Volume 126, Issue 17, 07 May 2007
 ARTICLES

 Theoretical Methods and Algorithms

Quantum chemistry calculations for molecules coupled to reservoirs: Formalism, implementation, and application to benzenedithiol
View Description Hide DescriptionModern quantum chemistry calculations are usually implemented for isolated systems—big molecules or atom clusters; total energy and particle number are fixed. However, in many situations, like quantum transport calculations or molecules in a electrochemical environment, the molecule can exchange particles (and energy) with a reservoir. Calculations for such cases require to switch from the canonical to a grand canonical description, where one fixes the chemical potential rather than particle number. To achieve this goal, the authors propose an implementation in standard quantum chemistry packages. An application to the nonlinear charge transport through 1,4benzenedithiol will be presented. They explain the leading finite bias effect on the transmission as a consequence of a nonequilibrium Stark effect and discuss the relation to earlier work.

Relativistic calculation of nuclear magnetic shielding tensor using the regular approximation to the normalized elimination of the small component. II. Consideration of perturbations in the metric operator
View Description Hide DescriptionA previous relativistic shielding calculation theory based on the regular approximation to the normalized elimination of the small component approach is improved by the inclusion of the magnetic interaction term contained in the metric operator. In order to consider effects of the metric perturbation, the selfconsistent perturbation theory is used for the case of perturbationdependent overlap integrals. The calculation results show that the secondorder regular approximation results obtained for the isotropic shielding constants of halogen nuclei are well improved by the inclusion of the metric perturbation to reproduce the fully relativistic fourcomponent DiracHartreeFock results. However, it is shown that the metric perturbation hardly or does not affect the anisotropy of the halogen shielding tensors and the protonmagnetic shieldings.

Development of a sumoverstates density functional theory for both electric and magnetic static response properties
View Description Hide DescriptionIt is shown that it is possible to formulate a sumoverstates (SOS) response theory for static perturbations based directly on the KohnSham formulation of density functional theory(DFT). The SOSDFT response theory affords expressions analogous to those obtained from the classical RaleighSchrödinger perturbation theory, where use is made of a complete set of ground and excited state energies and wave functions. The static SOSDFT response theory is applicable for both real and imaginary perturbations. The theory is established by making use of timedependent DFT taken to zero frequency with the use of the adiabatic approximation. In the SOSDFT formulation the expression for electric (e.g., polarization) and magnetic (e.g., magnetization) response properties are symmetrical.

Wave packet theory of dynamic stimulated Raman spectra in femtosecond pumpprobe spectroscopy
View Description Hide DescriptionThe quantum theory for stimulated Raman spectroscopy from a moving wave packet using the thirdorder density matrix and polarization is derived. The theory applies, in particular, to the new technique of femtosecond broadband stimulated Raman spectroscopy (FSRS). In the general case, a femtosecond actinic pump pulse first prepares a moving wave packet on an excited statesurface which is then interrogated with a coupled pair of picosecond Raman pump pulse and a femtosecond Raman probe pulse and the Raman gain in the direction of the probe pulse is measured. It is shown that the thirdorder polarization in the time domain, whose Fourier transform governs the Raman gain, is given simply by the overlap of a firstorder wave packet created by the Raman pump on the upper electronic state with a secondorder wave packet on the initial electronic state that is created by the coupling of the Raman pump and probe fields acting on the molecule. Calculations are performed on model potentials to illustrate and interpret the FSRS spectra.

Effect of removing the novirtualpair approximation on the correlation energy of the He isoelectronic sequence
View Description Hide DescriptionThe correlationenergies (CEs) for the Helike ions are studied with the virtualpair approximation (VPA) and with the novirtualpair approximation (NVPA). In contrast to the nonrelativistic CEs, the CEs calculated with relativity fell sharply as the nuclear charge increased, although the CE calculated with the NVPA was considerably lower than with the VPA for the heavier atoms. It is shown that CE calculated with a Hylleraastype function implicitly includes the effects of the excitations into negativeenergy states, which corresponds to the VPA. The present results verify that the strong dependence on of the CE of Helike ions is an essential effect of the relativity.

