bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of the rotational distribution for the OD product from the reaction ND3++D2O→ND4++OD under translationally thermal conditions
Authors: Green Richard J. , Zare Richard N.
Published: July 2002

The state-to-state ion-molecule reaction ND3+(ν2=1)+D2O→ND4++OD(v=0,N) is investigated. A slowly flowing, 2:1 mixture of ND3 and D2O at a total pressure of 50 mTorr is irradiated with a two-color sequence of laser pulses that prepares ND3+ in either the ν2=1 umbrella bending mode or the ground vibrational state by 1+1+1 resonance-enhanced multiphoton ionization via the ND3 Ã and B̃ states. After a delay of 200 ns to allow product buildup, the rotational distribution of the OD(v=0) product is measured by recording the OD A2Σ+−X2Π laser-induced fluorescence spectrum on the (1,1) band following excitation of the (1,0) band. Rotational distributions are presented for the Π23/2 and Π21/2 fine-structure states of the OD product for the reaction of the vibrationally excited reactant ion; for the experimentally difficult case of the reactant ion in the ground state, a rotational distribution is presented for the Π23/2 fine-structure state of the OD product. For the case of the reaction with excited ND3+, the relative rotational populations are fit to a Boltzmann distribution to yield temperatures of 990±30 K and 890±70 K for the OD Π23/2 and Π21/2 fine-structure components, respectively. For the ground state ion, such a fit yields a temperature of 700±100 K for the OD Π23/2 fine-structure component. The results are compared to an RRKM-type model that predicts a rotational distribution of 800 K, and 940 K for the reaction of ion with ν2=0 and ν2=1, respectively. The excellent agreement is evidence for reaction through a long-lived complex.
CH-stretching overtone spectra and internal methyl rotation in 2,6-difluorotoluene
Authors: Zhu Chenxi , Kjaergaard Henrik G. , Henry Bryan R.
Published: July 2002

Vapor phase overtone spectra of 2,6-difluorotoluene are recorded in the ΔvCH=2 and 3 regions by conventional near-infrared spectroscopy and in the ΔvCH=4–6 regions by intracavity dye/titanium: sapphire laser photoacoustic spectroscopy. The spectra are interpreted on the basis of ab initio calculations at the HF/6-31G* level. The methyl regions of the spectra are complex due to coupling between the nearly freely rotating methyl rotor and CH stretching. A model has been developed to predict the methyl spectral profiles, which uses the harmonically coupled anharmonic oscillator local mode model and the rigid rotor model for stretching and torsion, respectively. A dipole moment function is formulated which combines a Taylor series for CH stretching and a Fourier series for torsion. The dipole moment function includes both angular dependence and higher order expansion terms in the CH-stretching coordinate. The model is successful in predicting the methyl overtone spectral profiles and attributes these profiles to a very large number of transitions that arise from terms involving torsion–stretching coupling, both in the Hamiltonian and in the dipole moment function.
Spin–lattice relaxation study of the photoexcited triplet states in solid C60
Authors: Prilutski Yu. I.
Published: July 2002

The spin–lattice relaxation (SLR) of the photoexcited triplet states in fullerene C60 crystal is investigated at low temperatures and high magnetic fields. The mixing of the translational and rotational motions of the molecules (TRM relaxation mechanism) is chosen as the major phenomenon, leading to the coupling of the spin system with acoustic vibrations. The general expressions of the SLR rates for the direct one-phonon processes are obtained. The results of calculation are in accordance with available experimental data.
Polarization of emission from asymmetric rotors. II. Vector reorientation through intramolecular coupling and inelastic collisions
Authors: Truhins Kaspars , McCaffery Anthony J. , Alwahabi Zeyad T. , Rawi Zaid
Published: July 2002

We report measurements of the linear and circular polarization ratios from fully resolved rotational levels of the asymmetric rotor NH2 populated by collisions with H atoms. The results compare well with a theoretical model that includes the depolarizing effects of intramolecular coupling of rotational angular momentum N to nuclear and to electron spin. These have a very significant influence on fluorescence polarization. The model also incorporates the tilting of the N vector in the molecule frame that occurs when inter-k stack transitions take place. Changes in N vector orientation are described with the aid of the angular momentum sphere, a classical representation of the motion of the N vector in a molecule fixed frame. The theoretical treatment assumes the classically impulsive limit for the collisional interaction with conservation of the m quantum number along the kinematic apse. This description of the fate of the N vector under the influence of intra- and intermolecular interactions allows stereodynamical conclusions to be drawn from experimental observations of fluorescence polarization.
Proton and hydrogen atom transfer in hydrogen bonded clusters: Ammonia as a paradigm
Authors: Snyder E. M. , Castleman A. W.
Published: July 2002

