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28 Sep 2011

Volume 135, Issue 12, Articles (12xxxx)

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J. Chem. Phys. 135, 124501 (2011); http://dx.doi.org/10.1063/1.3626839 (19 pages)

Viktor Szalay, Krisztián Lengyel, László Kovács, Vicente Timón, and Alfonso Hernández-Laguna
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Communication: Strong excitonic and vibronic effects determine the optical properties of Li2O2

J. M. Garcia-Lastra, J. D. Bass, and K. S. Thygesen

J. Chem. Phys. 135, 121101 (2011); http://dx.doi.org/10.1063/1.3645544 (4 pages) | Cited 2 times

Online Publication Date: 28 September 2011

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The band structure and optical absorption spectrum of lithium peroxide (Li2O2) is calculated from first-principles using the G0W0 approximation and the Bethe-Salpeter equation, respectively. A strongly localized (Frenkel type) exciton corresponding to the π*→σ* transition on the O2−2 peroxide ion gives rise to a narrow absorption peak around 1.2 eV below the calculated bandgap of 4.8 eV. In the excited state, the internal O2−2 bond is significantly weakened due to the population of the σ* orbital. As a consequence, the bond is elongated by almost 0.5 Å leading to an extreme Stokes shift of 2.6 eV. The strong vibronic coupling entails significant broadening of the excitonic absorption peak in good agreement with diffuse reflectance data on Li2O2 which shows a rather featureless spectrum with an absorption onset around 3.0 eV. These results should be important for understanding the origin of the high potential losses and low current densities, which are presently limiting the performance of Li-air batteries.
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71.35.Aa Frenkel excitons and self-trapped excitons
78.55.Hx Other solid inorganic materials
71.20.Ps Other inorganic compounds
71.15.-m Methods of electronic structure calculations
78.40.Ha Other nonmetallic inorganics
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Communication: Drift velocity of Brownian particle in a periodically tapered tube induced by a time-periodic force with zero mean: Dependence on the force period

V. Yu. Zitserman, A. M. Berezhkovskii, A. E. Antipov, and Yu. A. Makhnovskii

J. Chem. Phys. 135, 121102 (2011); http://dx.doi.org/10.1063/1.3647873 (4 pages)

Online Publication Date: 30 September 2011

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We study the drift of a Brownian particle in a periodically tapered tube, induced by a longitudinal time-periodic force of amplitude |F| that alternates in sign every half-period. The focus is on the velocity dependence on the force period, which is usually considered not tractable analytically. For large |F| we derive an analytical solution that gives the velocity as a function of the amplitude and the period of the force as well as the geometric parameters of the tube. The solution shows how the velocity decreases from its maximum value to zero as the force period decreases from infinity (adiabatic regime) to zero. Our analytical results are in excellent agreement with those obtained from 3D Brownian dynamics simulations.
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05.40.Jc Brownian motion
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back to top Theoretical Methods and Algorithms

Mayer-sampling Monte Carlo calculations of uniquely flexible contributions to virial coefficients

Katherine R. S. Shaul, Andrew J. Schultz, and David A. Kofke

J. Chem. Phys. 135, 124101 (2011); http://dx.doi.org/10.1063/1.3635773 (9 pages)

Online Publication Date: 22 September 2011

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We present methods for computing contributions to the virial coefficients uniquely associated with molecular flexibility, and we demonstrate their use with application to the third, fourth, and fifth virial coefficients of united-atom models of linear alkanes and methanol belonging to the suite of transferrable potentials for phase equilibria (TraPPE-UA). We find that these uniquely flexible contributions are more difficult to compute than the remainder of the coefficient, especially for the conditions at which they appear to be most important. The significance of these contributions relative to the full virial coefficient grows with the number of sites (the size of the molecule), the number of molecules, and, to a certain extent, the temperature. The nature of the site-site interactions is of great importance: the significance of the uniquely flexible contribution at third and fourth order is orders of magnitude larger for TraPPE-UA methanol, which has Coulombic interactions, than for TraPPE-UA propane, which does not, even though both models have three sites per molecule and comparable bending potentials. While the uniquely flexible contribution of TraPPE-UA propane has a negligible impact on its third-order virial-equation-of-state estimate of the critical point, the uniquely flexible contribution of TraPPE-UA methanol increases this estimate of its critical pressure by about 5%.
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64.30.Jk Equations of state of nonmetals
64.60.Kw Multicritical points
33.15.Bh General molecular conformation and symmetry; stereochemistry
02.70.Uu Applications of Monte Carlo methods

