Spectroscopic and spin-orbit calculations on the SO+ radical cation

Highly correlated ab initio methods were used in order to generate the potential-energy curves of the SO+ electronic states correlating to S+(4Su)+O(3Pg) and S+(2Du)+O(3Pg). These curves were used for deducing accurate spectroscopic properties for these electronic states. Our calculations predict the existence of a 2Phi state lying close in energy to the well-characterized b 4Sigma- state and several weakly bound quartet and doublet states located in the 6-9 eV internal energy range not identified yet. The spin-orbit integrals between these electronic states were evaluated using these highly correlated wave functions, allowing the discussion of the metastability and the predissociation processes forming S+ +O in their electronic ground states. Multistep spin-orbit-induced predissociation pathways are suggested. More specifically, the experimentally determined dissociative potential-energy curve [H. Bissantz et al., Z. Phys. D 22, 727 (1992)] proposed to explain the rapid SO+(b 4Sigma-, v> or =13)-->S+(4Su)+O(3Pg) reaction is found to coincide with the 2 4Pi potential-energy curve for short internuclear distances and with the repulsive 1 6Pi state for longer internuclear separations.     

Ab initio adiabatic and diabatic energies and dipole moments of the CaH+ molecular ion

The diabatic and adiabatic potential-energy curves and permanent and transition dipole moments of the highly excited states of the CaH(+) molecular ion have been computed as a function of the internuclear distance R for a large and dense grid varying from 2.5 to 240 au. The adiabatic results are determined by an ab initio approach involving a nonempirical pseudopotential for the Ca core, operatorial core-valence correlation, and full valence configuration interaction. The molecule is thus treated as a two-electron system. The diabatic potential energy curves have been calculated using an effective metric combined to the effective Hamiltonian theory. The diabatic potential-energy curves and their permanent dipole moments for the (1)∑(+) symmetry are examined and corroborate the high imprint of the ionic state in the adiabatic representation. Taking the benefit of the diabatization approach, correction of hydrogen electron affinity was taken into account leading to improved results for the adiabatic potentials but also the permanent and transition electric dipole moments.     

Heavier alkali-metal monosulfides (KS, RbS, CsS, and FrS) and their cations

The heavier alkali-metal monosulfides (KS, RbS, CsS, and FrS) have been studied by high-level ab initio calculations. The RCCSD(T) method has been employed, combined with large flexible valence basis sets. All-electron basis sets are used for potassium and sulfur, with effective core potentials being used for the other metals, describing the core electrons. Potential-energy curves are calculated for the lowest two neutral and cationic states: all neutral monosulfide species have a (2)Pi ground state, in contrast with the alkali-metal monoxide species, which undergo a change in the electronic ground state from (2)Pi to (2)Sigma(+) as the group is descended. In the cases of KS, RbS, and CsS, spin-orbit curves are also calculated. We also calculate potential-energy curves for the lowest (3)Sigma(-) and (3)Pi states of the cations. From the potential-energy curves, spectroscopic constants are derived, and for KS the spectroscopic results are compared to experimental spectroscopic values. Ionization energies, dissociation energies, and heats of formation are also calculated; for KS, we explore the effects of relativity and basis set extrapolation on these values.     

A novel 1,3,5-tris(alpha,beta,beta-trifluorovinyl)benzene monomer

The reaction of the perfluoroalkenylzinc reagent, CF2=CFZnBr, with 1,3,5-tribromobenzene in the presence of a catalytic amount of Pd(Ph3)4 yielded a novel trifunctional monomer 1,3,5-tris(alpha,beta,beta-trifluorovinyl)benzene (1).     

Synthesis of 1,3,5-tris(2-chloroethyl)- and 1,3,5-tris(2-iodoethyl)benzenes

A new method has been elaborated for the synthesis of 1,3,5-tris(2-chloroethyl)- and 1,3,5-tris(2-iodoethyl)benzenes based on the commercially available 1,3,5-tribromobenzene.     