Optimized effective potential method for individual lowlying excited states
View Description Hide DescriptionThis paper presents an optimized effective potential (OEP) approach based on density functional theory(DFT) for individual excited states that implements a simple method of taking the necessary orthogonality constraints into account. The amended KohnSham (KS) equations for orbitals of excited states having the same symmetry as the ground one are proposed. Using a variational principle with some orthogonality constraints, the OEP equations determining a local exchange potential for excited states are derived. Specifically, local potentials are derived whose KS determinants minimize the total energies and are simultaneously orthogonal to the determinants for states of lower energies. The parametrized form of an effective DFT potential expressed as a direct mapping of the external potential is used to simplify the OEP integral equations. A performance of the presented method is examined by exchangeonly calculations of excited state energies for simple atoms and molecules.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Total dissociative electron attachment cross sections of selected amino acids
View Description Hide DescriptionTotal dissociative electron attachment cross sections are presented for the amino acids, glycine, alanine, proline, phenylalanine, and tryptophan, at energies below the first ionization energy. Cross section magnitudes were determined by observation of positive ion production and normalization to ionization cross sections calculated using the binaryencounterBethe method. The prominent feature in the cross sections of the amino acids and the closely related HCOOH molecule is widely attributed to the attachment into the –COOH orbital. The authors discuss evidence that direct attachment to the lowest orbital may instead be responsible. A close correlation between the energies of the coreexcited anion states of glycine, alanine, and proline and the ionization energies of the neutral molecules is found. A prominent feature in the total dissociative electron attachment cross section of these compounds is absent in previous studies using mass analysis, suggesting that the missing fragment is energetic .

Cavity ringdown spectroscopy measurements of single aerosol particle extinction. I. The effect of position of a particle within the laser beam on extinction
View Description Hide DescriptionA continuous wave distributed feedback diode laser operating in the near infrared at wavelengths close to has been used to measure the extinction of light by single aerosol particles. The technique of optical feedback cavity ringdown spectroscopy (CRDS) was used for measurement of CRDS events at a repetition rate of . This very high repetition rate enabled multiple measurements of the extinction of light by single aerosol particles for the first time and demonstrated the dependence of light scattering on the position of a particle within the laser beam. A model is proposed to explain quantitatively this phenomenon. The minimum detectable dimensionless extinction coefficient was determined to be . Extinction values obtained for single spherical polymer beads from a monodisperse sample of particles of diameter of are in nearquantitative agreement with the values calculated by the Mie scattering theory. The deviations from the Mie theory expected for measurement of extinction by CRDS using a continuous wave laser are discussed in the companion paper.

Cavity ringdown spectroscopy measurement of single aerosol particle extinction. II. Extinction of light by an aerosol particle in an optical cavity excited by a cw laser
View Description Hide DescriptionThe authors present an analytical derivation of the scattered power from a spherical, homogeneous, nonabsorbing particle in a plane standing wave. The scattered power changes significantly with the position of the particle with respect to the peaks and nodes of the standing wave, even for particles whose diameters are many times the wavelength of the light. The analysis is applicable to continuouswave cavity ringdown spectroscopy on aerosol particles, and the structure of the standing wave is expected to affect both the measured ringdown time and the shape of the ringdown trace. The dependence of the extinction on the phase of the standing wave at the location of the particle is captured in a parameter which connects the current treatment to standard Mie scatteringtheory. Methods for calculating are presented.

Probing the valence character of O excited by participator Auger decay measurements and partial ion yield spectroscopy following xray absorption
View Description Hide DescriptionThe valence character of O excited is investigated by means of participator Auger decay spectroscopy, performed at selected photonenergies across the shell resonance region, and by means of partial ion yield xray absorption spectroscopy. For several of the excitation energies studied, the authors find substantial valence character being mixed with and Rydberg states. An experimental indication of a coupling between the channels associated with quartet and doublet ion cores is considered and discussed. New spectroscopic constants are derived for the singly ionized state of based on the observation of at least 20 vibrational sublevels.

Calculation of argon trimer rovibrational spectrum
View Description Hide DescriptionRovibrational spectra of are computed for total angular momenta up to using roworthonormal hyperspherical coordinates and an expansion of the wave function on hyperspherical harmonics. The sensitivity of the spectra to the twobody potential and to the threebody corrections is analyzed. First, the best available semiempirical pair potential (HFDID1) is compared with our recent ab initio twobody potential. The ab initio vibrational energies are typically higher than the semiempirical ones, which is related to the slightly larger dissociation energy of the semiempirical potential. Then, the AxilrodTeller asymptotic expansion of the threebody correction is compared with our newly developed ab initio threebody potential. The difference is found smaller than . In addition, we define approximate quantum numbers to describe the vibration and rotation of the system. The vibration is represented by a hyperradial mode and a twodegreeoffreedom hyperangular mode, including a vibrational angular momentum defined in an Eckart frame. The rotation is described by the total angular momentum quantum number, its projection on the axis perpendicular to the molecular plane, and a hyperangular internal momentum quantum number, related to the vibrational angular momentum by a transformation between Eckart and principalaxesofinertia frames. These quantum numbers provide a qualitative understanding of the spectra and, in particular, of the impact of the nuclear permutational symmetry of the system (bosonic with zero nuclear spin). Rotational constants are extracted from the spectra and are shown to be accurate only for the ground hyperangular mode.