Hierarchical expansion of the kinetic energy operator in curvilinear coordinates for the vibrational self-consistent field method

D. Strobusch and Ch. Scheurer

J. Chem. Phys. 135, 124102 (2011); http://dx.doi.org/10.1063/1.3637629 (11 pages) | Cited 1 time

Online Publication Date: 22 September 2011

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A new hierarchical expansion of the kinetic energy operator in curvilinear coordinates is presented and modified vibrational self-consistent field (VSCF) equations are derived including all kinematic effects within the mean field approximation. The new concept for the kinetic energy operator is based on many-body expansions for all G matrix elements and its determinant. As a test application VSCF computations were performed on the H2O2 molecule using an analytic potential (PCPSDE) and different hierarchical approximations for the kinetic energy operator. The results indicate that coordinate-dependent reduced masses account for the largest part of the kinetic energy. Neither kinematic couplings nor derivatives of the G matrix nor its determinant had significant effects on the VSCF energies. Only the zero-point value of the pseudopotential yields an offset to absolute energies which, however, is irrelevant for spectroscopic problems.
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33.20.Tp Vibrational analysis
31.15.xr Self-consistent-field methods
31.50.-x Potential energy surfaces

A generalized mean field theory of coarse-graining

Vinod Krishna and Luca Larini

J. Chem. Phys. 135, 124103 (2011); http://dx.doi.org/10.1063/1.3638044 (12 pages) | Cited 1 time

Online Publication Date: 22 September 2011

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A general mean field theory is presented for the construction of equilibrium coarse-grained models. Inverse methods that reconstruct microscopic models from low resolution experimental data can be derived as particular implementations of this theory. The theory also applies to the opposite problem of reduction, where relevant information is extracted from available equilibrium ensemble data. Additionally, a complementary approach is presented and problems of representability in coarse-grained modeling analyzed using information theoretic arguments. These problems are central to the construction of coarse-grained representations of complex systems, and commonly used coarse-graining methods and variational principles for coarse-graining are derived as particular cases of the general theory.
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36.20.-r Macromolecules and polymer molecules
61.41.+e Polymers, elastomers, and plastics

Marginal states in a cubic autocatalytic reaction

Debojyoti Das, Pushpita Ghosh, and Deb Shankar Ray

J. Chem. Phys. 135, 124104 (2011); http://dx.doi.org/10.1063/1.3640012 (7 pages)

Online Publication Date: 23 September 2011

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Marginal steady state belongs to a special class of states in nonlinear dynamics. To realize this state we consider a cubic autocatalytic reaction A + 2B → 3B in a continuous-stirred-tank-reactor, where the flow rate of the reactant A can be controlled to manipulate the dynamical behavior of the open system. We demonstrate that when the flow rate is weakly noisy the autocatalytic reaction admits of a steady state which is marginal in nature and is surrounded by infinite number of periodic trajectories. When the uncatalyzed reaction A → B is included in the reaction scheme, there exists a marginal steady state which is a critical state corresponding to the point of transition between the flow branch and the equilibrium branch, similar to gas-liquid critical point of transition. This state loses its stability in the weak noise limit.
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82.40.Bj Oscillations, chaos, and bifurcations
82.30.Vy Homogeneous catalysis in solution, polymers and zeolites
05.45.-a Nonlinear dynamics and chaos

Bond energy analysis revisited and designed toward a rigorous methodology

Hiromi Nakai, Hideaki Ohashi, Yutaka Imamura, and Yasuaki Kikuchi

J. Chem. Phys. 135, 124105 (2011); http://dx.doi.org/10.1063/1.3636387 (12 pages)

Online Publication Date: 28 September 2011

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The present study theoretically revisits and numerically assesses two-body energy decomposition schemes including a newly proposed one. The new decomposition scheme is designed to make the equilibrium bond distance equivalent with the minimum point of bond energies. Although the other decomposition schemes generally predict the wrong order of the C–C bond strengths of C2H2, C2H4, and C2H6, the new decomposition scheme is capable of reproducing the C–C bond strengths. Numerical assessment on a training set of molecules demonstrates that the present scheme exhibits a stronger correlation with bond dissociation energies than the other decomposition schemes do, which suggests that the new decomposition scheme is a reliable and powerful analysis methodology.
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31.15.X- Alternative approaches
33.15.Dj Interatomic distances and angles
33.15.Fm Bond strengths, dissociation energies