Photodissociation of bromobenzene in solution

The photodissociation of bromobenzene in solution was investigated with ultrafast transient absorption spectroscopy following excitation at 266 nm. Ab initio calculations of lower singlet and triplet states were performed in order to guide the interpretations. The main feature of the kinetics measured between 300 and 930 nm in acetonitrile is a 9±1 ps decay, which we mainly assign to predissociation. Similar decays were observed in hexane, dichloromethane and tetrachloromethane at 400 and 800 nm. Other features in acetonitrile, such as complicated short-time dynamics between 420 and 620 nm and a long-lived component, might indicate the involvement of lower triplet states.     

Diatomics-in-molecules calculations of potential-energy surfaces for B+(3P) + H2(X 1Σg+)

Collisions of B⁺(³P) with H₂(X ¹Σ_g⁺) have been studied repeatedly using molecular-beam techniques. The theoretical interpretation of the results suffered from missing information about the potential-energy surfaces for this system. This paper reports on diatomics-in-molecules calculations for a wide range of nuclear geometries. The stationary points on the minimum-energy paths are determined. Symmetrically orthogonalized and non-hermitean diatomics-in-molecules versions differ only slightly at the minimum-energy path. The potential-energy surfaces of the 1³A′ and 2³A′ states show dramatical changes with the BHH angle, leading possibly to adiabatic as well as non-adiabatic elementary processes. The 1³A′' and 1³A′ states are nearly degenerate for geometries ranging from the entrance channel to the interaction region. The most favourable configurations of approach on these potential-energy surfaces are those with C₂, symmetry.     

Extensive theoretical study on the low-lying electronic states of silicon monofluoride cation including spin-orbit coupling

Ab initio calculations on the low-lying electronic states of SiF+ are performed using the internally contracted multireference configuration interaction method with the Davidson correction and entirely uncontracted aug-cc-pV5Z basis set. The effects of spin-orbit coupling are accounted for by the state interaction approach with the full Breit-Pauli Hamiltonian. The entire 23 Omega states generated from the 12 valence Lambda-S states, which correlate with the first dissociation channel are studied for the first time. Good agreement is found between the calculated results and the available experimental data. The spin-orbit coupling effects on the potential energy curves and spectroscopic properties are studied. Various curve crossings are revealed, which could lead to the predissociation of the a3Pi, A1Pi, and (2)3Sigma+ states and the predissociation pathways are analyzed based upon the calculated spin-orbit matrix elements. The calculated ionization potentials of the ground-state SiF to a few states of SiF+ are in good agreement with the available experimental measurements. Moreover, the transition dipole moments of the dipole-allowed transitions and the transition properties for the A3Pi0+ -X1Sigma+ 0+ and B3Pi1-X1Sigma+ 0+ transitions are predicted, including the Franck-Condon factors and the radiative lifetimes.     

Conical intersection between the lowest spin-aligned Li3(4A') potential-energy surfaces

We have calculated new potential-energy surfaces for the lowest two spin-aligned (4)A(') states of the Li(3) trimer. This calculation shows a seam of conical intersections between these states resulting from the extra symmetry of the system when the atoms are in a collinear arrangement. This seam is especially important because of its proximity to the three-body dissociation limit of the system; ultracold scattering calculations and the bound-state energies of the system will be affected by the presence of this conical intersection. In this paper we discuss the calculation of the potential-energy surface and the location of the conical intersection seam.     

Dynamical consequences of symmetry breaking in benzene and difluorobenzene

The systems benzene/benzene-d(1) and o-/m-/p-difluorobenzene were studied in the dense gas phase with ultrafast transient absorption spectroscopy to investigate the effect of symmetry reduction through monodeuteration and constitutional isomerism on the timescales of intramolecular vibrational energy redistribution (IVR). In both systems IVR proceeds faster in the molecules of lower symmetry. In addition the dynamics were simulated in vibrational quantum number space using a simple model based on scaling state-to-state interactions by coupling order and the energy gap law. These simulations (semi-) quantitatively reproduce the experimental data for benzene and benzene-d(1) without incorporating further molecular symmetry restrictions. The relative impact of molecular symmetry and vibrational state space structure on IVR is discussed.     

Temperature dependence of rotational correlation times in tribromobenzene

The ¹³C relaxation times and nuclear Overhauser enhancements of the protonated carbons in 1,3,5-tribromobenzene were measured as a function of temperature in the solvent benzene-d₆. Rotational correlation times, τ_C(CH), calculated by the Microviscosity/Free Rotor and Hu-Zwanzig “slip” models are substantially below the measured values. In contrast, correlation times predicted by the Hynes—Kapral—Weinberg model are in near quantitative agreement with experiment at all temperatures studied.     

