Volume 121, Issue 15, 15 October 2004
 COMMUNICATIONS


Tunable resonance hyperRaman spectroscopy of secondorder nonlinear optical chromophores
View Description Hide DescriptionTwophotonresonant hyperRaman spectra are reported for three “push–pull” conjugated organic chromophores bearing acceptor groups, two dipolar and one octupolar. The excitation source is an unamplified picosecond modelocked Ti:sapphire laser tunable from 720 to 950 nm. The linear resonanceRaman spectra of the same molecules are measured using excitation from the laser second harmonic. Excitation on resonance with the lowestlying band in the linear absorptionspectrum yields nearly identical resonance Raman and resonance hyperRaman spectra. However, excitation into a region that appears to contain more than one electronic transition gives rise to different intensity patterns in the linear and nonlinear spectra, indicating that different transitions contribute differently to the onephoton and twophotonoscillator strength. The promise of the hyperRaman technique for examining electronic transitions that are both one and twophoton allowed is discussed.

Rovibrationalstateselected pulsed field ionizationphotoelectron study of methyl iodide using twocolor infraredvacuum ultraviolet lasers
View Description Hide DescriptionThe preparation of methyl iodide in selected rovibrational states (CH stretch); J] by infrared (IR) excitation prior to vacuum ultraviolet (VUV)photoionization has greatly simplified the observed pulsed field ionizationphotoelectron (PFIPE) spectra, allowing the direct determination of the rotational constants for and the ionizationenergy (76 896.9±0.2 cm^{−1}) for The IRVUVPFIPE and IRVUVphotoion measurements also provide relative statetostate J) cross sections for the photoionization process.

Evidence of dissociative collision induced diatomic and triatomic hydrogen ion formation from hydrocarbon ion interaction with silicon surface
View Description Hide DescriptionA singly charged hydrocarbon ion (x=0,1,2,3,4) was extracted from an electron bombardment type ion source using methane as the reagent gas and irradiated onto the Si(100) surface at glancing angle. Scattered ion spectrometry using an electrostatic energy analyzer revealed that and ions were clearly formed at the scattering angle of 15°, associated with dissociative collisions of hydrocarbon ion species of incidence energy of 1000 eV. The formation of was tentatively interpreted as resulting from combination of excited atomic hydrogen produced by dissociative collisions of ions with Si(100) surface.
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 ARTICLES

 Theoretical Methods and Algorithms

Interpreting nonlinear vibrational spectroscopy with the classical mechanical analogs of doublesided Feynman diagrams
View Description Hide DescriptionObservables in coherent, multiplepulse infrared spectroscopy may be computed from a vibrational nonlinear response function. This response function is conventionally calculated quantummechanically, but the challenges in applying quantum mechanics to large, anharmonic systems motivate the examination of classical mechanical vibrational nonlinear response functions. We present an approximate formulation of the classical mechanical thirdorder vibrational response function for an anharmonic solute oscillator interacting with a harmonic solvent, which establishes a clear connection between classical and quantum mechanical treatments. This formalism permits the identification of the classical mechanical analog of the pure dephasing of a quantum mechanical degree of freedom, and suggests the construction of classical mechanical analogs of the doublesided Feynman diagrams of quantum mechanics, which are widely applied to nonlinear spectroscopy. Application of a rotating wave approximation permits the analytic extraction of signals obeying particular spatial phase matching conditions from a classicalmechanical response function. Calculations of the thirdorder response function for an anharmonic oscillator coupled to a harmonic solvent are compared to numerically correct classical mechanical results.

Excited electronic state calculations by the transcorrelated variational Monte Carlo method: Application to a helium atom
View Description Hide DescriptionWe have implemented the excited electronic state calculations for a helium atom by the transcorrelated variational Monte Carlo (TCVMC) method. In this method, JastrowSlatertype wave function is efficiently optimized not only for the Jastrow factor but also for the Slater determinant. Since the formalism for the TCVMC method is based on the variance minimization, excited states as well as the ground state calculations are feasible. It is found that both the first and the second excitation energies given by TCVMC are much closer to the experimental data than those given by the variational Monte Carlo method with using the Hartree–Fock orbitals. The successful results in the TCVMC method are considered to be due to the nodal optimization of the wave functions.