Monte Carlo and event-driven dynamics of Brownian particles with orientational degrees of freedom

Flavio Romano, Cristiano De Michele, Davide Marenduzzo, and Eduardo Sanz

J. Chem. Phys. 135, 124106 (2011); http://dx.doi.org/10.1063/1.3629452 (7 pages)

Online Publication Date: 28 September 2011

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Recently, a simple scaling argument was introduced that allows us to map, with some precautions, Brownian and Monte Carlo dynamics for spherical particles. Here, we extend the scaling to study systems that have orientational degrees of freedom and carefully asses its validity over a wide region of temperature and density. Our work allows us to devise a Brownian Monte Carlo algorithm that produces, to a good approximation, physically meaningful trajectories with a minimum programming effort, although at the expense of some sampling efficiency.
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05.40.Jc Brownian motion
05.10.Ln Monte Carlo methods

Mathematical analysis of the boundary-integral based electrostatics estimation approximation for molecular solvation: Exact results for spherical inclusions

Jaydeep P. Bardhan and Matthew G. Knepley

J. Chem. Phys. 135, 124107 (2011); http://dx.doi.org/10.1063/1.3641485 (10 pages)

Online Publication Date: 28 September 2011

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We analyze the mathematically rigorous BIBEE (boundary-integral based electrostatics estimation) approximation of the mixed-dielectric continuum model of molecular electrostatics, using the analytically solvable case of a spherical solute containing an arbitrary charge distribution. Our analysis, which builds on Kirkwood's solution using spherical harmonics, clarifies important aspects of the approximation and its relationship to generalized Born models. First, our results suggest a new perspective for analyzing fast electrostatic models: the separation of variables between material properties (the dielectric constants) and geometry (the solute dielectric boundary and charge distribution). Second, we find that the eigenfunctions of the reaction-potential operator are exactly preserved in the BIBEE model for the sphere, which supports the use of this approximation for analyzing charge-charge interactions in molecular binding. Third, a comparison of BIBEE to the recent GBε theory suggests a modified BIBEE model capable of predicting electrostatic solvation free energies to within 4% of a full numerical Poisson calculation. This modified model leads to a projection-framework understanding of BIBEE and suggests opportunities for future improvements.
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87.15.R- Reactions and kinetics
82.30.Nr Association, addition, insertion, cluster formation
02.60.Nm Integral and integrodifferential equations
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Projected quasiparticle theory for molecular electronic structure

Gustavo E. Scuseria, Carlos A. Jiménez-Hoyos, Thomas M. Henderson, Kousik Samanta, and Jason K. Ellis

J. Chem. Phys. 135, 124108 (2011); http://dx.doi.org/10.1063/1.3643338 (16 pages) | Cited 1 time

Online Publication Date: 30 September 2011

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We derive and implement symmetry-projected Hartree-Fock-Bogoliubov (HFB) equations and apply them to the molecular electronic structure problem. All symmetries (particle number, spin, spatial, and complex conjugation) are deliberately broken and restored in a self-consistent variation-after-projection approach. We show that the resulting method yields a comprehensive black-box treatment of static correlations with effective one-electron (mean-field) computational cost. The ensuing wave function is of multireference character and permeates the entire Hilbert space of the problem. The energy expression is different from regular HFB theory but remains a functional of an independent quasiparticle density matrix. All reduced density matrices are expressible as an integration of transition density matrices over a gauge grid. We present several proof-of-principle examples demonstrating the compelling power of projected quasiparticle theory for quantum chemistry.
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31.15.xm Quasiparticle methods
31.15.xr Self-consistent-field methods

Instanton calculations of tunneling splittings for water dimer and trimer

Jeremy O. Richardson, Stuart C. Althorpe, and David J. Wales

J. Chem. Phys. 135, 124109 (2011); http://dx.doi.org/10.1063/1.3640429 (12 pages)

Online Publication Date: 30 September 2011

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We investigate the ability of the recently developed ring-polymer instanton (RPI) method [J. O. Richardson and S. C. Althorpe, J. Chem. Phys. 134, 054109 (2011)]10.1063/1.3530589 to treat tunneling in water clusters. We show that the RPI method is easy to extend to treat tunneling between more than two minima, using elementary graph theory. Tests of the method on water dimer and trimer yield a set of instanton periodic orbits which correspond to all known tunneling pathways in these systems. Splitting patterns obtained from the orbits are in good overall agreement with experiment. The agreement is closer for the deuterated than for the protonated clusters, almost certainly because the main approximation in the calculations is neglect of anharmonicity perpendicular to the tunneling path. All the calculations were performed on a desktop computer, which suggests that similar calculations will be possible on much larger clusters.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
31.50.-x Potential energy surfaces
31.15.X- Alternative approaches