Orientational order of rod- and disk-like solutes in the nematic liquid crystal 5CB

Proton and deuteron nuclear magnetic resonance has been used to investigate orientational order in binary liquid crystalline mixtures. Results obtained from small rigid molecules (2-butyne, 2,4-hexadiyne, propyne, 1,2-propadiene, acetylene, 1,3,5-trichlorobenzene and 1,3,5-tribromobenzene) dissolved in the nematic phase of selectively deuterated 4-n-pentyl-4'-cyanobiphenyl (5CB-d^αβ₄), indicate that the solute and liquid-crystal molecules have different order parameters. Such observations are consistent with the predictions of mean-field theory, which, under certain assumptions, indicates that for a given mixture a plot of one order parameter, S⁽¹⁾, against that of the other parameter, S⁽²⁾, should yield a universal curve independent of temperature and composition. Fitting the experimental NMR results to these theoretical curves provides a virtual nematic-isotropic transition temperature, T^*_NI, for the pure solute.     

丙烯酸分子的激发态超快预解离动力学

利用飞秒泵浦-探测技术结合飞行时间质谱(TOF-MS),研究了丙烯酸分子被200nm泵浦光激发到第二电子激发态(S2)后的超快预解离动力学.采集了母体离子和碎片离子的时间分辨质谱信号,并利用动力学方程对时间分辨离子质谱信号进行拟合和分析,揭示了预解离通道的存在.布居在S2激发态的分子通过快速的内转换弛豫到第一电子激发态(S1),时间常数为210fs,随后再经内转换从S1态弛豫到基态(S0)的高振动态,时间常数为1.49ps.分子最终在基态高振动态势能面上发生C-C键和C-O键的断裂,分别解离生成H2C=CH和HOCO、H2C=CHCO和OH中性碎片,对应的预解离时间常数分别约为4和3ps.碎片离子的产生有两个途径,分别来自于母体离子的解离和基态高振动态势能面上中性碎片的电离.     

Intensity and wavelength control of a single molecule reaction: simulation of photodissociation of cold-trapped MgH+

Photodissociation of cold magnesium hydride ions MgH(+) leading to either Mg(+)+H or Mg+H(+) is simulated from first principles. The purpose is to study the possibility of single molecule control of the products in the presence of two laser fields. The system evolves on four electronic potential-energy curves, X(1) Sigma, A(1) Sigma, B(1) Pi, and C(1) Sigma. These potential-energy curves are calculated from first principles using multireference self-consistent field theory. The accuracy of the electronic potential curves has been checked by calculating the energies of the rovibrational eigenstates and comparing them to experimental findings. The photodissociation dynamics has furthermore been simulated by solving the time-dependent Schrodinger equation. It is shown that the branching ratio of the two dissociation channels, Mg(+)+H or Mg+H(+), can be controlled by changing the intensity and wavelength of the two driving laser fields.     

Relativistic DMRG calculations on the curve crossing of cesium hydride

Over the past few years, it has been shown in various studies on small molecules with only a few electrons that the density-matrix renormalization group (DMRG) method converges to results close to the full configuration-interaction limit for the total electronic energy. In order to test the capabilities of the method for molecules with complex electronic structures, we performed a study on the potential-energy curves of the ground state and the first excited state of 1sigma+ symmetry of the cesium hydride molecule. For cesium relativistic effects cannot be neglected, therefore we have used the generalized arbitrary-order Douglas-Kroll-Hess protocol up to tenth order, which allows for a complete decoupling of the Dirac Hamiltonian. Scalar-relativistic effects are thus fully incorporated in the calculations. The potential curves of the cesium hydride molecule feature an avoided crossing between the ground state and the first excited state, which is shown to be very well described by the DMRG method. Compared to multireference configuration-interaction results, the potential curves hardly differ in shape, for both the ground state and the excited state, but the total energies from the DMRG calculations are in general consistently lower. However, the DMRG energies are as accurate as corresponding coupled cluster energies at the equilibrium distance, but convergence to the full configuration-interaction limit is not achieved.     