Statistical mechanical theory for the structure of steady state systems: Application to a LennardJones fluid with applied temperature gradient
View Description Hide DescriptionThe constrained entropy and probability distribution are given for the structure that develops in response to an applied thermodynamic gradient, as occurs in driven steady state systems. The theory is linear but is applicable to gradients with arbitrary spatial variation. The phase space probability distribution is also given, and it is surprisingly simple with a straightforward physical interpretation. With it, all of the known methods of equilibrium statistical mechanics for inhomogeneous systems may now be applied to determining the structure of nonequilibrium steady state systems. The theory is illustrated by performing Monte Carlo simulations on a LennardJones fluid with externally imposed temperature and chemical potential gradients. The induced energy and density moments are obtained, as well as the moment susceptibilities that give the rate of change of these with imposed gradient and which also give the fluctuations in the moments. It is shown that these moment susceptibilities can be written in terms of bulk susceptibilities and also that the Soret coefficient can be expressed in terms of them.

Accurate prediction of heat of formation by combining Hartree–Fock/density functional theory calculation with linear regression correction approach
View Description Hide DescriptionA linear regression correction approach has been developed successfully to account for the electron correlation energy missing in Hartree–Fock calculation and to reduce the calculation errors of density functional theory. The numbers of lonepair electrons, bonding electrons and inner layer electrons in molecules, and the number of unpaired electrons in the composing atoms in their ground states are chosen to be the most important physical descriptors to determine the correlation energy unaccounted by Hartree–Fock method or to improve the results calculated by B3LYP density functional theory method. As a demonstration, this proposed linear regression correction approach has been applied to evaluate the standard heats of formation of 180 smallsized to mediumsized organic molecules at 298.15 K. Upon correction, the mean absolute deviation for the 150 molecules in the training set decreases from 351.0 to 4.6 kcal/mol and 360.9 to 4.6 kcal/mol for and methods, respectively. For B3LYP method, the mean absolute deviations are reduced from 9.2 and 18.2 kcal/mol to 2.7 and 2.4 kcal/mol for and basis sets, respectively.

The role of the reaction force to characterize local specific interactions that activate the intramolecular proton transfers in DNA basis
View Description Hide DescriptionMP2/6311G^{*} ^{*} and B3LYP/6311G^{*} ^{*} studies of the intramolecular proton transfer in adenine, cytosine, guanine, and thymine has been performed, with the aim of evaluating the role of the reaction force as a global descriptor of the process. It turns out that the reaction force profile is quite an interesting tool to characterize reaction mechanisms. Indeed, in adenine and cytosine the proton transfer is assisted by an increasing electronic delocalization in the adjacent ring, whereas in guanine and thymine the attractive electrostatic interaction with the acceptor oxygen atom is strong enough to promote the transfer.

Full configuration interaction calculation of singlet excited states of
View Description Hide DescriptionThe full configuration interaction (FCI) study of the singlets vertical spectrum of the neutral beryllium trimer has been performed using atomic natural orbitals basis set. The FCI triangular equilibrium structure of the ground state has been used to calculate the FCI vertical excitation energies up to 4.8 eV. The FCI vertical ionization potential for the same geometry and basis set amounts to 7.6292 eV. The FCI dipole and quadrupoletransition moments from the ground state are reported as well. The FCI electric quadrupole moment of the ground state has been also calculated with the same basis set Twelve of the 19 calculated excited singlets are doubly excited states. Most of the states have large multiconfigurational character. These results provide benchmark values for electronic correlation multireference methods. values for the same energies and properties are also reported.

Correlated geminal wave function for molecules: An efficient resonating valence bond approach
View Description Hide DescriptionWe show that a simple correlated wave function, obtained by applying a Jastrow correlation term to an antisymmetrized geminal power, based upon singlet pairs between electrons, is particularly suited for describing the electronic structure of molecules, yielding a large amount of the correlationenergy. The remarkable feature of this approach is that, in principle, several resonating valence bonds can be dealt simultaneously with a single determinant, at a computational cost growing with the number of electrons similar to more conventional methods, such as HartreeFock or density functional theory. Moreover we describe an extension of the stochastic reconfiguration method, which was recently introduced for the energy minimization of simple atomic wave functions. Within this extension the atomic positions can be considered as further variational parameters, which can be optimized together with the remaining ones. The method is applied to several molecules from to benzene by obtaining total energies,bond lengths and binding energies comparable with much more demanding multiconfiguration schemes.