A parallelizable block cellular automaton for the study of diffusion of binary mixtures containing CO2 in microporous materials

Alberto M. Pintus, Federico G. Pazzona, Pierfranco Demontis, and Giuseppe B. Suffritti

J. Chem. Phys. 135, 124110 (2011); http://dx.doi.org/10.1063/1.3643764 (8 pages)

Online Publication Date: 30 September 2011

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We applied a method based on a block cellular automaton (BCA) algorithm to the study of diffusion of various binary mixtures adsorbed in a model microporous material, such as zeolite ZK4. Our aim was to test the capability of our model to cope with systems in which more than one species is present, using a set of parameters based on heuristic considerations from the molecular dynamics (MD) results present in the literature. A rigorous methodology for the assignment of suitable adsorption energies and diffusion activation barriers for our BCA has not been developed yet, nonetheless the results were quite interesting at this stage and we obtained a good qualitative agreement with MD data in the literature. The mixtures we investigated contain CO2, which causes the so-called segregation-effect, a strong suppression of self-diffusivity of co-adsorbed species. This effect gives rise to relevant problems in the application of some well established and robust methods, while our model proved to be able to reproduce both the common features and the segregation anomaly in the trends of diffusion.
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68.43.Mn Adsorption kinetics
66.30.H- Self-diffusion and ionic conduction in nonmetals
61.43.Gt Powders, porous materials
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Ab initio potential energy surface and bound states for the Kr–OCS complex

Eryin Feng, Chunyan Sun, Chunhua Yu, Xi Shao, and Wuying Huang

J. Chem. Phys. 135, 124301 (2011); http://dx.doi.org/10.1063/1.3640415 (6 pages) | Cited 1 time

Online Publication Date: 22 September 2011

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The first ab initio potential energy surface of the Kr–OCS complex is developed using the coupled-cluster singles and doubles with noniterative inclusion of connected triples [CCSD(T)]. The mixed basis sets, aug-cc-pVTZ for the O, C, and S atom, and aug-cc-pVQZ-PP for the Kr atom, with an additional (3s3p2d1f) set of midbond functions are used. A potential model is represented by an analytical function whose parameters are fitted numerically to the single point energies computed at 228 configurations. The potential has a T-shaped global minimum and a local linear minimum. The global minimum occurs at R = 7.146 a0, θ = 105.0° with energy of −270.73 cm−1. Bound state energies up to J = 9 are calculated for three isotopomers 82Kr–OCS, 84Kr–OCS, and 86Kr–OCS. Analysis of the vibrational wavefunctions and energies suggests the complex can exist in two isomeric forms: T-shaped and quasi-linear. The calculated transition frequencies and spectroscopic constants of the three isotopomers are in good agreement with the experimental values.
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31.15.A- Ab initio calculations
31.50.-x Potential energy surfaces
31.15.bw Coupled-cluster theory
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.15.Mt Rotation, vibration, and vibration-rotation constants

Pulsed discharge jet electronic spectroscopy of the aluminum dicarbide (AlC2) free radical

Jie Yang, Richard H. Judge, and Dennis J. Clouthier

J. Chem. Phys. 135, 124302 (2011); http://dx.doi.org/10.1063/1.3638049 (7 pages) | Cited 1 time

Online Publication Date: 22 September 2011

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Laser-induced fluorescence and wavelength resolved emission spectra of the math 2B2–math 2A1 band system of the gas phase aluminum dicarbide free radical have been obtained using the pulsed discharge jet technique. The radical was produced by electron bombardment of a precursor mixture of trimethylaluminum in high-pressure argon. The three vibrational frequencies of T-shaped AlC2 have been determined in both the combining states along with several of the anharmonicity constants. The 000 band has been recorded with high resolution and rotationally analyzed. The spectrum is complicated by partially resolved spin-rotation and aluminum hyperfine splittings. Where necessary, we have fixed the spin-rotation constants used in the rotational analysis at the values predicted by density functional theory. The derived molecular structures are: r0′′(C–C) = 1.271(2) Å, r0′′(Al–C) = 1.926(1) Å, θ″(C–Al–C) = 38.5(2)°, r0′(C–C) = 1.323(2) Å, r0′(Al–C) = 1.934(1) Å, and θ′(C–Al–C) = 40.0(2)°. Unlike SiC2, aluminum dicarbide shows no spectroscopic evidence of facile isomerization to the linear structure in the ground electronic state.
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33.50.Dq Fluorescence and phosphorescence spectra
31.30.Gs Hyperfine interactions and isotope effects
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.20.Kf Visible spectra
33.20.Tp Vibrational analysis
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions

Physical properties of small water clusters in low and moderate electric fields

S. Acosta-Gutiérrez, J. Hernández-Rojas, J. Bretón, J. M. Gomez Llorente, and D. J. Wales

J. Chem. Phys. 135, 124303 (2011); http://dx.doi.org/10.1063/1.3640804 (11 pages)

Online Publication Date: 26 September 2011

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Likely candidates for the lowest minima of water clusters (H2O)N for N ⩽ 20 interacting with a uniform electric field strength in the range E ⩽ 0.6 V/Å have been identified using basin-hopping global optimization. Two water-water model potentials were considered, namely TIP4P and the polarizable Dang-Chang potential. The two models produce some consistent results but also exhibit significant differences. The cluster internal energy and dipole moment indicate two varieties of topological transition in the structure of the global minimum as the field strength is increased. The first takes place at low field strengths (0.1 V/Å<E < 0.2 V/Å) and reorganizes the hydrogen-bonds to orient the water permanent dipoles along the field. The second type of transition occurs at larger field strengths (0.3 V/Å<E < 0.5 V/Å) and corresponds to an extensive structural reorganization, where several hydrogen-bonds break as the cluster stretches along the field direction, the larger clusters (N > 10) usually forming helical structures.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Fm Bond strengths, dissociation energies

Optical purification of a mixture of chiral forms by dimer formation

Asaf Eilam and Moshe Shapiro

J. Chem. Phys. 135, 124304 (2011); http://dx.doi.org/10.1063/1.3641283 (5 pages)

Online Publication Date: 26 September 2011

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We introduce a readily executable method for the optical purification of “scalemic” (non 50%-50%) mixtures of chiral molecules of opposite handedness (“enantiomers”). The method relies on the formation of two types of dimers, (R-R or S-S) “homodimers” and (R-S) “heterodimers.” The selectivity is linked to the difference in sign recently discovered by us to exist between certain transition-dipole matrix elements of opposite enantiomers. This sign difference results in differences in spectral propensity rules: In homodimers, transitions from the ground state can only take place to inversion symmetric excited states, while in the heterodimer the transitions are much more likely to proceed to antisymmetric excited states (although for heterodimers weak transitions to symmetric states might exist). These opposing propensity rules fully explain the observed large differences in the spectra of homodimers vs. heterodimers, which exist despite the almost identical energy levels positions. We illustrate the general concepts by computationally demonstrating the optically induced enantio-purification of scalemic mixtures of the hydropropionic C3H6O3 (lactic) acid.
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33.80.Wz Other multiphoton processes
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Intramolecular vibrational dynamics in S1 p-fluorotoluene. I. Direct observation of doorway states

Julia A. Davies and Katharine L. Reid

J. Chem. Phys. 135, 124305 (2011); http://dx.doi.org/10.1063/1.3638689 (14 pages)

Online Publication Date: 27 September 2011

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Picosecond time-resolved photoelectron spectroscopy is used to investigate intramolecular vibrational redistribution (IVR) following excitation of S1 18a1 in p-fluorotoluene (pFT) at an internal energy of 845 cm−1, where ν18a is a ring bending vibrational mode. Characteristic oscillations with periods of 8 ps and 5 ps are observed in the photoelectron signal and attributed to coupling between the initially excited zero-order bright state and two doorway states. Values for the coupling coefficients connecting these three vibrational states have been determined. In addition, an exponential change in photoelectron signal with a lifetime of 17 ps is attributed to weaker couplings with a bath of dark states that play a more significant role during the latter stages of IVR. A tier model has been used to assign the most strongly coupled doorway state to S1 17a1 6a2′, where ν17a is a CH out-of-plane vibrational mode and 6a2′ is a methyl torsional level. This assignment signifies that a torsion-vibration coupling mechanism mediates the observed dynamics, thus demonstrating the important role played by the methyl torsional mode in accelerating IVR.
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33.60.+q Photoelectron spectra
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.80.-b Photon interactions with molecules