The shape of the potential energy curves for NHN(+) hydrogen bonds and the influence of non-linearity

The potential energy curves for proton motion in NHN(+) hydrogen bonds have been calculated to investigate whether different methods of evaluation give different results: for linear H bonds most curves calculated along the NH direction are, as expected, identical with those along NN; for intramolecular H bonds it is very important to take into account the non-linearity and the potential energy curve calculated along the NH direction can be very far from the curve correctly describing the proton transfer. Other factors which influence the proton-transfer process are steric hindrance and presence of anions which modify the proton motion. In the analysis of the proton transfer process it is very important to take changes in the structure of the rest of the molecule into account, which is connected with exchange of energy with the surroundings. Comparison of adiabatic and non-adiabatic curves shows that they are significantly different for very bent hydrogen bonds and for hydrogen bonds with steric constraints for which the proton transfer process must be accompanied with relaxation of the whole molecule. Comparison of the potential-energy curves for compounds with very short H bonds emphasizes that the term 'strong H bond' needs to be qualified. For intermolecular H bonds shortening of the bond is connected with linearization. But for intramolecular H bonds the NN distance cannot be used as the only measure of H bond strength.     

Cold and ultracold NH-NH collisions: the field-free case

We present elastic and inelastic spin-changing cross sections for cold and ultracold NH(X (3)Σ(-)) + NH(X (3)Σ(-)) collisions, obtained from full quantum scattering calculations on an accurate ab initio quintet potential-energy surface. Although we consider only collisions in zero field, we focus on the cross sections relevant for magnetic trapping experiments. It is shown that evaporative cooling of both fermionic (14)NH and bosonic (15)NH is likely to be successful for hyperfine states that allow s-wave collisions. The calculated cross sections are very sensitive to the details of the interaction potential, due to the presence of (quasi)bound state resonances. The remaining inaccuracy of the ab initio potential-energy surface therefore gives rise to an uncertainty in the numerical cross-section values. However, based on a sampling of the uncertainty range of the ab initio calculations, we conclude that the exact potential is likely to be such that the elastic-to-inelastic cross-section ratio is sufficiently large to achieve efficient evaporative cooling. This likelihood is only weakly dependent on the size of the channel basis set used in the scattering calculations.     

Predissociation and autoionization of triplet Rydberg states in molecular hydrogen

We present single-photon spectroscopy in molecular hydrogen starting from the metastable c3Piu- state to a number of triplet nd-Rydberg states (v = 0 - 4, n = 12 - 20). Using fast beam spectroscopy both the autoionization channel and the predissociation channel are quantified, field free, as well as with small electric fields. Coupling with the i3Pig state is assumed to be responsible for field-free predissociation of the v = 0 Rydberg levels. The stronger observed predissociation channel of the v = 1 Rydberg levels is due to the nonadiabatic interaction with the h3Sigmag+ state in combination with l mixing due to an external electric field. No direct evidence is found for possible electric field induced predissociation of the gerade Rydberg states by low lying ungerade states. The competition between autoionization and predissociation is discussed in terms of possible consequences for dissociative recombination involving low energy electron collisions with the H2+ molecular ion.     

A calculation of the rovibronic energies and spectrum of the B1A1 electronic state of SiH2

The B1A1 electronic state of silylene (SiH2) is the second excited singlet state of the molecule and, like the analogous c state of methylene (CH2), it is quasilinear with symmetry 1sigmag+ at linearity. This state dissociates to Si(1D) + H2(1sigmag+). At equilibrium, the B state of SiH2 has an energy that we calculate to be 0.71 eV above that of the dissociation products. However, there is a barrier to dissociation that allows quasibound rovibrational levels to occur, and some have been observed recently [Y. Muramoto et al., J. Chem. Phys. 122, 154302 (2005)]. Starting with our analytical ab initio potential-energy surface, we adjusted it in a fitting to the experimental term values in order to determine the optimum potential-energy function in the bound region. This potential has a C2v equilibrium structure with a SiH bond length of 1.459 angstroms and a bond angle of 165.4 degrees; the barrier to linearity is only 129 cm(-1). Using the optimized potential-energy surface we calculate B-state term values, and using our calculated y and z dipole moment surfaces, we simulate the rotation-vibration spectrum of the state in order to assist in the detection of the matrix isolation spectrum.