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28 Dec 2012

Volume 137, Issue 24, Articles (24xxxx)

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J. Chem. Phys. 137, 244102 (2012); http://dx.doi.org/10.1063/1.4769879 (11 pages)

Kuang Yu, Jesse G. McDaniel, and J. R. Schmidt
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Communication: Rovibrationally selected absolute total cross sections for the reaction H2O+(X2B1; v1+v2+v3+ = 000; N+Ka+Kc+) + D2: Observation of the rotational enhancement effect

Yuntao Xu, Bo Xiong, Yih Chung Chang, and C. Y. Ng

J. Chem. Phys. 137, 241101 (2012); http://dx.doi.org/10.1063/1.4773099 (4 pages) | Cited 1 time

Online Publication Date: 26 December 2012

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By employing the newly established vacuum ultraviolet laser pulsed field ionization-photoion (PFI-PI) double quadrupole-double octopole ion guide apparatus, we have measured the rovibrationally selected absolute total cross sections of the ion-molecule reaction H2O+(X2B1; v1+v2+v3+ = 000; N+Ka+Kc+) + D2 → H2DO+ + D in the center-of-mass collision energy (Ecm) range of 0.05–10.00 eV. The pulsing scheme used for the generation of PFI-PIs has made possible the preparation of reactant H2O+(X2B1; v1+v2+v3+ = 000) ions in single N+Ka+Kc+ rotational levels with high kinetic energy resolutions. The absolute total cross sections observed in different N+Ka+Kc+ levels with rotational energies in the range of 0–200 cm−1 were found to exhibit a significant rotational enhancement on the reactivity for the titled reaction. In contrast, the measured cross sections reveal a decreasing trend with increasing Ecm, indicating that the rotational enhancement observed is not a total energy effect, but a dynamical effect. Furthermore, the rotational enhancement is found to be more pronounced as Ecm is decreased. This experiment provided evidence that the coupling of the core rotational angular momentum with the orbital angular momentum could play a role in chemical reactivity, particularly at low Ecm.
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34.50.Ez Rotational and vibrational energy transfer
36.20.Ng Vibrational and rotational structure, infrared and Raman spectra
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
33.80.Eh Autoionization, photoionization, and photodetachment
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NVU dynamics. III. Simulating molecules at constant potential energy

Trond S. Ingebrigtsen and Jeppe C. Dyre

J. Chem. Phys. 137, 244101 (2012); http://dx.doi.org/10.1063/1.4768957 (10 pages) | Cited 1 time

Online Publication Date: 26 December 2012

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This is the final paper in a series that introduces geodesic molecular dynamics at constant potential energy. This dynamics is entitled NVU dynamics in analogy to standard energy-conserving Newtonian NVE dynamics. In the first two papers [T. S. Ingebrigtsen, S. Toxvaerd, O. J. Heilmann, T. B. Schrøder, and J. C. Dyre, J. Chem. Phys. 135, 104101 (2011)10.1063/1.3623585 ; T. S. Ingebrigtsen, S. Toxvaerd, T. B. Schrøder, and J. C. Dyre, J. Chem. Phys. 135, 104102 (2011)10.1063/1.3623586] , a numerical algorithm for simulating geodesic motion of atomic systems was developed and tested against standard algorithms. The conclusion was that the NVU algorithm has the same desirable properties as the Verlet algorithm for Newtonian NVE dynamics, i.e., it is time-reversible and symplectic. Additionally, it was concluded that NVU dynamics becomes equivalent to NVE dynamics in the thermodynamic limit. In this paper, the NVU algorithm for atomic systems is extended to be able to simulate the geodesic motion of molecules at constant potential energy. We derive an algorithm for simulating rigid bonds and test this algorithm on three different systems: an asymmetric dumbbell model, Lewis-Wahnström o-terphenyl (OTP) and rigid SPC/E water. The rigid bonds introduce additional constraints beyond that of constant potential energy for atomic systems. The rigid-bond NVU algorithm conserves potential energy, bond lengths, and step length for indefinitely long runs. The quantities probed in simulations give results identical to those of Nosé-Hoover NVT dynamics. Since Nosé-Hoover NVT dynamics is known to give results equivalent to those of NVE dynamics, the latter results show that NVU dynamics becomes equivalent to NVE dynamics in the thermodynamic limit also for molecular systems.
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31.15.xv Molecular dynamics and other numerical methods
31.50.-x Potential energy surfaces
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Dj Interatomic distances and angles

An efficient multi-scale lattice model approach to screening nano-porous adsorbents

Kuang Yu, Jesse G. McDaniel, and J. R. Schmidt

J. Chem. Phys. 137, 244102 (2012); http://dx.doi.org/10.1063/1.4769879 (11 pages) | Cited 1 time

Online Publication Date: 26 December 2012

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We present a multi-scale, hierarchical, approach for developing lattice models to estimate adsorption in nano-porous sorbents, derived on the basis of underlying atomistic potentials. This approach is a generalization of earlier work in zeolites (where the specific adsorption sites are easily definable) to encompass both specific as well as diffuse adsorption; the latter often dominates in the case of nano-porous metal-organic frameworks (MOFs). In conjunction with appropriately coarse grained guest-guest interactions, we demonstrate that our lattice approach offers semi-quantitative to quantitative agreement as compared to fully atomistic simulation from the low pressure regime through saturation. However, it also yields orders-of-magnitude acceleration versus the latter, thus enabling high-throughput screenings of both non-polar and polar adsorbates with high efficiency. We also show how our lattice model can be extended to facilitate rapid, qualitative screening of transport properties via appropriate calibration. Although our example applications focus on CO2 adsorption in MOFs, this approach is readily generalizable to various nano-porous materials (MOFs, zeolites…) and guest adsorbates (CO2, H2, hydrocarbons).
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68.43.Mn Adsorption kinetics