Spectroscopy and thermochemistry of a jet-cooled open-shell polyene: 1,4-pentadienyl radical

Nahid Chalyavi, George B. Bacskay, Ambili S. Menon, Tyler P. Troy, Nathaniel J. L. K. Davis, Leo Radom, Scott A. Reid, and Timothy W. Schmidt

J. Chem. Phys. 135, 124306 (2011); http://dx.doi.org/10.1063/1.3640475 (7 pages)

Online Publication Date: 28 September 2011

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The 1,4-pentadienyl (vinylallyl) radical has been observed for the first time by optical spectroscopy. An excitation spectrum is recorded on m/z 67 by resonant two-color two-photon ionization spectroscopy. Several bands are observed with the origin transition identified at 19 449 cm−1. The spectrum is assigned by a comparison with ab initio frequencies calculated at the CASPT2/cc-pVTZ level of theory, with an accompanying Franck-Condon calculation of the excitation spectrum, including Dushinsky mixing. The b1 and a2 outer C–C bond torsional modes are calculated to halve in frequency upon electronic excitation, bringing about their appearance in the excitation spectrum. This can be readily understood by considering the torsional sensitivity of the frontier molecular orbital energies. High-level quantum chemical calculations of the radical stabilization energy, resulting in a value of nearly 120 kJ mol−1, provide quantitative confirmation that this radical is highly stabilized.
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36.20.Kd Electronic structure and spectra
36.20.Ey Conformation (statistics and dynamics)
36.20.Ng Vibrational and rotational structure, infrared and Raman spectra
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.20.Kf Visible spectra
31.15.xp Perturbation theory

Low energy (0–10 eV) electron driven reactions in the halogenated organic acids CCl3COOH, CClF2COOH, and CF3CHNH2COOH (trifluoroalanine)

Janina Kopyra, Constanze König-Lehmann, and Eugen Illenberger

J. Chem. Phys. 135, 124307 (2011); http://dx.doi.org/10.1063/1.3641479 (7 pages)

Online Publication Date: 28 September 2011

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Negative ion formation following resonant electron attachment to the three title molecules is studied by means of a beam experiment with mass spectrometric detection of the anions. All three molecules exhibit a pronounced resonance in the energy range around 1 eV which decomposes by the loss of a neutral hydrogen atom thereby generating the closed shell anion (M–H)− (or RCOO−), a reaction which is also a common feature in the non-substituted organic acids. The two chlorine containing molecules CCl3COOH and CClF2COOH exhibit an additional strong and narrow resonance at very low energy (close to 0 eV) which decomposes by the cleavage of the C–Cl bond with the excess charge finally localised on either of the two fragments Cl− and (M–Cl)−. This reaction is by two to three orders of magnitude more effective than hydrogen loss. Apart from these direct bond cleavages (C–Cl, O–H) resonant attachment of subexcitation electrons trigger additional remarkably complex unimolecular decompositions leading, e.g., to the formation of the bihalide ions ClHCl− and ClHF− from CCl3COOH and CClF2COOH, respectively, or the loss of a neutral CF2 unit from trifluoroalanine thereby generating the fluoroglycine radical anion. These reactions require substantial rearrangement in the transitory negative ion, i.e., the cleavage of different bonds and formation of new bonds. F− from both chlorodifluoroacetic acid and trifluoroalanine is formed at comparatively low intensity (more than three orders of magnitude less than Cl− from the chlorine containing molecules) and predominantly within a broad resonant feature around 7–8 eV characterised as core excited resonance.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
34.80.Ht Dissociation and dissociative attachment
34.80.Lx Recombination, attachment, and positronium formation

Interplay between charge and vibrational delocalization in cationic helium clusters

F. Calvo, F. Y. Naumkin, and D. J. Wales

J. Chem. Phys. 135, 124308 (2011); http://dx.doi.org/10.1063/1.3641895 (7 pages)

Online Publication Date: 28 September 2011

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The stable structures and low temperature thermodynamics of cationic helium clusters are investigated theoretically using a diatomics-in-molecules model for the potential energy surfaces and a computational framework in which both electronic and nuclear degrees of freedom are treated on a quantum mechanical footing. While the charge is generally carried by two atoms, vibrational delocalization significantly spreads out the charge over multiple isomers for clusters containing five or more helium atoms. Our calculations indicate that large clusters are essentially fluid with a well-defined solvation shell around the charged core.
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36.40.Wa Charged clusters
36.40.Cg Electronic and magnetic properties of clusters
36.40.Mr Spectroscopy and geometrical structure of clusters
33.20.Tp Vibrational analysis
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
31.50.-x Potential energy surfaces