A refined model of the double exchange phenomenon: Test on the stretched molecule
View Description Hide DescriptionThe molecule is studied at different interatomic distances as a model molecule for the double exchange mechanism. The energy spectrum as well as the wave functions of the lowest states are analyzed and confronted both with the usual model of double exchange and with a recently proposed refined model. It is shown that the usual model fails to reproduce the energy spacings while the refined model is valid on a large domain of interatomic distances (in the magnetic regime). The study of a model molecule on a large domain of interatomic distances makes it possible to systematically investigate several regimes associated with different energetic state orderings. The perfect agreement between the refined model and the computed energies in the whole domain of stretched distances shows its applicability to a large number of real compounds. Finally, the respective contributions of dynamical and nondynamical correlations are analyzed.

Semiquantal timedependent Hartree approach to condensed phase chemical dynamics: Application to the systembath model
View Description Hide DescriptionA semiquantal analysis of condensed phase chemical dynamics, outlined recently for a doublewell linearly coupled to dissipative harmonic bath [K. Ando, Chem. Phys. Lett. 376, 532 (2003)], is formulated in detail to clarify its general features as well as the specifics of the linear and quadratic coupling cases. The theory may be called a “semiquantal timedependent Hartree (SQTDH)” approach, as it assumes a factorized product of the squeezed coherent state wave packets for the variational subspace of the manydimensional timedependent wave function. Due to this assumption, it straightforwardly satisfies the canonicity condition introduced by Marumori et al. [Prog. Theor. Phys. 64, 1294 (1980)] and is described by a set of Hamilton equations of motion in an extended phase space that includes auxiliary coordinates representing the wave packet widths. The potential in the extended phase space provides a pictorial understanding of the quantum effects affected due to the bath coupling, e.g., suppression of the wave packet spreading in terms of the potential wall developing along the auxiliary coordinates. The idea is illustrated by prototypical models of quartic doublewell and cubic metastable potentials linearly and quadratically coupled to the bath. Further applications and extensions, where the SQTDH method will offer a practical approach for introducing quantum effects into realistic molecular dynamics simulations, are also discussed.

Interpretation of Hund’s multiplicity rule for the carbon atom
View Description Hide DescriptionHund’s multiplicity rule is investigated for the carbon atom using quantum Monte Carlo methods. Our calculations give an accurate account of electronic correlation and obey the virial theorem to high accuracy. This allows us to obtain accurate values for each of the energy terms and therefore to give a convincing explanation of the mechanism by which Hund’s rule operates in carbon. We find that the energy gain in the triplet with respect to the singlet state is due to the greater electronnucleus attraction in the higher spin state, in accordance with HartreeFock calculations and studies including correlation. The method used here can easily be extended to heavier atoms.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

New RydbergRydberg transitions in Identification of the state
View Description Hide DescriptionUse of a special Penningionization source allowed Fouriertransform recording of two previously nonobserved IR emission bands spectra at 5480 and 7630 cm^{−1} arising from neutral The first of these bands is the transition, both states involved being previously known by direct vacuum (UV)absorption spectroscopy. The second band corresponds to a transition, in which the upper level belongs to an up to now unidentified Rydberg state. Both the upper and lower levels are perturbed by neighboring valence state levels.

Dissociation energies of six isotopologues by laser induced fluorescence spectroscopy and zero point energy of some triatomic molecules
View Description Hide DescriptionWe have measured the rotationless photodissociation threshold of six isotopologues of containing and isotopes using laser induced fluorescence detection and jet cooled (to avoid rotational congestion). For each isotopologue, the spectrum is very dense below the dissociation energy while fluorescence disappears abruptly above it. The six dissociation energies ranged from 25 128.56 cm^{−1} for to 25 171.80 cm^{−1} for The zero point energy for the isotopologues was determined from experimental vibrational energies, application of the Dunham expansion, and from canonical perturbation theory using several potential energy surfaces. Using the experimentally determined dissociation energies and the calculated zero point energies of the parent isotopologue and of the NO product(s) we determined that there is a common using the BornOppenheimer approximation. The canonical perturbation theory was then used to calculate the zero point energy of all stable isotopologues of and which are compared with previous determinations.

Dynamics of Rydberg electron transfer to Velocity dependent studies
View Description Hide DescriptionThe dynamics of freeion production through electron transfer in collisions are examined through measurements using velocityselected Rydberg atoms. The data show that Rydbergelectron transfer leads to the creation of two groups of dipolebound ions, one long lived (τ>85 μs), the other short lived (τ<1 μs). The velocity dependences associated with the production of both groups of ions are similar, the ion formation rate decreasing markedly with decreasing Rydberg atom velocity, principally as a consequence of postattachment electrostatic interactions between the product ions. The results are in reasonable accord with the predictions of a Monte Carlo collision model that considers the effect of crossings between the diabatic potential curves for the covalent system and the ion pair. This model also accounts for the relatively small reaction rate constants, associated with the formation of longlived ions. No velocity dependence in the lifetime of the ions is observed.