Low-rank spectral expansions of two electron excitations for the acceleration of quantum chemistry calculations

Christine A. Schwerdtfeger and David A. Mazziotti

J. Chem. Phys. 137, 244103 (2012); http://dx.doi.org/10.1063/1.4770278 (8 pages)

Online Publication Date: 26 December 2012

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Treatment of two-electron excitations is a fundamental but computationally expensive part of ab initio calculations of many-electron correlation. In this paper we develop a low-rank spectral expansion of two-electron excitations for accelerated electronic-structure calculations. The spectral expansion differs from previous approaches by relying upon both (i) a sum of three expansions to increase the rank reduction of the tensor and (ii) a factorization of the tensor into geminal (rank-two) tensors rather than orbital (rank-one) tensors. We combine three spectral expansions from the three distinct forms of the two-electron reduced density matrix (2-RDM), (i) the two-particle 2D, (ii) the two-hole 2Q, and the (iii) particle-hole 2G matrices, to produce a single spectral expansion with significantly accelerated convergence. While the resulting expansion is applicable to any quantum-chemistry calculation with two-particle excitation amplitudes, it is employed here in the parametric 2-RDM method [D. A. Mazziotti, Phys. Rev. Lett. 101, 253002 (2008)]10.1103/PhysRevLett.101.253002. The low-rank parametric 2-RDM method scales quartically with the basis-set size, but like its full-rank version it can capture multi-reference correlation effects that are difficult to treat efficiently by traditional single-reference wavefunction methods. Applications are made to computing potential energy curves of HF and triplet OH+, equilibrium bond distances and frequencies, the HCN-HNC isomerization, and the energies of hydrocarbon chains. Computed 2-RDMs nearly satisfy necessary N-representability conditions. The low-rank spectral expansion has the potential to expand the applicability of the parametric 2-RDM method as well as other ab initio methods to large-scale molecular systems that are often only treatable by mean-field or density functional theories.
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34.80.Gs Molecular excitation and ionization
31.15.A- Ab initio calculations
31.15.V- Electron correlation calculations for atoms, ions and molecules
33.15.Dj Interatomic distances and angles
33.15.Fm Bond strengths, dissociation energies
31.15.E- Density-functional theory

Performance of recent and high-performance approximate density functionals for time-dependent density functional theory calculations of valence and Rydberg electronic transition energies

Miho Isegawa, Roberto Peverati, and Donald G. Truhlar

J. Chem. Phys. 137, 244104 (2012); http://dx.doi.org/10.1063/1.4769078 (17 pages) | Cited 4 times

Online Publication Date: 27 December 2012

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We report a test of 30 density functionals, including several recent ones, for their predictions of 69 singlet-to-singlet excitation energies of 11 molecules. The reference values are experimental results collected by Caricato et al. for 30 valence excitations and 39 Rydberg excitations. All calculations employ time-dependent density functional theory in the adiabatic, linear-response approximation. As far as reasonable, all of the assignments are performed by essentially the same protocol as used by Caricato et al., and this allows us to merge our mean unsigned errors (MUEs) with the ones they calculated for both density functional and wave function methods. We find 21 of the 30 density functionals calculated here have smaller MUEs for the 30 valence states than what they obtained (0.47 eV) for the state-of-the-art EOM-CCSD wave function. In contrast, for all of density functionals the MUE for 39 Rydberg states is larger than that (0.11 eV) of EOM-CCSD. Merging the 30 density functionals calculated here with the 26 calculated by Caricato et al. makes a set of 56 density functionals. Averaging the unsigned errors over both the valence excitations and the Rydberg excitations, none of the 56 density functionals shows a lower mean unsigned error than that (0.27 eV) of EOM-CCSD. Nevertheless, two functionals are successful in having an overall mean unsigned error of 0.30 eV, and another nine are moderately successful in having overall mean unsigned errors in the range 0.32–0.36 eV. Successful or moderately successful density functionals include seven hybrid density functionals with 41% to 54% Hartree–Fock exchange, and four range-separated hybrid density functionals in which the percentage of Hartree–Fock exchange increases from 0% to 19% at small interelectronic separation to 65%–100% at long range.
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31.15.ee Time-dependent density functional theory
31.15.xr Self-consistent-field methods
31.15.xw Valence bond calculations
31.15.es Applications of density-functional theory (e.g., to electronic structure and stability; defect formation; dielectric properties, susceptibilities; viscoelastic coefficients; Rydberg transition frequencies)

Gaussian attenuation hybrid scheme applied to the Ernzerhof-Perdew exchange hole model (Gau-PBEh)