The competition between proton and hydrogen atom transfer in ammonia clusters is studied in the excited C̃′ state using femtosecond pump-probe spectroscopy. Little effect of solvation is seen for the case of proton transfer, while the hydrogen transfer processes display significant dependence on the degree of clustering. The former have lifetimes from 85 to 135 fs for (NH3)3–40 while the latter display values ranging from 300 to 1500 fs. Similar effects are found for deuterated systems, with a relatively large isotope effect for the atom transfer process. The results are consistent with the formation of ion pairs in the excited state and published findings for related phenomena in isolated ion-molecule reactions [Conaway et al., J. Chem. Phys. 87, 3453 (1987)].
Fluorescence excitation spectroscopy of the Ar–HCO(X̃ 2A′,B̃ 2A′) van der Waals complex
Authors: Wright Scott A. , Dagdigian Paul J.
Published: July 2002

The fluorescence excitation spectrum of the Ar–HCO van der Waals complex in the vicinity of the B̃ 2A′–X̃ 2A′ 000 band of free HCO is reported. At least eight bands associated with the complex have been detected. From the spectral shift of the lowest energy Ar–HCO band from the origin of the free HCO transition, we estimate the Ar–HCO binding energy in the excited electronic state to be at least 13 cm−1 greater than that in the ground state. Rotational analysis of some of the bands has been carried out, and average Ar–HCO separations (∼3.7 Å) in both electronic states determined. Several of the bands were assigned as hot bands from the first excited bend–stretch level (K″=1) in the ground electronic state. From the derived A rotational constants, we conclude that the ArCO framework has an approximately T-shaped geometry in both electronic states. The decay lifetime of the upper state of the strongest Ar–HCO band was measured and was found to be somewhat smaller than those previously measured for low rotational levels of free HCO. With the high signal-to-noise ratio in this study, it was also possible to observe transitions in the free H13CO isotopomer. A rotational analysis of the B̃ 2A′–X̃ 2A′ 000 band of the H13CO isotopomer was carried out. The isotopic shifts of the origins of the 301 and 201 bands were also measured, and a normal mode analysis of HCO(B̃) was carried out.
A nontypical atom-diatom van der Waals interaction: Ar–C2
Authors: Naumkin F. Y. , McCourt F. R. W.
Published: July 2002

The ground state Ar–C2 potential energy surface has been calculated ab initio and fitted by means of an effective ArC pair interaction model. The global minimum of energy is predicted to occur for the linear Ar–C2 geometry, while a saddle point is predicted for the T-shaped geometry. This behaviour is distinct from that found for rare gas complexes with heavier symmetric diatomic molecules of the second row atoms. The results are analysed in terms of effective ArC potentials, which are perturbed by intramolecular forces in C2, and differ considerably from the Lennard-Jones potentials normally used to model these interactions. The present model is applied to small ArnC2 (n=2–4) clusters and used to predict the microwave spectrum of ArC2.
The observation of strong pseudo-Jahn–Teller activity in the benzene cation B̃ 2E2g state
Authors: Goode Jon G. , Hofstein Jason D. , Johnson Philip M.
Published: July 2002

Photoinduced Rydberg ionization (PIRI) spectroscopy has been applied to the problem of recording vibrationally resolved spectra of the dipole-forbidden B̃ 2E2g←X̃ 2E1g transition of C6H6+ and C6D6+. PIRI spectra of the B̃ state have been recorded via a number of vibrational states of the ionic ground state. A combination of Herzberg–Teller and pseudo-Jahn–Teller vibronic coupling between the B̃ state and a close lying C̃ state were used to explain the complex B̃ state vibrational structure. The two pseudo-Jahn–Teller vibrational modes, ν16 and ν17, were both found to be active in the PIRI spectra. In addition, evidence for a strong pseudo-Jahn–Teller interaction within each of these modes was found. The perturbed vibronic bands resulting from this pseudo-Jahn–Teller activity were successfully modeled using a two-mode pseudo-Jahn–Teller vibronic coupling model. Using a combination of the spectral results and the pseudo-Jahn–Teller calculations, we were able to generate absolute vibronic symmetry assignments for many of the lower B̃ state vibrational levels, from which vibrational assignments were made. The calculations also produced unperturbed vibrational frequencies for both pseudo-Jahn–Teller active modes along with values for their respective linear coupling constants.
Coupled cluster anharmonic force fields, spectroscopic constants, and vibrational energies of AIF3 and SiF3+
Authors: Pak Youngshang , Sibert Edwin L. , Woods R. Claude
Published: July 2002

