似文献
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