Energy levels, Auger branching ratios, and radiative rates of the core-excited states of B-like carbon

Yan Sun, Feng Chen, and Bing Cong Gou

J. Chem. Phys. 135, 124309 (2011); http://dx.doi.org/10.1063/1.3643334 (8 pages)

Online Publication Date: 29 September 2011

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Energy levels, Auger branching ratios, and radiative rates of the core-excited states of B-like carbon are calculated by the saddle-point variation and saddle-point complex-rotation methods. Relativistic and mass polarization corrections are included using first-order perturbation theory. Calculated Auger channel energies and branching ratios are used to identify high-resolution Auger spectrum in the 300-keV C+ → CH4 collision experiment. It is found that Auger decay of these five-electron core-excited states gives significant contributions to Auger spectrum in the range of 238–280 eV.
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32.80.Hd Auger effect (including Coster-Krönig transitions)
31.15.aj Relativistic corrections, spin-orbit effects, fine structure; hyperfine structure

Torsional vibrational structure of the propene radical cation studied by high-resolution photoelectron spectroscopy

K. Vasilatou and F. Merkt

J. Chem. Phys. 135, 124310 (2011); http://dx.doi.org/10.1063/1.3638182 (14 pages)

Online Publication Date: 30 September 2011

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The pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the math+ 2A″←math 1A′ transition of CH 3 CHCH 2 (propene), CD 3 CDCD 2, and several partially deuterated isotopomers have been recorded in the region of their adiabatic ionization thresholds and up to 2000 cm−1 of internal energy of the cations. The vibrational structure has been assigned on the basis of the frequency shifts resulting from deuteration of selected sites of the propene molecule. Two highly anharmonic progressions have been identified and assigned to the two torsional modes of the propene cation, the methyl and methylene torsions. The positions of the torsional levels could be approximately reproduced using one-dimensional models, allowing a semi-quantitative description of the potential energy surface along each torsional coordinate. The observation of forbidden vibrational bands and the analysis of their partially resolved rotational contours reveal the importance of the vibronic coupling between the math+ 2A″ and the math+ 2A′ states mediated by the methylene (ν20) and methyl (ν21) torsional modes.
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33.60.+q Photoelectron spectra
31.30.Gs Hyperfine interactions and isotope effects
31.50.Bc Potential energy surfaces for ground electronic states
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.80.Eh Autoionization, photoionization, and photodetachment
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions

The ν2 bending vibrational structure of the math 2Σ+ state of MgNC

Masaru Fukushima and Takashi Ishiwata

J. Chem. Phys. 135, 124311 (2011); http://dx.doi.org/10.1063/1.3640024 (11 pages)

Online Publication Date: 30 September 2011

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We have generated MgNC in supersonic free jet expansions and observed the laser induced fluorescence (LIF) of the math 2Π–math 2Σ+ transition. We measured the LIF dispersed spectra from the single vibronic levels of the math 2Π electronic state of MgNC, following excitation of each ν2 bending vibronic band observed, i.e., the κ series of the (0,v2′,0)–(0,0,0), v2′ = 0, 1, 2, 4, and 6 vibronic bands. In the vibrational structure in the dispersed fluorescence spectra measured, the long progression of the ν2 bending mode in the math 2Σ+ state is identified, e.g., up to v2′′ = 14 in the (0,6,0)–(0,v2′′,0) spectrum. This enables us to derive the potential curve of the ν2 bending mode in the math 2Σ+ state. We used two kinds of models to obtain the potential curve; (I) the customary formula expressed in the polynomial series of the (v2′′+(d2/2)) term and (II) the internal rotation model. The potential curve derived from model (I) indicates the convergence of the bending vibrational levels at about 800 cm−1 from the vibrationless level of MgNC, which may correspond to the barrier height of the isomerization reaction, MgNC ⇌ MgCN, in the math 2Σ+ state. Model (II) gives a simple picture for the isomerization reaction pathway with a barrier height of about 630 cm−1 from the vibrationless level of the more stable species, MgNC. This shows that the v2′′ = 8 bending vibrational level of MgNC is already contaminated by the v2′′ = 2 bending vibrational level of the isomer, MgCN, and implies that the isomerization reaction begins at the v2′′ = 8 level. The bending potential surface and the isomerization reaction pathway, MgNC ⇌ MgCN, in the math 2Σ+ state are discussed by comparing the potential derived in this study with the surface obtained by quantum chemical calculation.
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33.20.Tp Vibrational analysis
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
33.50.Dq Fluorescence and phosphorescence spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
31.50.Df Potential energy surfaces for excited electronic states