Anharmonic force fields and ro-vibrational spectroscopic properties of AlF3 and SiF3+ have been investigated in detail, using the coupled cluster method with single and double substitutions augmented by a perturbative treatment of triple excitations [CCSD(T)] with a triple zeta basis set of 124 contracted Gaussian-type orbitals (cGTOs). A complete set of ro-vibrational spectroscopic constants for each species has been calculated using second order perturbation theory. The geometry only was calculated with a quadruple zeta basis set (224 cGTOs). Our best estimates of the equilibrium bond distances re (Al–F) and re (Si–F) are 1.624 Å and 1.508 Å, respectively, based on the quadruple zeta CCSD(T) bond distances and corrections derived from spectroscopically related known molecules. The CCSD(T) fundamental frequencies are 689 cm−1(ν1),301 cm−1(ν2),951 cm−1(ν3), and 241 cm−1(ν4) for AlF3 and 853 cm−1(ν1),357 cm−1(ν2),1187 cm−1(ν3), and 307 cm−1(ν4) for SiF3+. Energies of a number of low lying vibrational states (⩽2400 cm−1) have been calculated by three methods: (1) standard second order perturbation theory formulas; (2) Van Vleck perturbation theory based on raising and lowering operators to second order and to fourth order; and (3) a variational method.
Analytic energy gradients with frozen molecular orbitals in coupled-cluster and many-body perturbation theory methods: Systematic study of the magnitude and trends of the effects of frozen molecular orbitals
Authors: Baeck Kyoung K. , Watts John D. , Bartlett Rodney J.
Published: July 2002

Analytic coupled-cluster (CC) and many-body perturbation theory (MBPT) energy gradient methods with restricted Hartree–Fock (RHF), unrestricted Hartree–Fock (UHF), restricted open-shell Hartree–Fock (ROHF), and quasi-RHF(QRHF) reference functions are extended to permit dopping core and excited orbitals. By using the canonical property of the semicanonical ROHF orbitals and the RHF orbitals from which the QRHF reference function is constructed, it is shown that a general procedure can be established not only for RHF and UHF, but also for ROHF and QRHF reference functions. The basic theory and implementation are reported. To provide a systematic study of the trends and magnitudes of the effects of dropped molecular orbitals (MOs) on the structures, harmonic frequencies, and ir intensities, we study HCN, C2H2, CO2, HO2, and C2H4 at increasing levels of correlation and basis sets. The effects of the dropped MOs with the largest basis sets are about 0.003 Å and 0.1° in structures and about 1% on harmonic frequencies and ir intensities. The magnitude and the direction of the drop-MO effect tend to be almost constant from MBPT(2) to CCSD(T) methods. The two isomers of S3 are studied by the drop-MO-method, yielding very accurate results.
Quantum and semiclassical calculations of H2-He vibrational line-shape parameters at high temperature
Authors: Joubert P. , Dubernet M.-L. , Bonamy J. , Robert D.
Published: July 2002

Quantum close-coupling (CC) and semiclassical (SC) calculations of broadening and shifting coefficients of Q(j) rovibrational lines have been performed using an ab initio potential energy surface. The agreement between the theoretical results and experimental data available up to 1000 K is very good for the broadening coefficient γ and reasonable for the shift δ. The main interest is to test the validity of the semiclassical method versus CC calculation on a wide range of temperature to allow confident application of the SC method to more complex systems. The agreement is very satisfactory. Further the SC model permits a detailed analysis of the mechanisms involved in the temperature dependence of the molecular parameters γ and δ.
Classical trajectory study of product state vector correlations: A model for the photodissociation of CF3NO
Authors: Fukui Kazuhiko , Frederick John H. , Cline Joseph I.
Published: July 2002