Design of UV laser pulses for the preparation of matrix isolated homonuclear diatomic molecules in selective vibrational superposition states
View Description Hide DescriptionThe preparation of matrix isolated homonuclear diatomic molecules in a vibrational superposition state , with large plus small contributions of the ground and specific low excited vibrational eigenstates, respectively, in the electronic ground state, and without any net population transfer to electronic excited states, is an important challenge; it serves as a prerequisite for coherent spin control. For this purpose, the authors investigate two scenarios of laser pulse control, involving sequential or intrapulse pump and dumptype transitions via excited vibronic states with a dominant singlet or triplet character. The mechanisms are demonstrated by means of quantum simulations for representative nuclear wave packets on coupled potential energy surfaces, using as an example a onedimensional model for in an Ar matrix. A simple threestate model (including , and ) allows illuminating analyses and efficient determinations of the parameters of the laser pulses based on the values of the transition energies and dipole couplings of the transient state which are derived from the absorption spectra.

Anisotropic nonadditive ab initio force field for noncovalent interactions of
View Description Hide DescriptionA quantum mechanical polarizable force field (QMPFF) has been applied to the noncovalent interactions of molecular hydrogen as well as closedshell monoatomic species (CSMS): rare gases, alkali cations, and halide anions. The importance of all the main energy components is demonstrated: electrostatics (including penetration effect), exchange repulsion, dispersion, and induction. As the MP2 level of quantum mechanics, which is used to parametrize QMPFF, significantly underestimates the dimer binding energy, the force field was refined using stateoftheart CCSD(T) data. The approach demonstrates excellent transferability, which is confirmed by accurate reproduction of mixed dimers and the second virial coefficient of hydrogen vapor.

Vibrational overtone spectrum of matrix isolated cis, cisHOONO
View Description Hide DescriptionCis, cisperoxynitrous acid is known to be an intermediate in atmospheric reactions between OH and as well as HOO and NO. The infrared absorption spectra of matrixisolated and in argon have been observed in the range of . Besides the seven fundamental vibrational modes that have been assigned earlier for this molecule [Zhang et al., J. Chem. Phys.124, 084305 (2006)], more than 50 of the overtone and combination bands have been observed for and . Ab initio CCSD(T)/atomic natural orbital anharmonic force field calculations were used to help guide the assignments. Based on this study of the vibrational overtone transitions of cis, cisHOONO that go as high as and the earlier paper on the vibrational fundamentals, we conclude that the CCSD(T)/ANO anharmonic frequencies seem to correct to . The success of the theoretically predicted anharmonic frequencies in assigning overtone spectra of HOONO up to suggests that the CCSD(T)/ANO method is producing a reliable potential energy surface for this reactive molecule.

Isomers of and free energy criteria for cluster growth
View Description Hide DescriptionA molecular dynamics procedure is developed to search for cluster isomers and is used to study the isomer spectrum of with the Brenner potential. Beginning with isolated carbon atom, the procedure quickly arrives at the cage with the lowest potential and produces other 410 isomers. Among these isomers, we selected ones of typical cage, bowl, and sheet structures to calculate their free energies at and performed molecular dynamics simulations starting either from 36 free carbon atoms diluted in He buffer gas kept at or from the cage under the same conditions, which show that the microsystem reaches a kinetic equilibrium within about and that the isomer of the lowest free energy rather than the cage of the lowest potential energy dominates in the resultant cluster.

