Theoretical investigation on o-, m-, and p-chlorotoluene photodissociations at 193 and 266 nm

Photodissociations of the o-, m-, and p-chlorotoluene at 193 and 266 nm were investigated by ab initio calculations with and without spin-orbit interaction. The experimentally observed photodissociation channels were clearly assigned by multistate second order multiconfigurational perturbation theory (MS-CASPT2) calculated potential energy curves. The dissociation products with spin-orbit-coupled states of Cl*(2P1/2) and Cl(2P3/2) were identified by MS-CASPT2 in conjunction with spin-orbit interaction through complete active space state interaction (MS-CASPT2/CASSI-SO) calculations. The effects of methyl rotation and substituent on the photodissociation mechanism were discussed in detail.     

Spin-orbit Ab initio investigation of the photodissociation of dibromomethane in the gas and solution phases

A clear and reliable theoretical investigation on dibromomethane (CH(2)Br(2)) photodissociation is desired. The calculation must consider: (i) relativistic effects; (ii) the potential energy curves (PECs) of spin-orbit coupling states; (iii) geometry optimization by the method with both static and dynamic electron correlations; (iv) solvent effects on the photodissociation in the solution. All these have been considered in this study by state-of-the-art quantum chemical calculations. The experimentally observed photodissociation in the gas phase with products of spin-orbit-coupled states, Br((2)P(3/2)) and Br*((2)P(1/2)), was assigned by multi-state second order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space state interaction (MS-CASPT2/CASSI-SO) PECs. The mechanisms of the experimentally observed photodissociation and photoisomerization in solvent were elucidated by the MS-CASPT2/CASSI-SO method combined with polarized continuum model of the solvent.     

Spin-orbit ab initio investigation of the photodissociation of C2H5Br

The photodissociation of ethyl bromide (C₂H₅Br) has been investigated by spin-orbit (SO) ab initio calculations. The vertical excitation energies of some excited states for C₂H₅Br were calculated. The potential energy curves of C₂H₅Br along the C–Br dissociation coordinate were calculated by multistate second-order multiconfigurational perturbation theory in conjunction with spin-orbit (SO) interaction through complete active space state interaction (MS-CASPT2/CASSI-SO). The calculated results clearly assigned the experimentally observed photodissociation channels leading to C₂H₅ + Br (²P_3/2) and C₂H₅ + Br*(²P_1/2).     

Relativistic multireference calculation of photodissociation of o-, m-, and p-bromofluorobenzene

Quantum chemical calculations with relativistic effects were performed on the photodissociation of o-, m-, and p-bromofluorobenzene (o-, m-, and p-BrFPh) at 266 nm. The method of multistate second-order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space state interaction was employed to calculate the potential energy curves for the ground and low-lying excited states of o-, m-, and p-BrFPh along their photodissociation reaction coordinates. The dissociation mechanisms with products of Br((2)P(3∕2)) and Br(?)((2)P(1∕2)) states were clarified with the computed potential energy curves and the surface crossings. The current calculations augmented previous theoretical investigations by including relativistic effects and resolved some differences of experimental assignment regarding the dissociation channels of o-, m-, and p-BrFPh.     

Spin-orbit ab initio investigation of the ultraviolet photolysis of diiodomethane.

The UV photodissociation (<5 eV) of diiodomethane (CH(2)I(2)) is investigated by spin-orbit ab initio calculations. The experimentally observed photodissociation channels in the gas and condensed phases are clearly assigned by multi-state second-order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space-state interaction potential energy curves. The calculated results indicate that the fast dissociations of the first two singlet states of CH(2)I(2) and CH(2)I--I lead to geminate-radical products, CH(2)I (.)+I((2)P(3/2)) or CH(2)I (.)+ I*((2)P(1/2)). The recombination process from CH(2)I--I to CH(2)I(2) is explained by an isomerization process and a secondary photodissociation reaction of CH(2)I--I. Finally, the study reveals that spin-orbits effects are significant in the quantitative analysis of the electronic spectrum of the CH(2)I--I species.     