The photodissociation dynamics of CF3NO are modeled by three-dimensional classical trajectory calculations on the S0 and T1 electronic potential surfaces that include all vibrational and rotational degrees of freedom. The intramolecular potential energy surface for CF3NO was obtained by optimizing parameters in an analytical potential function to best fit spectroscopic data and the results of UMP2/6-31+G* level ab initio calculations. Initial parent molecule energy and angular momentum distributions are computed using a new method that simulates the conditions of molecular beam experiments. The calculations are directed at interpreting experimental measurements of correlated NO product state distributions in the photodissociation of the nitrosoalkanes. The results show that the v–j correlation of NO photofragment is primarily determined by the C–N–O bending force constant and exit channel barrier on the T1 surface.
Structure of the metal-aqueous electrolyte solution interface
Authors: Bérard D. R. , Kinoshita M. , Cann N. M. , Patey G. N.
Published: July 2002

Theoretical results are given for aqueous electrolyte solutions in contact with uncharged metallic surfaces. The metal is modeled as a jellium slab and is treated using local density functional theory. The solution structure is obtained using the reference hypernetted-chain theory. The two phases interact electrostatically and the coupled theories are iterated to obtain fully self-consistent results for the electron density of the metal and surface-particle correlation functions. The metal-induced structure of pure water and aqueous electrolyte solutions as well as the electrostatic potential drop across the interface are discussed in detail. The results are compared with those for ions in simple dipolar solvents. It is found that the water molecules are ordered by the metal field and that the surface-induced solvent structure strongly influences the ion distributions.
Mean field calculation of polymer segment depletion and depletion induced demixing in ternary systems of globular proteins and flexible polymers in a common solvent
Authors: Schaink H. M. , Smit J. A. M.
Published: July 2002

We present a new method for calculating the depletion of polymer segments and the depletion induced demixing in solutions of flexible polymers and small globular proteins. The proteins are modelled as hard spheres. The method consists of a mean field calculation of the local polymer volume fraction with the use of a spherical cell-model. Two different types of polymer contribution to the free energy are considered in this calculation. Most of the results presented here are obtained for a low volume fraction expansion of the polymer free energy and an expression similar to the well-known Flory-Huggins free energy. The free energy also contains a squared expression of the gradient in the polymer volume fraction accounting for the steric hindrance suffered by the polymer segments from the spheres and reflecting the entropy loss of the segments in the neighborhood of the surfaces of the spheres. For dilute solutions containing long polymers and either very small or very large spheres, the local polymer volume fractions φ2(r) are obtained analytically. Intermediate cases are studied numerically. It is found that φ2(r) in the small sphere limit depends on the sphere volume fraction η, whereas in the large sphere limit φ2(r) is η-independent and tends to the φ2(r) near a flat plate. Using the local polymer volume fraction we calculate the macroscopic polymer volume fraction and the free energy. In this calculation we consider besides the cell model a more refined version which also accounts for sphere-sphere correlations. Both cell models yield qualitatively the same results for the spinodals, the quantitative differences between these models being of the order of 5–25%. The influence of parameters as the polymer chain length, the polymer segment excluded volume, the polymer concentration, the sphere radius on both the depletion profile and the spinodal is discussed. The spinodal, calculated without adaptable parameters, is shown to lie in the neighborhood of the experimental binodal.
On the origin of the heavy atom effect in the fine-structure splitting of the 1 2Π state of alkali metal P2-rare gas van der Waals molecules
Authors: Sohlberg Karl , Yarkony David R.
Published: July 2002

The fine-structure splitting of the (1 2ΠΩ,ν) levels in Li(1s 22p,2P)Ar and Li(1s 22p,2P)Ne is determined. Analysis of the electronic wave functions demonstrates that the heavy atom contribution to the fine-structure splitting results from the antibonding mixture of valence pπ orbitals on the rare gas and metal atoms. Rydberg orbitals do not contribute significantly to the heavy atom effect.
Infrared induced isomerization efficiency for CH2D–CH2D isolated in rare gas matrices: Influence of the vibrational excitation and of the medium
Authors: Roubin P. , Varin S. , Verlaque P. , Coussan S. , Berset J.-M. , Ortéga J.-M. , Peremans A. , Zheng W.-Q.
Published: July 2002