Fragmentation and conformation study of ephedrine by low and highresolution mass selective UV spectroscopy
View Description Hide DescriptionThe neurotransmitter molecule, ephedrine, has been studied by massselective low and highresolution UV resonance enhanced twophotonionization spectroscopy. Under all experimental conditions we observed an efficient fragmentation upon ionization. The detected vibronic peaks in the spectrum are classified according to the efficiency of the fragmentation, which leads to the conclusion that there exist three different species in the molecular beam: ephedrinewater cluster and two distinct conformers. The twocolor twophotonionization experiment with a decreased energy of the second photon leads to an upper limit of 8.3 eV for the ionization energy of ephedrine. The highresolution (70 MHz) spectrum of the strongest vibronic peak in the spectrum measured at the fragment mass channel displays a pronounced and rich rotational structure. Its analysis by the use of a specially designed computeraided rotational fit process yields accurate rotational constants for the and states and the transition moment ratio, providing information on the respective conformational structure.

The dynamics of the reaction at 0.32 eV
View Description Hide DescriptionRotational state resolved centerofmass angular scattering and kinetic energy release distributions have been determined for the HCl products of the reaction of chlorine with nbutane using the photoninitiated reaction technique, coupled with velocitymap ion imaging. The angular and kinetic energy release distributions derived from the ion images are very similar to those obtained previously for the Cl plus ethane reaction. The angular distributions are found to shift from forward scattering to more isotropic scattering with increasing HCl rotational excitation. The kinetic energy release distributions indicate that around 30% of the available energy is channeled into internal excitation of the butyl radical products. The data analysis also suggests that Hatom abstraction takes place from both primary and secondary carbon atom sites, with the primary site producing rotationally cold, forward scattered products, and the secondary site yielding more isotropically scattered possessing higher rotational excitation. The mechanisms leading to these two product channels are discussed in the light of the present findings, and in comparison with studies of other Cl plus alkane reactions.

A theoretical study of the staggered and eclipsed forms of the dinuclear complex
View Description Hide DescriptionTwo possible conformers of the dinuclear complex each of symmetry, with eclipsed and staggered conformations, have been analyzed theoretically. Using both the B3LYP and BP86 density functionals we find that the staggered form is lower in energy. A determination of the B3LYP potential energy surface as a function of the MnRe distance is presented for both conformers. The computed bond lengths, bond angles, and rotational constant for the staggered conformation compare favorably with the results from microwave experiments. The harmonic frequencies for the staggered structure have been determined using several basis sets, with both analytical and finite difference methods. These unscaled vibrational frequencies, together with their intensities for both infrared and Raman activity, are used to assign the three most intense experimental IR and Raman bands, and in particular, the region. The lowest vibration was calculated to occur at 41 cm^{−1} in the staggered conformer; this frequency becomes imaginary in the (saddle point) eclipsed form. Several fundamentals remain to be observed experimentally.

Lowlying electronic states of the dimer: Electronic absorption spectroscopy in rare gas matrices in concert with quantum chemical calculations
View Description Hide DescriptionAbsorption spectra were measured for in Ne and Ar matrices. The spectra give evidence for several electronic transitions in the region between 4000 and 10 000 cm^{−1} and provide important information about some excited electronic states of in proximity to the ground state. The vibrational fine structure measured for these transitions allowed to calculate the force constants and the anharmonicity of the potential energy curves of the excited states, and to estimate changes in the internuclear TiTi distances relative to the electronic ground state. The quantum chemical studies confirm the previously suggested state as the ground state of The equilibrium bond distance is calculated to be 195.4 pm. The calculated harmonic frequency of 432 cm^{−1} is in good agreement with the experimental value of 407.0 cm^{−1}. With the aid of the calculations it was possible to assign the experimentally observed transitions in the region between 4000 and 10 000 cm^{−1} to the and excitations (in the order of increasing energy). The calculated relative energies and harmonic frequencies are in pleasing agreement with the experimentally obtained values, with deviations of less than 5% and 2%, respectively. The bond distances estimated on the basis of the experimental spectra tally satisfactorily with the predictions of our calculations.