Jong-Won Song, Koichi Yamashita, and Kimihiko Hirao

J. Chem. Phys. 137, 244105 (2012); http://dx.doi.org/10.1063/1.4772401 (7 pages)

Online Publication Date: 27 December 2012

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Recently, we developed a Gaussian attenuation (Gau) scheme for solid-state bandgap calculation that uses a two-electron Gaussian function operator to include short-range Hartree-Fock exchange and combined it with the long-range Perdew-Burke-Ernzerhof (PBE) exchange correlation functional (Gau-PBE). Here, we apply the Ernzerhof-Perdew exchange hole (EP) model of PBE (PBEh) as a long-range density functional theory (DFT) exchange part to the Gau scheme (Gau-PBEh). We found that applying the EP model to the Gau scheme improves atomization energies and solid-state lattice constants and that the exact exchange included using the Gau scheme plays a critical role in simultaneously reproducing solid-state bandgaps and barrier heights. In addition, Gau-PBEh takes nearly the same computation time for bandgap calculations as Gau-PBE, implying less than 60% of the time taken in Heyd-Scuseria-Ernzerhof hybrid DFT functional calculations.
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71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.20.-b Electron density of states and band structure of crystalline solids

Reaction dynamics with the multi-layer multi-configurational time-dependent Hartree approach: H + CH4 → H2 + CH3 rate constants for different potentials

Ralph Welsch and Uwe Manthe

J. Chem. Phys. 137, 244106 (2012); http://dx.doi.org/10.1063/1.4772585 (9 pages) | Cited 2 times

Online Publication Date: 27 December 2012

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The multi-layer extension of the multi-configurational time-dependent Hartree (MCTDH) approach is applied to the investigation of elementary bimolecular chemical reactions. Cumulative reaction probabilities and thermal rate constants of the H + CH4 → H2 + CH3 reaction are calculated using flux correlation functions and the quantum transition state concept. Different coordinate systems and potential energy surfaces (PESs) are studied. The convergence properties of different layerings are investigated and the efficiency of multi-layer MCTDH approach is compared to the standard MCTDH approach. It is found that the multi-layer approach can decrease the numerical effort by more than an order of magnitude. The increased efficiency resulting from the multi-layer MCTDH approach is crucial for quantum dynamical calculations on recent global H + CH4 → H2 + CH3 PESs, e.g., the ZBB3-PES [Z. Xie, J. M. Bowman, and X. Zhang, J. Chem. Phys. 125, 133120 (2006)10.1063/1.2238871] based on permutational invariant polynomials, which are numerically more demanding than earlier PESs. The results indicate that an accurate description of all transition state frequencies is important to obtain accurate thermal rate constants.
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82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Xr Quantum effects in rate constants (tunneling, resonances, etc.)
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
31.15.xr Self-consistent-field methods
82.20.Kh Potential energy surfaces for chemical reactions

First passage times in homogeneous nucleation and self-assembly

Romain Yvinec, Maria R. D'Orsogna, and Tom Chou

J. Chem. Phys. 137, 244107 (2012); http://dx.doi.org/10.1063/1.4772598 (16 pages)

Online Publication Date: 28 December 2012

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Motivated by nucleation and molecular aggregation in physical, chemical, and biological settings, we present a thorough analysis of the general problem of stochastic self-assembly of a fixed number of identical particles in a finite volume. We derive the backward Kolmogorov equation (BKE) for the cluster probability distribution. From the BKE, we study the distribution of times it takes for a single maximal cluster to be completed, starting from any initial particle configuration. In the limits of slow and fast self-assembly, we develop analytical approaches to calculate the mean cluster formation time and to estimate the first assembly time distribution. We find, both analytically and numerically, that faster detachment can lead to a shorter mean time to first completion of a maximum-sized cluster. This unexpected effect arises from a redistribution of trajectory weights such that upon increasing the detachment rate, paths that take a shorter time to complete a cluster become more likely.
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36.20.Hb Configuration (bonds, dimensions)
02.50.Cw Probability theory
36.20.Ey Conformation (statistics and dynamics)
36.20.Fz Constitution (chains and sequences)
back to top Advanced Experimental Techniques

Slow photoelectron velocity-map imaging spectroscopy of cold negative ions

Christian Hock, Jongjin B. Kim, Marissa L. Weichman, Tara I. Yacovitch, and Daniel M. Neumark

J. Chem. Phys. 137, 244201 (2012); http://dx.doi.org/10.1063/1.4772406 (6 pages) | Cited 1 time

Online Publication Date: 26 December 2012

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Anion slow photoelectron velocity-map imaging (SEVI) spectroscopy is a high-resolution variant of photoelectron spectroscopy used to study the electronic and geometric structure of atoms, molecules, and clusters. To benefit from the high resolution of SEVI when it is applied to molecular species, it is essential to reduce the internal temperature of the ions as much as possible. Here, we describe an experimental setup that combines a radio-frequency ion trap to store and cool ions with the high-resolution SEVI spectrometer. For C5–, we demonstrate ion temperatures down to 10 ± 2 K after extraction from the trap, as measured by the relative populations of the two anion spin-orbit states. Vibrational hot bands and sequence bands are completely suppressed, and peak widths as narrow as 4 cm−1 are seen due to cooling of the rotational degrees of freedom.
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33.60.+q Photoelectron spectra
37.10.Mn Slowing and cooling of molecules
37.10.Pq Trapping of molecules
37.10.Rs Ion cooling
37.10.Ty Ion trapping
33.15.Mt Rotation, vibration, and vibration-rotation constants
back to top Atoms, Molecules, and Clusters