The resonant excitation of 13 vibrational states of the gauche and the trans forms of CH2D–CH2D isolated in a xenon matrix has been performed with the laser radiation provided by the free electron laser CLIO and by an optical parametric oscillator configuration. The determination of the sample composition by infrared spectroscopy gives the energy dependence of the conformational inversion quantum yield in the 1000–3000 cm−1 domain. This reaction proves the coupling of the molecular internal rotation with the laser excited vibrational mode. We observe a global increase of the isomerization efficiency with the excitation energy, but with large fluctuations in the energy dependence. The increase is interpreted as due to the addition of new coupling channels when higher states of the vibrational ladder are excited, whereas the fluctuations are a signature of specific molecular couplings. The influence of the temperature and of the nature of the rare gas matrix on the isomerization rate emphasizes the key role of the intermolecular interactions along the reaction path.
High resolution vibrational overtone studies of HOD and H2O with single mode, injection seeded ring optical parametric oscillators
Authors: Votava Ondrej , Fair Joanna R. , Plusquellic David F. , Riedle Eberhard , Nesbitt David J.
Published: July 2002

Design, performance, and applications of a pulsed, single mode optical parametric oscillator (OPO) for studies of high resolution spectroscopy and photodissociation dynamics are presented. Single mode operation is achieved by resonantly seeding a four-mirror OPO ring cavity with a tunable, continuous wave (cw) ring dye laser, providing continuous scanning capability at near Fourier transform limited resolution [Δν=160(20) MHz] with peak output energies ⩾10 mJ. The high spectral brightness of this OPO light source is sufficient to saturate Δv=3 stretching transitions in OH, NH, and CH vibrational manifolds, which makes feasible quantum state-selected multiple resonance spectroscopies at 0.005 cm−1 resolution. The capability of this single mode OPO is explicitly demonstrated via (i) vOH=3←0 overtone spectroscopy of HOD, (ii) near-IR optical saturation studies of H2O in the |03−〉 overtone vibrational manifold, (iii) high resolution LIF Dopplerimetry of OH radicals, and (iv) IR/UV multiple resonance spectroscopy of Ar–H2O molecular clusters.
Time- and frequency-domain properties of light emitted in slow ion–atom collisions
Authors: DaCosta Herbert F. M. , Micha David A. , Runge Keith
Published: July 2002

We describe light emission from the complex formed during ion–atom collisions, that arises from electronic charge transfer and the related oscillatory dipole of the complex. This is treated in an eikonal/time-dependent Hartree–Fock approach which accounts for coupling of electronic and nuclear motions and generates the time-dependent dipole of the complex. Calculations were done for H++H with a basis set of travelling atomic functions, for collision energies of 100, 250, and 1000 eV, to obtain the energy emitted per solid angle versus both time and the light frequency. Results are presented for the intensity components of the light emitted parallel and perpendicular to the incoming projectile H+ velocity. Light emission is found to last several femtoseconds, and to be distributed over ultraviolet frequencies. The intensity of light emitted by the complex H2+ increases as collision energies are lowered.
Molecular geometry and vibrational frequencies of ozone from compact variational wave functions explicitly including triple and quadruple substitutions
Authors: Leininger Matthew L. , Schaefer Henry F.
Published: July 2002

The performance of a multireference CISD method, CISD[TQ], is compared to that of other approaches which include a large degree of electron correlation, including Brueckner methods. The CISD[TQ] method selects as references all single and double substitutions within an active orbital space. Certain triple and quadruple substitutions from the Hartree–Fock reference are included in the CISD[TQ] wave function as singles and doubles from the selected reference set. This wave function has previously been shown in simpler cases to provide results near to those predicted by the configuration interaction wave function, including all single, double, triple, and quadruple substitutions (CISDTQ). For the challenging multireference case of ozone, the CISD[TQ] wave function yields geometries and harmonic vibrational frequencies with an accuracy similar to the full CCSDT method. These promising results suggest that for difficult multireference problems the CISD[TQ] wave function provides an efficient and accurate approach for approximating the complete CISDTQ.
Restricted molecular rotation in hexatic B and crystalline B mesophases as studied by the electron paramagnetic resonance method
Authors: Bikchantaev I. , Szdłowska J. , Pociecha D. , Krówcyński A. , Górecka E.
Published: July 2002

Molecular motions have been examined by the electron paramagnetic resonance method in orthogonal smectic phases for materials with SmA–HexB–CryB phase sequence, doped by a paramagnetic copper (II) complex. For the hexatic as well as crystalline B phases, two types of subphases were found: with free or constrained rotational molecular motion. The rotation becomes hindered either at the SmA–HexB phase transition or a few degrees below it. In both cases the order parameter describing the restricted rotation increases continuously upon decreasing temperature and does not change appreciably at the HexB–CryB phase transition point. The restricted rotation is attributed to a local herringbone order of molecules.
Protonated hydrogen clusters
Authors: Štich I. , Marx D. , Parrinello M. , Terakura K.
Published: July 2002