Franck-Condon simulation, including anharmonicity, of the photodetachment spectrum of P2H−: Restricted-spin coupled-cluster single-double plus perturbative triple and unrestricted-spin coupled-cluster single-double plus perturbative triple -F12x potential energy functions of P2H and P2H−

Daniel K. W. Mok, Edmond P. F. Lee, Foo-tim Chau, and John M. Dyke

J. Chem. Phys. 135, 124312 (2011); http://dx.doi.org/10.1063/1.3640037 (11 pages)

Online Publication Date: 30 September 2011

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Geometry optimization and harmonic vibrational frequency calculations have been carried out on the math2A′ state of P2H and the math1A′ state of P2H− using the restricted-spin coupled-cluster single-double plus perturbative triple excitation [RCCSD(T)] and explicitly correlated unrestricted-spin coupled-cluster single-double plus perturbative triple excitation [UCCSD(T)-F12x] methods. For RCCSD(T) calculations, basis sets of up to the augmented correlation-consistent polarized valence quintuple-zeta (aug-cc-pV5Z) quality were employed, and contributions from extrapolation to the complete basis set limit and from core correlation of the P 2s22p6 electrons were also included. For UCCSD(T)-F12x calculations, different atomic orbital basis sets of triple-zeta quality with different associated complementary auxiliary basis sets and different geminal Slater exponents were used. When the P 2s22p6 core electrons were correlated in these F12x calculations, appropriate core-valence basis sets were employed. In addition, potential energy functions (PEFs) of the math2A′ state of P2H and the math1A′ state of P2H− were computed at different RCCSD(T) and UCCSD(T)-F12x levels, and were used in variational calculations of anharmonic vibrational wavefunctions, which were then utilized to calculate Franck-Condon factors (FCFs) between these two states, employing a method which includes allowance for anharmonicity and Duschinsky rotation. The photodetachment spectrum of P2H− was then simulated using the computed FCFs. Simulated spectra obtained using the RCCSD(T)/aug-cc-pV5Z and UCCSD(T)-F12x(x = a or b)/aug-cc-pCVTZ PEFs are compared and found to be essentially identical. Based on the computed FCFs, a more detailed assignment of the observed vibrational structure than previously reported, which includes “hot bands,” has been proposed. Comparison between simulated and available experimental spectra has been made, and the currently most reliable sets of equilibrium geometrical parameters for P2H and its anion have been derived. The photodetachment spectrum of P2D, yet to be recorded, has also been simulated.
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33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.80.Eh Autoionization, photoionization, and photodetachment
31.15.bw Coupled-cluster theory
33.20.Tp Vibrational analysis
33.20.Sn Rotational analysis
31.15.xt Variational techniques

Rotational predissociation of extremely weakly bound atom-molecule complexes produced by Feshbach resonance association

Alisdair O. G. Wallis and Roman V. Krems

J. Chem. Phys. 135, 124313 (2011); http://dx.doi.org/10.1063/1.3641643 (7 pages)

Online Publication Date: 30 September 2011

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We study the rotational predissociation of atom–molecule complexes with very small binding energy. Such complexes can be produced by Feshbach resonance association of ultracold molecules with ultracold atoms. Numerical calculations of the predissociation lifetimes based on the computation of the energy dependence of the scattering matrix elements become inaccurate when the binding energy is smaller than the energy width of the predissociating state. We derive expressions that represent accurately the predissociation lifetimes in terms of the real and imaginary parts of the scattering length and effective range for molecules in an excited rotational state. Our results show that the predissociation lifetimes are the longest when the binding energy is positive, i.e., when the predissociating state is just above the excited state threshold.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
31.50.Df Potential energy surfaces for excited electronic states
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
34.50.Gb Electronic excitation and ionization of molecules
31.15.-p Calculations and mathematical techniques in atomic and molecular physics
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