Electron correlation and relativistic effects in atomic structure calculations of the thorium atom

Relativistic two-component ab initio calculations have been performed for the Th atom. The spin free low lying states have been calculated at state-averaged complete active space self-consistent field (SA-CASSCF) and multi-state complete active space second-order perturbation (MS-CASPT2) level of theories using different sets of active orbitals. The spin-orbit states have been computed using Douglas-Kroll type of atomic mean-field integral approach. The effects of dynamic electron correlation have been studied at the MS-CASPT2 level. The energy levels of spin-orbit states below 30,000 cm(-1) obtained by the inclusion of dynamic electron correlation are in very good agreement with the experimental values. The radiative properties such as weighted transition probabilities (gA) and oscillator strengths (gf) among several spin-orbit states have been calculated at the SA-CASSCF and MS-CASPT2 levels and are expected to be very helpful for future experiments.     

Spin-orbit ab initio investigation of the photolysis of bromoiodomethane.

The photodissociation of bromoiodomethane has been investigated by spin-orbit ab initio calculations. The experimentally observed A- and B-bands and the corresponding photoproducts were assigned by multistate second-order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space state interaction potential energy curves, vertical excitation energies, and oscillator strengths of low-lying excited states. The present conclusions with respect to the dissociation process in the B-band are different compared with those of previous studies. The reaction between the iso-CH(2)Br-I and iso-CH(2)I-Br species has also been studied. Finally, a set of stable excited states was identified for both isomers. These species might be of importance in the recombination process that follows the photodissociation in a solvent.     

A detailed experimental and theoretical study of the femtosecond A-band photodissociation of CH(3)I

The real time photodissociation dynamics of CH(3)I from the A band has been studied experimentally and theoretically. Femtosecond pump-probe experiments in combination with velocity map imaging have been carried out to measure the reaction times (clocking) of the different (nonadiabatic) channels of this photodissociation reaction yielding ground and spin-orbit excited states of the I fragment and vibrationless and vibrationally excited (symmetric stretch and umbrella modes) CH(3) fragments. The measured reaction times have been rationalized by means of a wave packet calculation on the available ab initio potential energy surfaces for the system using a reduced dimensionality model. A 40 fs delay time has been found experimentally between the channels yielding vibrationless CH(3)(nu=0) and I((2)P(32)) and I(*)((2)P(12)) that is well reproduced by the calculations. However, the observed reduction in delay time between the I and I(*) channels when the CH(3) fragment appears with one or two quanta of vibrational excitation in the umbrella mode is not well accounted for by the theoretical model.     

用共振增强多光子电离方法研究几种含溴化合物的光解

利用飞行时间质谱仪(TOF)和共振增强多光子电离(REMPI)方法, 研究了两种正一溴代烷烃(C₂H₅Br, n-C₃H₇Br)和溴苯(C₆H₅Br)在234及267 nm附近的光解. 测出了这几种含溴化合物在不同波长下光解产物Br^*和Br的分支比N(Br*)/N(Br), 并根据从头计算结果, 解释了这几种含溴化合物光解产物的分支比随光解波长变化的趋势及几个低激发态势能面之间的关系.     

A two-dimensional wavepacket study of the nonadiabatic dynamics of CH2BrCl

The nonadiabatic photodissociation dynamics of CH2BrCl into CH2Br + Cl or CH2Cl + Br is studied using two-dimensional wavepacket propagations on ab initio multiconfigurational MS-CASPT2 potential energy surfaces. Using a three-state diabatic model, we investigate the electronic states responsible for the two competing fragmentation channels and how the conical intersection present between the two lowest excited states affects the dissociation rate. Within this model, we find that the Br/Cl branching ratio depends on the irradiation wavelength. Predominant C-Br fragmentation occurs for wavelengths longer than 200 nm, while nonadiabatic C-Cl dissociation with a constant branching ratio of 0.4 is predicted upon absorption of photons in the range of 170-180 nm. Additionally, we observe complete nonadiabatic population transfer in less than 100 fs, that is, before the wavepacket can reach the conical intersection. As a consequence, there is no three-body CH2 + Br + Cl dissociation.     