The effect of protonation of pure hydrogen clusters is investigated at low temperature using a combination of path integral simulations and first-principles density functional electronic structure calculations. These odd n Hn+ clusters are shown to lose the quantum-liquid properties of their unprotonated counterparts. The added proton gets trapped as a very localized and strongly bound H3+ impurity in the cluster core, surrounded by stable shells of more spatially delocalized solvating H2 molecules. The clusters are frozen with respect to the translational degrees of freedom, while the H2 ligands undergo large-amplitude rotations. The rotational delocalization is found to increase in successive solvation shells. The combination of translational rigidity and rotational floppiness, which is akin to plastic behavior in crystals, is a quantum induced phenomenon absent in the classical approximation for the nuclei.
Raman spectroscopy of the N–C–O symmetric (ν3) and antisymmetric (ν2) stretch fundamentals in HNCO
Authors: Brown Steven S. , Berghout H. Laine , Crim F. Fleming
Published: July 2002

We report the first gas-phase Raman spectra of the N–C–O stretching fundamentals in isocyanic acid. Using stimulated Raman excitation to prepare vibrationally excited molecules, we record spectra via two different techniques, photoacoustic Raman spectroscopy and action spectroscopy. The former detects the sound wave generated as the Stokes laser tunes through resonances and deposits heat in the gas sample. The latter detects transitions by photodissociating the vibrationally excited states prepared in the vibrational excitation step and detecting the photofragments by laser induced fluorescence. In analogy with the stretching modes in CO2, the N–C–O symmetric stretch (ν3) Raman fundamental in HNCO is strong while the antisymmetric stretch (ν2) is weak, although neither is symmetry forbidden. Both vibrational states are strongly perturbed. The symmetric stretch interacts with combination states that contain two quanta of bending excitation, and the antisymmetric stretch interacts with several different combination states. Both Raman spectra have strong QQ branch rotational structure in which the band origins for different K sublevels in this near-prolate symmetric top follow no simple pattern. Photodissociation of the vibrationally excited states demonstrates the influence of the initial state preparation on the rotational resonances, photofragment appearance thresholds, and Franck–Condon factors in the transition to a dissociative excited electronic state.
Photodissociation of C2H at 193 nm: Branching ratios for the formation of C2 in the X 1Σg+, A 1Πu, and B′ 1Σg+ states
Authors: Sorkhabi Osman , Blunt Victor M. , Lin Hua , Xu Dadong , Wrobel Jacek , Price Roosevelt , Jackson William M.
Published: July 2002

The ratio of the nascent population of C2 (X 1Σg+) to C2 (A 1Πu) to C2 (B′ 1Σg+) produced from the photodissociation of C2H at 193 nm has been measured using laser induced fluorescence. This technique is typically used to measure rotational and vibrational distributions in a given electronic state. Here, we have extended the technique to measure the relative electronic distributions in the C2 photofragment. From the simultaneous measurement of the Mulliken (X 1Σg+–D 1Σu+) and Freymark (A 1Πu–E 1Σg+) systems, the nascent population ratio of C2 (A 1Πu) to C2 (X 1Σg+) molecules was determined. Similarly, from the measurement of the Deslandres–D’Azumbuja (A 1Πu–C 1Πg) and the LeBlanc (B′ 1Σg+–D 1Σu+) systems, the nascent population ratio of C2 (A 1Πu) to C2 (B′ 1Σg+) was determined. The overall ratio for the production of C2 in the X:A:B′ electronic states was found to be 1:19:1.4. These results along with the results of high quality ab initio calculations of Cui and Morokuma (unpublished) are used to discuss the photodissociation dynamics of C2H at 193 nm. Furthermore, these results should aid in the analysis and modeling of cometary spectra of C2 .
Correlated vibrational frequencies of polymers: MBPT(2) for all-trans polymethineimine
Authors: Sun Jun-Qiang , Bartlett Rodney J.
Published: July 2002