Spectroscopic investigation of the A and 3 1Σ+ states of 39K85Rb

Jin-Tae Kim, Yonghoon Lee, Bongsoo Kim, Dajun Wang, Phillip L. Gould, Edward E. Eyler, and William C. Stwalley

J. Chem. Phys. 137, 244301 (2012); http://dx.doi.org/10.1063/1.4771661 (8 pages) | Cited 1 time

Online Publication Date: 26 December 2012

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By using a combination of molecular beam (MB) excitation spectra and two distinct ultracold molecule excitation spectra (UM+ and UM−), we have assigned high vibrational levels of the A and 3 1Σ+ states from absorption spectra of the mutually strongly perturbed A 1Σ+ − 3 1Σ+ − 1 1Π − 2 3Σ+ − b 3Π states of ultracold 39K85Rb molecules in the energy region between 15 116 and 16 225 cm−1 above the minimum of the ground X 1Σ+ state. The ultracold molecules (UM+ and UM−) are formed by radiative decay following photoassociation (PA) to a specific level of the 3(0+) state (UM+) or to a specific level of the 3(0−) state (UM−). We observe that the A and 3 1Σ+ states are observable in the UM+ spectra, but absent from the UM− spectra. This is explained by considering Hund's case (c) selection rules and transition dipole moments between the upper excited A 1Σ+ (2(0+)) state and the three Ω components (0+, 0−, and 1) at the ground-state dissociation limit. We propose further investigations of the extended potential wells of the A and 3 1Σ+ states by combining short-range MB excitation spectra in a narrow Franck-Condon (FC) window near Re of the X 1Σ+ state, and long-range UM (and PA) excitation spectra, which have much larger FC windows.
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33.20.Lg Ultraviolet spectra
33.20.Tp Vibrational analysis
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Kf Visible spectra

Characterizing the excess electron of Li(NH3)4

Thomas Sommerfeld and Katelyn M. Dreux

J. Chem. Phys. 137, 244302 (2012); http://dx.doi.org/10.1063/1.4772018 (7 pages)

Online Publication Date: 26 December 2012

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Small lithium ammonia clusters are model systems for the dissociation of metals into solvated cations and electrons in ammonia. Metal–ammonia solutions display a complex behavior with increasing metal concentration including a phase change from a paramagnetic to a metallic diamagnetic phase, and small clusters should be useful models in the low concentration regime, where one may expect the ammoniated electron to show a behavior similar to that of the hydrated electron. Yet, even in the low concentration regime the nature of the ammoniated electron is still controversial with cavity models supported by optical and density measurements whereas localized radical models have been invoked to explain magnetic measurements. Small clusters can shed light on these open questions, and in particular the Li–NH3 tetramer represents the smallest cluster with a complete solvation shell for the Li+ cation. In view of the controversies about the character of the excess electron, the first question investigated is whether different theoretical characterizations of the “excess electron” lead to different conclusions about it. Only small differences are found between orbital-based and spin density-based and between self-consistent-field and coupled-cluster-based methods. Natural orbitals from equation-of-motion coupled-cluster calculations are then used to analyze the excess electron's distribution of Li(NH3)4 with particular emphasis on the portion of the excess electron's density that is closely associated with the N atoms. Three different comparisons show that only about 6% of the excess electron's density are closely associated with the atoms, with about 1% being closely associated with any N atom, and that the electron is best characterized as a Rydberg-like electron of the whole cluster. Finally, it is shown that in spite of the small amount of density close to the N atoms, the spin-density at the N nuclei is substantial, and that the magnetic observations can plausibly be explained within the cavity model.
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36.40.Ei Phase transitions in clusters
36.40.Jn Reactivity of clusters
31.15.xr Self-consistent-field methods
31.50.Df Potential energy surfaces for excited electronic states
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Geometries and electronic structures of the ground and low-lying excited states of FeCO: An ab initio study

Tsuneo Hirano, Rei Okuda, Umpei Nagashima, and Per Jensen

J. Chem. Phys. 137, 244303 (2012); http://dx.doi.org/10.1063/1.4769283 (13 pages)

Online Publication Date: 26 December 2012

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FeCO is a molecule of astrophysical interest. We report here theoretical calculations of its geometrical parameters, electronic structures, and molecular constants (such as dipole moment and spin-orbit coupling constant) in the electronic ground state math3Σ− and the low-lying triplet and quintet excited states. The calculations were made at the MR-SDCI+Q_DK3/[5ZP ANO-RCC (Fe, C, O)] and MR-AQCC_DK3/[5ZP ANO-RCC (Fe, C, O)] levels of theory. A multi-reference calculation was required to describe correctly the wavefunctions of all states studied. For all triplet states, the σ-donation through the 10σ molecular orbital (MO) as well as the π-back-donation through the 4π MO are observed, and the dipole moment vector points from O toward Fe as expected. However, in the excited quintet states 5Π, 5Φ, and 5Δ, the almost negligible contribution of Fe 4s to the 10σ MO makes the dipole moment vector point from Fe toward O, i.e., in the same direction as in CO. In the math3Σ− state, the electron provided by the σ-donation through the 10σ MO is shared between the Fe atom and the C end of the CO residue to form a coordinate-covalent Fe–C bond. In the math5Σ− state (the high-spin counterpart of math3Σ−), the σ-donation through the 10σ MO is not significant and so the Fe–C bond is rather ionic. The π-back-donation through the 4π MO is found to be of comparable importance in the two electronic states; it has a slightly larger magnitude in the math3Σ− state. The difference in the molecular properties of the low-spin math3Σ− and the high-spin math5Σ− states can be understood in terms of the dynamical electron correlation effects.
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31.15.ae Electronic structure and bonding characteristics
31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations
31.15.ap Polarizabilities and other atomic and molecular properties
31.15.xr Self-consistent-field methods
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