Photodissociation dynamics of allyl bromide at 234, 265, and 267 nm

The photodissociation dynamics of allyl bromide was investigated at 234, 265, and 267 nm. A two-dimensional photofragment ion velocity imaging technique coupled with a [2+1] resonance-enhanced multiphoton ionization scheme was utilized to obtain the angular and translational energy distributions of the nascent Br* (2P1/2) and Br (2P3/2) atoms. The Br fragments show a bimodal translational energy distribution, while the Br* fragments reveal one translational energy distribution. The vertical excited energies and the mixed electronic character of excited states were calculated at ab initio configuration interaction method. It is presumed that the high kinetic energy bromine atoms are attributed to the predissociation from 1(pipi*) or 1(pisigma*) state to the repulsive 1(nsigma*) state, and to the direct dissociation from 3(nsigma*) and 3(pisigma*) states, while the low kinetic energy bromine atoms stem from internal conversion from the lowest 3(pipi*) state to 3(pisigma*) state.     

Theoretical study of the UV photodissociation of Cl2: potentials, transition moments, extinction coefficients, and Cl*/Cl branching ratio

Potential energy curves for the X (1)Sigma(g) (+) ground state and Omega=0(u) (+), 1(u) valence states and dipole moments for the 0(u) (+), 1(u)-X transitions are obtained in an ab initio configuration interaction study of Cl(2) including spin-orbit coupling. In contrast to common assumptions, it is found that the B (3)Pi(0(+)u)-X transition moment strongly depends on internuclear distance, which has an important influence on the Cl(2) photodissociation. Computed energy curves and transition moments are employed to calculate the A, B, C<--X extinction coefficients, the total spectrum for the first absorption band, and the Cl(*)((2)P(1/2))/Cl((2)P(3/2)) branching ratio as a function of excitation wavelength. The calculated data are shown to be in good agreement with available experimental results.     

Reinvestigation of the electronic spectroscopy of the Au-Ar complex

The Au-Ar complex is reinvestigated employing resonance-enhanced multiphoton ionization spectroscopy. Spectra are reported, corresponding to the atomic transition Au(6p<--6s). This electronic excitation yields (2)Pi and (2)Sigma(+) states of Au-Ar, which interact under the influence of spin-orbit coupling. The spectra are consistent with strong sigma-pi mixing induced by the large spin-orbit coupling of Au, leading to strong interaction of the two Omega=12 states, which arise from the Ar((1)S(0))+Au((2)P(12,32)) asymptotes, and the consequent formation of a "shelf" on the outer wall of the lowest Omega=12 state. In addition, high-level ab initio calculations are reported on the ground electronic state, X (2)Sigma(+), including extrapolation to the basis set limit.     

Accurate ab initio potential for the Na+...I* complex

High-level ab initio calculations employing the multireference configuration interaction and coupled clusters methods with a correlation-consistent sequence of basis sets have been used to obtain accurate potential energy curves for the complex of the sodium cation with the iodine atom. Potential curves for the first two electronic Lambda-S states have very different characters: the potential for the 2pi state has a well depth of approximately 10 kcal/mol, while the 2sigma state is essentially unbound. This difference is rationalized in terms of the anisotropic interaction of the quadrupole moment of the iodine atom with the sodium cation, which is stabilizing in the case of the 2pi state and destabilizing in the case of the 2sigma state. The effects of spin-orbit coupling have been accounted for with both ab initio and semiempirical approaches, which have been found to give practically the same results. Inclusion of spin-orbit interactions does not affect the X(omega = 32) ground state, which retains its 2pi character, but it results in two omega = 12 spin-orbit states, with mixed 2sigma and 2pi characters and binding energies roughly half of that of the ground spin-orbit state. Complete basis set (CBS) extrapolations of potential curves, binding energies, and equilibrium geometries were also performed, and used to calculate a number of rovibronic parameters for the Na+...I* complex and to parameterize model potentials. The final CBS-extrapolated and zero-point vibrational energy-corrected binding energy is 10.2 kcal/mol. Applications of the present results for simulations of NaI photodissociation femtosecond spectroscopy are discussed.     