Polymethineimine is a conjugated polymer, isovalent with polyacetylene, which was first synthesized in 1971. It has been found to be a semiconductor, though prior theory did not support that. Except for its vibrational frequencies which have been measured, little about the polymer is known. MBPT(2) is applied to calculate its equilibrium structure, band gap, and vibrational frequencies. The latter represents the first time an ab initio correlated method has been applied to this problem for an infinite system. The calculations for three basis sets, STO-3G, 6-31G, and 6-31G**   demonstrate that both basis and electron correlation have a strong influence on its optimized geometry. The MBPT(2) band gap with a polarized basis set (6-31G** ) is 4.7826 eV, compared to 8.54 for Hartree-Fock. According to our MBPT(2) calculations, the band gap of all-trans polymethineimine would be estimated to be about 1 eV larger than that of polyacetylene, i.e. ∼2.75±0.5 eV. Electron correlation is demonstrated to have an important effect on the computed vibrational frequencies, which we find to be in good agreement with experiment.
Vacuum-UV fluorescence spectroscopy of PF3 in the range 9–20 eV
Authors: Biehl H. , Boyle K. J. , Seccombe D. P. , Tuckett R. P. , Baumgärtel H. , Jochims H. W.
Published: July 2002

The vacuum-UV and visible spectroscopy of PF3 using fluorescence excitation and dispersed emission techniques is reported. The fluorescence excitation spectrum has been recorded following photoexcitation with monochromatized synchrotron radiation from the Daresbury, UK source in the energy range 9–20 eV with an average resolution of ∼0.015 eV. Transitions to the three lowest-energy bands in the Rydberg spectra show resolved vibrational structure, they are assigned to transitions to the (8a1)−1 4p, 5p, and 6p Rydberg states of PF3, and fluorescence is due to valence transitions in the PF2 radical. From a Franck–Condon analysis of the vibrational structure, it is shown that the FPF bond angle in PF3 increases by ∼14±1° upon photoexcitation. The use of optical filters shows that at least three excited electronic states of PF2 are responsible for the induced emission. Dispersed emission spectra in the UV/visible region have been recorded with an optical resolution of 8 nm at the BESSY 1, Germany synchrotron source at the energies of all the peaks in the excitation spectrum. Four different decay channels are observed: (a) PF2 Ã 2A1–X̃ 2B1 fluorescence in the wide range 320–550 nm for photon energies around 9.8 eV, (b) PF2 Ã–X̃, and B̃ 2B2–X̃ 2B1 fluorescence at ∼300 nm for photon energies around 11.0 eV, (c) PF2 C̃ 2A1–X̃ 2B1 and Ẽ 2B1 (2Π)–Ã 2A1 fluorescence at ∼222 and 325 nm, respectively, for photon energies around 14.4 eV, and (d) PF A 3Π–X 3Σ− fluorescence between 300–380 nm for photon energies around 16.1 eV. These assignments are confirmed by action spectra in which the excitation energy of the vacuum-UV radiation is scanned with detection of the fluorescence at a fixed, dispersive wavelength. Using the single-bunch mode of the BESSY 1 source, we have attempted to measure the lifetimes of the emitting states, but the timing profile of the source imposes an upper limit on lifetimes that can be measured of ∼500 ns. We have therefore only been able to measure values for the bent C̃ 2A1 and linear Ẽ 2B1 (2Π) states of PF2 of 14.7 and 7.9 ns, respectively; the lifetimes of the other emitters are too long to measure by this method. Our assignments in PF2 are heavily dependent on recent ab initio calculations on the geometries and energies of the valence electronic states of this species. Our knowledge on the electronic spectroscopy of this free radical is reviewed.
Thermodynamic scaling Monte Carlo study of the liquid–gas transition in the square–well fluid
Authors: Brilliantov Nikolai V. , Valleau John P.
Published: July 2002

A Temperature–and–Density–Scaling Monte Carlo (TDSMC) study of the square-well fluid was carried out. The relative excess free energy was obtained directly from the simulations with a standard deviation less then 0.5%. Energy, pressure, compressibility and heat capacity were calculated from the excess free energy data. For the system of 256 particles a reduced density interval of 0.1-0.68 and a reduced temperature interval of 1.053-1.29 was covered. In the simulations a “zoom focus” on the temperature–density grid was used: a “panoramic view” of the whole temperature–density region, on a relatively coarse grid, was combined with a “close-up” of the critical–point region, using a finer (T,ρ) grid. The coexistence curve was obtained with use of the double–tangent construction on the free energy, and the critical temperature and critical density were estimated. The critical parameters found by the TDSMC method are compared with those of other Monte-Carlo techniques.
Modeling the concentration dependence of diffusion in zeolites. III. Testing mean field theory for benzene in Na–Y with simulation
Authors: Saravanan Chandra , Jousse Fabien , Auerbach Scott M.
Published: July 2002