Melting behavior of Ag14 cluster: An order parameter by instantaneous normal modes

Ping-Han Tang, Ten-Ming Wu, P. J. Hsu, and S. K. Lai

J. Chem. Phys. 137, 244304 (2012); http://dx.doi.org/10.1063/1.4772096 (13 pages)

Online Publication Date: 27 December 2012

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This paper studies the melting behavior of Ag14 cluster employing the instantaneous normal mode (INM) analysis that was previously developed for bimetallic cluster Ag17Cu2. The isothermal Brownian-type molecular dynamics simulation is used to generate atom configurations of Ag14 at different temperatures up to 1500 K. At each temperature, these atomic configurations are then analyzed by the INM technique. To delve into the melting behavior of Ag14 cluster which differs from Ag17Cu2 by the occurrence of an anomalous prepeak in the specific heat curve in addition to the typical principal peak, we appeal to examining the order parameter τ(T) defined in the context of the INM method. Two general approaches are proposed to calculate τ(T). In one, τ(T) is defined in terms of the INM vibrational density of states; in another, τ(T) is defined considering the cluster as a rigid body with its rotational motions described by three orthogonal eigenvectors. Our results for Ag14 by these two methods indicate the mutual agreement of τ(T) calculated and also the consistent interpretation of the melting behavior with the specific heat data. The order parameter τ(T) provides in addition an insightful interpretation between the melting of clusters and the concept of broken symmetry which has been found successful in studies of the melting transition of bulk systems.
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64.70.dj Melting of specific substances
61.43.Bn Structural modeling: serial-addition models, computer simulation
65.40.Ba Heat capacity
63.20.D- Phonon states and bands, normal modes, and phonon dispersion

Strong-field induced XUV transmission and multiplet splitting in 4d−16p core-excited Xe studied by femtosecond XUV transient absorption spectroscopy

Ming-Fu Lin, Adrian N. Pfeiffer, Daniel M. Neumark, Stephen R. Leone, and Oliver Gessner

J. Chem. Phys. 137, 244305 (2012); http://dx.doi.org/10.1063/1.4772199 (8 pages)

Online Publication Date: 28 December 2012

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Light-induced coupling of core-excited states of Xe atoms is investigated by femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy with photon energies ranging from 50 eV to 72 eV. Coupling of the 4d−1(2D5/2)6p(2P3/2) (65.1 eV) and 4d−1(2D3/2)6p(2P1/2) (67.0 eV) core-excited states to nearby states by a strong infrared laser field leads to a threefold enhancement of XUV transmission. The transmission at 65.1 eV (67.0 eV) changes from 3.2 ± 0.4% (5.9 ± 0.5%) without the coupling laser to 9 ± 2% (22 ± 5%) at the maximum of the laser field. A strong-field induced broad XUV absorption feature between 60 eV and 65 eV is ascribed to splitting of the field-free absorption lines into multiple branches when the Rabi frequencies of the coupling transitions exceed the infrared laser frequency. This picture is supported by a comparison of the strong-field induced absorption spectrum with a numerical integration of the von Neumann equation for a few-level quantum system. The valence hole-alignment of strong-field ionized Xe is revisited, confirming the previously observed reduced alignment compared to theoretical predictions.
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32.30.Jc Visible and ultraviolet spectra
33.80.Eh Autoionization, photoionization, and photodetachment

The He + H2+ → HeH+ + H reaction: Ab initio studies of the potential energy surface, benchmark time-independent quantum dynamics in an extended energy range and comparison with experiments

Dario De Fazio, Miguel de Castro-Vitores, Alfredo Aguado, Vincenzo Aquilanti, and Simonetta Cavalli

J. Chem. Phys. 137, 244306 (2012); http://dx.doi.org/10.1063/1.4772651 (14 pages)

Online Publication Date: 28 December 2012

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In this work we critically revise several aspects of previous ab initio quantum chemistry studies [P. Palmieri et al., Mol. Phys. 98, 1835 (2000);10.1080/00268970009483387 C. N. Ramachandran et al., Chem. Phys. Lett. 469, 26 (2009)]10.1016/j.cplett.2008.12.035 of the HeH 2+ system. New diatomic curves for the H2+ and HeH+ molecular ions, which provide vibrational frequencies at a near spectroscopic level of accuracy, have been generated to test the quality of the diatomic terms employed in the previous analytical fittings. The reliability of the global potential energy surfaces has also been tested performing benchmark quantum scattering calculations within the time-independent approach in an extended interval of energies. In particular, the total integral cross sections have been calculated in the total collision energy range 0.955–2.400 eV for the scattering of the He atom by the ortho- and para-hydrogen molecular ion. The energy profiles of the total integral cross sections for selected vibro-rotational states of H2+ (v = 0, …,5 and j = 1, …,7) show a strong rotational enhancement for the lower vibrational states which becomes weaker as the vibrational quantum number increases. Comparison with several available experimental data is presented and discussed.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
34.50.Ez Rotational and vibrational energy transfer
31.50.Df Potential energy surfaces for excited electronic states
31.15.A- Ab initio calculations
33.20.Tp Vibrational analysis
33.20.Sn Rotational analysis