Ab initio treatment of the chemical reaction precursor complex Br(2P)-HCN. 1. Adiabatic and diabatic potential surfaces

The three adiabatic potential surfaces of the Br(2P)-HCN complex that correlate to the 2P ground state of the Br atom were calculated ab initio. With the aid of a geometry-dependent diabatic mixing angle, also calculated ab initio, these adiabatic potential surfaces were transformed into a set of four diabatic potential surfaces required to define the full 3 x 3 matrix of diabatic potentials. Each of these diabatic potential surfaces was expanded in terms of the appropriate spherical harmonics in the atom-linear molecule Jacobi angle theta. The dependence of the expansion coefficients on the distance R between Br and the HCN center of mass and on the CH bond length was fit to an analytic form. For HCN in its equilibrium geometry, the global minimum with De = 800.4 cm(-1) and Re = 6.908a0 corresponds to a linear Br-NCH geometry, with an electronic ground state of Sigma symmetry. A local minimum with De = 415.1 cm-1, Re = 8.730a0, and a twofold degenerate Pi ground state is found for the linear Br-HCN geometry. The binding energy, De, depends strongly on the CH bond length for the Br-HCN complex and much less strongly for the Br-NCH complex, with a longer CH bond giving stronger binding for both complexes. Spin-orbit coupling was included and diabatic states were constructed that correlate to the ground 2P3/2 and excited 2P1/2 spin-orbit states of the Br atom. For the ground spin-orbit state with electronic angular momentum j = (3/2) the minimum in the potential for projection quantum number omega = +/-(3/2) coincides with the local minimum for linear Br-HCN of the spin-free case. The minimum in the potential for projection quantum number omega = +/-(1/2) occurs for linear Br-NCH but is considerably less deep than the global minimum of the spin-free case. According to the lowest spin-orbit coupling included adiabatic potential the two linear isomers, Br-NCH and Br-HCN, are about equally stable. In the subsequent paper, we use these potentials in calculations of the rovibronic states of the Br-HCN complex.     

The effect of spin-orbit coupling on fast neutral chemical reaction O(3P)+CH3-->CH3O

The effect of nonadiabatic transitions through the spin-orbit couplings has been investigated on the fast neutral reaction, O((3)P)+CH(3)-->CH(3)O. Adiabatic potential energies and the spin-orbit coupling terms have been evaluated for the four electronic states of CH(3)O ((2)E, (2)A(2), (4)E, and (4)A(2)) that correlate with the O((3)P)+CH(3) asymptote, as a function of CO distance and OCH angle under the C(3v) symmetry, by ab initio electronic structure calculations using multireference internally contracted single and double excitation configuration interaction method with the 6-311G(2df,2pd) basis sets. Multistate quantum reactive scattering calculations have been carried out with the use of thus obtained potential energies and spin-orbit coupling matrices, based on the generalized R-matrix propagation method. The calculated thermal rate constants show a slight positive dependence on temperature in a range between 50 and 2000 K, supporting the previous experimental results. It is shown that the spin-orbit coupled excited states give rise to reflections over the centrifugal barrier due to the quantum interference. Classical capture calculations yield larger rate constants due to the neglect of quantum reflections. It is concluded that the effect of nonadiabatic transitions is of minor importance on the overall reactivity in this reaction.     

Measurement of Br photofragment orientation and alignment from HBr photodissociation: production of highly spin-polarized hydrogen atoms

The orientation and alignment of the (2)P(3/2) and (2)P(1/2) Br photofragments from the photodissociation of HBr is measured at 193 nm in terms of a(q) ((k))(p) parameters, using slice imaging. The A (1)Pi state is excited almost exclusively, and the measured a(q) ((k))(p) parameters and the spin-orbit branching ratio show that the dissociation proceeds predominantly via nonadiabatic transitions to the a (3)Pi and 1 (3)Sigma(+) states. Conservation of angular momentum shows that the electrons of the nascent H atom cofragments (recoiling parallel to the photolysis polarization) are highly spin polarized: about 100% for the Br((2)P(1/2)) channel, and 86% for the Br((2)P(3/2)) channel. A similar analysis is demonstrated for the photodissociation of HCl.     