Infrared absorption of gaseous ClCS detected with timeresolved Fouriertransform spectroscopy
View Description Hide DescriptionA transient infrared absorptionspectrum of gaseous ClCS was detected with a stepscan Fouriertransform spectrometer coupled with a multipass absorption cell. ClCS was produced upon irradiating a flowing mixture of and or with a KrF excimer laser at . A transient band in the region of , which diminished on prolonged reaction, is assigned to the C–S stretching mode of ClCS. Calculations with densityfunctional theory (B3P86 and B3LYP/augccpVTZ) predict the geometry, vibrational wave numbers, and rotational parameters of ClCS. The rotational contour of the spectrum of ClCS simulated based on predicted rotational parameters agrees satisfactorily with experimental observation; from spectral simulation, the band origin is determined to be at . Reaction kinetics involving ClCS, CS, and are discussed.

transition of in supersonic jets and rare gas matrices
View Description Hide DescriptionThe study of the transition of (, ) in supersonic jets and solid rare gas matrices is reported. In the jetcooled spectrum, the origin band position is located at , the assignment being supported by the analysis of vibrational shifts and rotational band contours. Except for the origin band, which is weak, all bands are attributed to the fundamental excitation of nontotally symmetric vibrational modes of . The intensity pattern is interpreted as a consequence of the weak oscillator strength of the electronic transition combined with intensityborrowing through vibronic interaction between the and states. The spectra of the and transitions have also been measured for in solid neon and argon matrices. The comparison of the redshifts determined for either transition reveals that the polarizability of is larger in its than in its state. Bandwidths of measured in supersonic jets, which provide conditions relevant for astrophysics, are similar to those of most diffuse interstellar bands. The electronic transitions of are found to lie outside the ranges covered by present databases. From the comparison between experimental spectra and theoretical computations, it is concluded that the accuracy of empirical and ab initio approaches in predicting electronic energies is still not sufficient to identify astrophysically interesting candidates for spectroscopic laboratory studies.
 Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Additive property of secondary relaxation processes in dioctyl and diisooctyl phthalates: Signature of nonJohariGoldstein relaxation
View Description Hide DescriptionBroadband dielectric spectroscopy was used to study relaxation dynamics of supercooled dioctyl phthalate, diisooctyl phthalate, and their mixtures. Additionally, low temperature measurements were performed to investigate the nature of the secondary relaxation processes in both glass formers. The authors found that the secondary relaxation observed in the mixture is the additive sum of the secondary relaxations of the two components. This experimental evidence indicates that these secondary relaxation processes are intramolecular in origin, and they are nonJohariGoldstein secondary relaxations.

Dependence on chain length of NMR relaxation times in mixtures of alkanes
View Description Hide DescriptionMany naturally occurring fluids, such as crude oils, consist of a very large number of components. It is often of interest to determine the composition of the fluids in situ. Diffusion coefficients and nuclear magnetic resonance(NMR)relaxation times can be measured in situ and depend on the size of the molecules. It has been shown [D. E. Freed et al., Phys. Rev. Lett.94, 067602 (2005)] that the diffusion coefficient of each component in a mixture of alkanes follows a scaling law in the chain length of that molecule and in the mean chain length of the mixture, and these relations were used to determine the chain length distribution of crude oils from NMRdiffusion measurements. In this paper, the behavior of NMRrelaxation times in mixtures of chain molecules is addressed. The author explains why one would expect scaling laws for the transverse and longitudinal relaxation times of mixtures of short chain molecules and mixtures of alkanes, in particular. It is shown how the power law dependence on the chain length can be calculated from the scaling laws for the translational diffusion coefficients. The author fits the literature data for NMR relaxation in binary mixtures of alkanes and finds that its dependence on chain length agrees with the theory. Lastly, it is shown how the scaling laws in the chain length and the mean chain length can be used to determine the chain length distribution in crude oils that are high in saturates. A good fit is obtained between the NMRderived chain length distributions and the ones from gas chromatography.

Charge and momentum transfer in supercooled melts: Why should their relaxation times differ?
View Description Hide DescriptionThe steadystate values of the viscosity and the intrinsic ionic conductivity of quenched melts are computed, in terms of independently measurable quantities. The frequency dependence of the ac dielectric response is estimated. The discrepancy between the corresponding characteristic relaxation times is only apparent; it does not imply distinct mechanisms, but stems from the intrinsic barrier distribution for relaxation in supercooled fluids and glasses. This type of intrinsic “decoupling” is argued not to exceed four orders in magnitude for known glassformers. The origin of the discrepancy between the stretching exponent , as extracted from and the dielectric modulus data, is explained. The actual width of the barrier distribution always grows with lowering the temperature. The contrary is an artifact of the large contribution of the dcconductivity component to the modulus data. The methodology allows one to single out other contributions to the conductivity, as in “superionic” liquids or when charge carriers are delocalized, implying that in those systems, charge transfer does not require structural reconfiguration.