Formation and relaxation of RbHe exciplexes on He nanodroplets studied by femtosecond pump and picosecond probe spectroscopy

C. Giese, T. Mullins, B. Grüner, M. Weidemüller, F. Stienkemeier, and M. Mudrich

J. Chem. Phys. 137, 244307 (2012); http://dx.doi.org/10.1063/1.4772749 (7 pages)

Online Publication Date: 28 December 2012

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Vibrationally resolved photoionization spectra of RbHe exciplexes forming on He nanodroplets are recorded using femtosecond pump-probe spectroscopy with amplitude-shaped probe pulses. The time-evolution of the spectra reveals an exciplex formation time ∼10 ps followed by vibrational relaxation extending up to ≳ 1 ns. This points to an indirect, time-delayed desorption process of RbHe off the He surface.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.80.Be Level crossing and optical pumping

Molecular modeling study of agglomeration of [6,6]-phenyl-C61-butyric acid methyl ester in solvents

S. M. Mortuza and Soumik Banerjee

J. Chem. Phys. 137, 244308 (2012); http://dx.doi.org/10.1063/1.4772759 (12 pages)

Online Publication Date: 28 December 2012

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The molecular interactions between solvent and nanoparticles during photoactive layer formation in organic photovoltaic (OPV) cells influence the morphology of the photoactive layer and hence determine the power conversion efficiency. Prediction of optimal synthesis parameters in OPVs, such as choice of solvent, processing temperature, and nanoparticle concentration, requires fundamental understanding of the mechanisms that govern the agglomeration of nanoparticles in solvents. In this study, we used molecular dynamics simulations to simulate a commonly used organic nanoparticle, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), in various solvents to correlate solvent-nanoparticle interactions with the size of the agglomerate structure of PCBM. We analyzed the effects of concentration of PCBM and operating temperature on the molecular rearrangement and agglomeration of PCBM in three solvents: (i) toluene, (ii) indane, and (iii) toluene-indane mixture. We evaluated the agglomeration behavior of PCBM by determining sizes of the largest clusters of PCBM and the corresponding size distributions. To obtain further insight into the agglomerate structure of PCBMs, we evaluated radial distribution functions (RDFs) and coordination numbers of the various moieties of PCBMs with respect to solvent atoms as well as with respect to that of other PCBMs. Our simulations demonstrate that PCBMs form larger clusters in toluene while they are relatively dispersed in indane, which indicates the greater solubility of PCBM in indane than in toluene. In toluene-indane mixture, PCBMs are clustered to a greater extent than in indane and less than that in toluene. To correlate agglomerate size to nanoparticle-solvent interactions, we also evaluated the potential of mean force (PMF) of the fullerene moiety of PCBM in toluene and indane. Our results also show that the cluster size of PCBM molecules increases with the increase of concentration of PCBM and the processing temperature. To correlate the PCBM agglomeration with the dynamics of solvents, we evaluated the rotational correlation functions of the solvents. Our results illustrate that toluene relaxes faster than indane in the simulated systems and relaxation time of solvent molecules decreases with the decrease of concentration of PCBM and increase of processing temperature. Results presented in this study provide fundamental insight that can help to choose favorable solvents for processing PCBMs in OPV applications.
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61.20.Ja Computer simulation of liquid structure
61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)
31.15.xv Molecular dynamics and other numerical methods

On the photophysics and photochemistry of the water dimer

Javier Segarra-Martí, Daniel Roca-Sanjuán, Manuela Merchán, and Roland Lindh

J. Chem. Phys. 137, 244309 (2012); http://dx.doi.org/10.1063/1.4772187 (6 pages)

Online Publication Date: 28 December 2012

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The photochemistry of the water dimer irradiated by UV light is studied by means of the complete active space perturbation theory//complete active space self-consistent field (CASPT2//CASSCF) method and accurate computational approaches like as minimum energy paths. Both electronic structure computations and ab initio molecular dynamics simulations are carried out. The results obtained show small shifts relative to a single water molecule on the vertical excitation energies of the dimer due to the hydrogen bond placed between the water donor (WD) and the water acceptor (WA). A red-shift and a blue-shift are predicted for the WD and WA, respectively, supporting previous theoretical and experimental results. The photoinduced chemistry of the water dimer is described as a process occurring between two single water molecules in which the effect of the hydrogen bond plays a minor role. Thus, the photoinduced decay routes correspond to two photodissociation processes, one for each water molecule. The proposed mechanism for the decay channels of the lowest-lying excited states of the system is established as the photochemical production of a hydrogen-bonded H2O…HO species plus a hydrogen H atom.
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82.50.Hp Processes caused by visible and UV light
31.15.A- Ab initio calculations
31.15.xr Self-consistent-field methods
33.15.Fm Bond strengths, dissociation energies
33.70.Jg Line and band widths, shapes, and shifts
33.80.Gj Diffuse spectra; predissociation, photodissociation
back to top Liquids, Glasses, and Crystals