Adiabatic states derived from a spin-coupled diabatic transformation: semiclassical trajectory study of photodissociation of HBr and the construction of potential curves for LiBr+

The development of spin-coupled diabatic representations for theoretical semiclassical treatments of photodissociation dynamics is an important practical goal, and some of the assumptions required to carry this out may be validated by applications to simple systems. With this objective, we report here a study of the photodissociation dynamics of the prototypical HBr system using semiclassical trajectory methods. The valence (spin-free) potential energy curves and the permanent and transition dipole moments were computed using high-level ab initio methods and were transformed to a spin-coupled diabatic representation. The spin-orbit coupling used in the transformation was taken as that of atomic bromine at all internuclear distances. Adiabatic potential energy curves, nonadiabatic couplings and transition dipole moments were then obtained from the diabatic ones and were used in all the dynamics calculations. Nonadiabatic photodissociation probabilities were computed using three semiclassical trajectory methods, namely, coherent switching with decay of mixing (CSDM), fewest switches with time uncertainty (FSTU), and its recently developed variant with stochastic decoherence (FTSU/SD), each combined with semiclassical sampling of the initial vibrational state. The calculated branching fraction to the higher fine-structure level of the bromine atom is in good agreement with experiment and with more complete theoretical treatments. The present study, by comparing our new calculations to wave packet calculations with distance-dependent ab initio spin-orbit coupling, validates the semiclassical trajectory methods, the semiclassical initial state sample scheme, and the use of a distance-independent spin-orbit coupling for future applications to polyatomic photodissociation. Finally, using LiBr(+) as a model system, it is shown that accurate spin-coupled potential curves can also be constructed for odd-electron systems using the same strategy as for HBr.     

Br2 elimination in 248-nm photolysis of CF2Br2 probed by using cavity ring-down absorption spectroscopy

By using cavity ring-down absorption spectroscopy technique, we have observed the channel of Br2 molecular elimination following photodissociation of CF2Br2 at 248 nm. A tunable laser beam, which is crossed perpendicular to the photolyzing laser beam in a ring-down cell, is used to probe the Br2 fragment in the B 3Piou+-X1Sigmag+ transition. The vibrational population is obtained in a nascent state, despite ring-down time as long as 500-1000 ns. The population ratio of Br2(v=1)/Br2(v=0) is determined to be 0.4+/-0.2, slightly larger than the value of 0.22 evaluated by Boltzmann distribution at room temperature. The quantum yield of the Br2 elimination reaction is also measured to be 0.04+/-0.01. This work provides direct evidence to support molecular elimination occurring in the CF2Br2 photodissociation and proposes a plausible pathway with the aid of ab initio potential-energy calculations. CF2Br2 is excited probably to the 1B1 and 3B2 states at 248 nm. As the C-Br bond is elongated upon excitation, the coupling of the 1A'(1B1) state to the high vibrational levels of the ground state X 1A'(1A1) may be enhanced to facilitate the process of internal conversion. After transition, the highly vibrationally excited CF2Br2 feasibly surpasses a transition barrier prior to decomposition. According to the ab initio calculations, the transition state structure tends to correlate with the intermediate state CF2Br+Br(CF2Br...Br) and the products CF2+Br2. A sequential photodissociation pathway is thus favored. That is, a single C-Br bond breaks, and then the free-Br atom moves to form a Br-Br bond, followed by the Br2 elimination. The formed Br-Br bond distance in the transition state tends to approach equilibrium such that the Br2 fragment may be populated in cold vibrational distribution. Observation of a small vibrational population ratio of Br2(v=1)Br2(v=0) agrees with the proposed mechanism.     

Photodissociation of molecular bromine in solid H2 and D2: spectroscopy of the atomic bromine spin-orbit transition

We report 355 nm photodissociation studies of molecular bromine (Br2) trapped in solid parahydrogen (pH2) and orthodeuterium (oD2). The product Br atoms are observed via the spin-orbit transition ((2)P(1/2)<-- (2)P(3/2)) of atomic bromine. The quantum yield (Phi) for Br atom photoproduction is measured to be 0.29(3) in pH2 and 0.24(2) in oD2, demonstrating that both quantum solids have minimal cage effects for Br2 photodissociation. The effective Br spin-orbit splitting increases when the Br atom is solvated in solid pH2 (+1.1%) and oD2 (+1.5%); these increases are interpreted as evidence that the solvation energy of the Br ground fine structure state ((2)P(3/2)) is significantly greater than the excited state ((2)P(1/2)). Molecular bromine induced H2 infrared absorptions are detected in the Q1(0) and S1(0) regions near 4150 and 4486 cm(-1), respectively, which allow the relative Br2 concentration to be monitored as a function of 355 nm photolysis.