Hydrogen bond effects in the vibrational spectra of 1,3-propanediol in acetonitrile: Ab initio and experimental study

Francesco Muniz-Miranda, Marco Pagliai, Gianni Cardini, and Roberto Righini

J. Chem. Phys. 137, 244501 (2012); http://dx.doi.org/10.1063/1.4770499 (10 pages)

Online Publication Date: 26 December 2012

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Hydrogen bond interactions strongly affect vibrational properties and frequencies, the most common consequence being a redshift of the stretching vibration involved; there are, however, few exceptions to this general trend. In previous works, we have proved the effectiveness of ab initio simulations combined with wavelet analysis to investigate these effects and put them into relation to structural environment. In this work, we investigate the hydrogen bond effects on the structural and vibrational properties of 1,3-propanediol in acetonitrile by a combined experimental and computational approach. We explain the appearance of two spectral components in the O−H stretching band on the basis of intra- and intermolecular hydrogen bond interactions. We also elucidate the blueshift of the C≡N stretching band as due to a hydrogen bond interaction between the glycol and acetonitrile that modify the electron density distribution inside the CN group. This effect is well reproduced by ab initio molecular dynamics simulations and density functional calculations reported in this work.
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33.20.Tp Vibrational analysis
33.70.Jg Line and band widths, shapes, and shifts
31.15.ae Electronic structure and bonding characteristics
31.15.E- Density-functional theory
31.15.xv Molecular dynamics and other numerical methods
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Fm Bond strengths, dissociation energies

Structural and thermodynamical properties of charged hard spheres in a mixture with core-softened model solvent

Miha Lukšič, Barbara Hribar-Lee, Vojko Vlachy, and O. Pizio

J. Chem. Phys. 137, 244502 (2012); http://dx.doi.org/10.1063/1.4772582 (10 pages)

Online Publication Date: 26 December 2012

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The canonical Monte Carlo computer simulations and integral equation theory were applied to examine the structural and thermodynamic properties of a mixture of ions and a core-softened fluid molecules. The positive and negative ions forming a +1:−1 salt were modeled as charged hard spheres, immersed in the dielectric medium. It was shown previously that the core-softened fluid under study is characterized by a set of structural, thermodynamic, and dynamic anomalies. The principal objective of this work was to elucidate how the presence of ions alters this behavior. The structural properties of the mixtures are discussed in terms of the pair distribution functions; in addition, the pair contribution to the excess entropy was calculated. Thermodynamic properties are investigated by using the dependencies of energy and compressibility factor on density, composition of the mixture, and reduced temperature. The heat capacity was also evaluated. Our principal findings concern the description of structural anomalies in the mixture, the dependence of the temperature of maximum density on the ionic concentration, and establishing the regions delimiting the structural and thermodynamic anomalies of the model mixture.
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65.20.-w Thermal properties of liquids
02.30.Rz Integral equations
02.50.Ng Distribution theory and Monte Carlo studies
64.75.Bc Solubility

Ice polyamorphism in the minimal Mercedes-Benz model of water

Julyan H. E. Cartwright, Oreste Piro, Pedro A. Sánchez, and Tomás Sintes

J. Chem. Phys. 137, 244503 (2012); http://dx.doi.org/10.1063/1.4772202 (7 pages)

Online Publication Date: 27 December 2012

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We investigate ice polyamorphism in the context of the two-dimensional Mercedes-Benz model of water. We find a first-order phase transition between a crystalline phase and a high-density amorphous phase. Furthermore, we find a reversible transformation between two amorphous structures of high and low density; however, we find this to be a continuous and not an abrupt transition, as the low-density amorphous phase does not show structural stability. We discuss the origin of this behavior and its implications with regard to the minimal generic modeling of polyamorphism.
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64.70.K- Solid-solid transitions
61.43.Er Other amorphous solids
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder

Nonlinear dynamic heat capacity of a bead-spring polymeric glass former

Jonathan R. Brown and John D. McCoy

J. Chem. Phys. 137, 244504 (2012); http://dx.doi.org/10.1063/1.4772467 (10 pages)

Online Publication Date: 27 December 2012

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Nonlinear dynamics of a simple bead-spring glass-forming polymer were studied with molecular dynamics simulations. The energy response to sinusoidal variations in the temperature was tracked in order to evaluate the dynamic heat capacity. The amplitude dependence of the response is the focus of the current paper where pronounced nonlinear behavior is observed for large amplitudes in the temperature “driving force.” We generalize the usual linear response analysis to the nonlinear regime so that higher order terms in the Fourier series of the energy response can be compactly analyzed. This is done by grouping all Fourier terms contributing to entropy generation into a “loss” contribution and the remainder yields the “storage” term. Finally, the bead-spring system is mapped onto three simpler models. First is a potential energy inspired “trap” model consisting of interconnected potential energy meta-basins and barriers. Second is the Tool-Narayanaswamy-Moynihan (TNM) model. Third is a version of the TNM model with a temperature dependent heat capacity. Qualitatively similar nonlinear behaviors are observed in all cases.
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65.40.Ba Heat capacity
61.43.Bn Structural modeling: serial-addition models, computer simulation
61.43.Fs Glasses

Femtosecond Raman spectra of cis-stilbene and trans-stilbene with isotopomers in solution

A. L. Dobryakov, I. Ioffe, A. A. Granovsky, N. P. Ernsting, and S. A. Kovalenko

J. Chem. Phys. 137, 244505 (2012); http://dx.doi.org/10.1063/1.4769971 (16 pages)

Online Publication Date: 27 December 2012

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Femtosecond stimulated Raman spectra of trans-stilbene (D0), its isotopomers D2, D10, D12, 13C2 and of cis-stilbene in hexane are measured in the ground (S0) and excited (S1) electronic states. The ground 13C2 and excited D12 spectra are presented for the first time; the excited cis-spectra differ substantially from previously published ones. S1 Raman bands of trans-stilbene are 20 cm−1 wide corresponding to ∼1 ps vibrational dephasing. For cis-stilbene the bands are broadened to 40 cm−1 reflecting a short excited-state lifetime of 0.3 ps, in agreement with transient absorption data. From a dynamic shift of the 1569 cm−1 band, pump-induced intramolecular cooling is estimated to be less than 20 K. Many S1 Raman lines are detected for the first time. Vibrational spectra are calculated at MP2/cc-pVTZ (for S0) and XMCQDPT2/cc-pVTZ (for S1) levels of theory. Experimental and computational results can be used for a re-evalution of Rice-Ramsberger-Kassel-Marcus (RRKM) predictions for this famous photoisomeration reaction.
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82.30.Qt Isomerization and rearrangement
31.15.xp Perturbation theory
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Fb Raman and Rayleigh spectra (including optical scattering)
33.70.Jg Line and band widths, shapes, and shifts
82.20.Tr Kinetic isotope effects including muonium

Glass transition in ultrathin films of amorphous solid water

A. Sepúlveda, E. Leon-Gutierrez, M. Gonzalez-Silveira, C. Rodríguez-Tinoco, M. T. Clavaguera-Mora, and J. Rodríguez-Viejo

J. Chem. Phys. 137, 244506 (2012); http://dx.doi.org/10.1063/1.4771964 (5 pages)

Online Publication Date: 28 December 2012

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Nanocalorimetry at ultrafast heating rates is used to investigate the glass transition of nanometer thick films of metastable amorphous solid water grown by vapor deposition in an ultrahigh vacuum environment. Apparent heat capacity curves exhibit characteristic features depending on the deposition temperature. While films grown at T ≥ 155 K are completely crystallized, those deposited at 90 K show a relaxation exotherm prior to crystallization. Films grown between 135 and 140 K and subsequently cooled down to 90 K reveal a clear endothermic feature before crystallization, which is compatible with a glass-to-liquid transition. The onset temperature is located at 174 K at a heating rate of 2.4 × 104 K/s and is independent of film thickness in the range of 16–150 nm. Comparison of our data with other calorimetric measurements at various heating rates suggests that water is a strong glass former in the deeply supercooled state.
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64.70.P- Glass transitions of specific systems
81.40.Gh Other heat and thermomechanical treatments
61.43.-j Disordered solids
68.55.jd Thickness
64.70.dg Crystallization of specific substances
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.

On the absolute thermodynamics of water from computer simulations: A comparison of first-principles molecular dynamics, reactive and empirical force fields

Tod A. Pascal, Daniel Schärf, Yousung Jung, and Thomas D. Kühne

J. Chem. Phys. 137, 244507 (2012); http://dx.doi.org/10.1063/1.4771974 (7 pages) | Cited 1 time

Online Publication Date: 28 December 2012

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We present the absolute enthalpy, entropy, heat capacity, and free energy of liquid water at ambient conditions calculated by the two-phase thermodynamic method applied to ab initio, reactive and classical molecular dynamics simulations. We find that the absolute entropy and heat capacity of liquid water from ab initio molecular dynamics (AIMD) is underestimated, but falls within the range of the flexible empirical as well as the reactive force fields. The origin of the low absolute entropy of liquid water from AIMD simulations is due to an underestimation of the translational entropy by 20% and the rotational entropy by 40% compared to the TIP3P classical water model, consistent with previous studies that reports low diffusivity and increased ordering of liquid water from AIMD simulations. Classical MD simulations with rigid water models tend to be in better agreement with experiment (in particular TIP3P yielding the best agreement), although the TIP4P-ice water model, the only empirical force field that reproduces the experimental melting temperature, has the lowest entropy, perhaps expectedly. This reiterates the limitations of existing empirical water models in simultaneously capturing the thermodynamics of solid and liquid phases. We find that the quantum corrections to heat capacity of water can be as large as 60%. Although certain water models are computed to yield good absolute free energies of water compared to experiments, they are often due to the fortuitous enthalpy-entropy cancellation, but not necessarily due to the correct descriptions of enthalpy and entropy separately.
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65.20.-w Thermal properties of liquids
61.20.Ja Computer simulation of liquid structure
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