We have performed kinetic Monte Carlo (KMC) simulations of benzene tracer diffusion in Na–Y for various loadings and temperatures to test the analytical diffusion theory presented in Paper I of this series. Our theory and simulations assume that benzene molecules jump among SII and W sites, located near Na+ ions in 6-rings and in 12-ring windows, respectively. Our diffusion theory is based on a mean field approximation (MFA) which yields Dθ=16kθaθ2, where aθ≅11 Å is the mean intercage jump length and 1/kθ is the mean supercage residence time. KMC simulations of D(θ), kθ, and aθ at 300 and 400 K show that our MFA is essentially exact for loadings that allow SII site vacancies, and that the concentration dependence is controlled by kθ. For higher loadings, the MFA error is independent of temperature, and increases roughly linearly with loading to a maximum value of ca. 25%, resulting from correlated motion. We present an analytical theory for such correlated motion at infinite vacancy dilution, which predicts the corresponding KMC simulated diffusivities to within statistical Monte Carlo error.
A study of genetic algorithm approaches to global geometry optimization of aromatic hydrocarbon microclusters
Authors: White Ronald P. , Niesse J. A. , Mayne Howard R.
Published: July 2002

We have carried out potential energy minimization calculations on benzene, naphthalene, and anthracene clusters using model potential energy functions. The primary purpose was to examine several techniques which use concepts from the field of genetic algorithms (GA). In particular, we compared the “traditional GA” in which the variables of the problem are coded into binary and genetic operations performed on these, and recent methods which use real-valued variables. Our primary technique, the “space-fixed modified GA” (SFMGA), also uses a conjugate gradient descent on the geometries generated by the GA. Our results show the convergence to the global minimum is greatly improved by the use of the descent minimization. In fact, it appears unlikely that the traditional GA’s are useful for any but the very simplest clusters. We have also compared the SFMGA with simulated annealing (SA) and Wales and Doye’s recent basin-hopping (BH) technique. We find our method to be superior to SA, and comparable to BH.
Photodissociation spectroscopy and dynamics of MgC2H4+
Authors: Chen J. , Wong T. H. , Cheng Y. C. , Montgomery K. , Kleiber P. D.
Published: July 2002

The weakly bound ion–molecule complex MgC2H4+ has been studied by photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer over the spectral range 218–510 nm. Mg+ is the major photofragment throughout this range, although for λ<270 nm, charge-transfer dissociation to C2H4+ is observed as a minor channel. We have identified five absorption bands of MgC2H4+. The spectral assignment is facilitated by results from ab initio calculations for the ground and low-lying excited states of MgC2H4+. Three of the bands, 1 2B2←1 2A1, 1 2B1←1 2A1, and 2 2A1←1 2A1, are based primarily in the metal-centered Mg+(3p 2P←3s 2S) atomic transition. One of the remaining bands is assigned as 2 2B2←1 2A1, a transition correlating with the a 3B1u←X 1Ag forbidden band of C2H4, with mixed charge-transfer character. The final band, 3 2A1←1 2A1, is assigned to a metal-to-ligand charge-transfer transition, enhanced by coupling with the nearby 2 2A1 state. The 1 2B2←1 2A1 band is a broad continuum, indicative of fast predissociation in the upper state. A nonadiabatic dissociation mechanism involving C=C π-bond activation by Mg+(3p) is suggested by ab initio calculations. The 1 2B1←1 2A1 band shows pronounced vibrational structure with a strong progression in the Mg+–CH4 intermolecular stretch (ν2), and weaker progressions assigned to combination bands built on the intermolecular out-of-plane wag (ν3), and a CH2–CH2 wag (ν7). The observed vibrational constants are ω2=329, x22=−2.3, ω3=439, and ω7=1024 cm−1. Measurement of the photofragment kinetic energy release determines the bond dissociation energies for the ground state [D0″(Mg+–C2H4)=0.7±0.2 eV], and for the 1 2B1 excited state, [D0′(Mg+–C2H4)=1.8±0.2 eV]. Spectroscopic constants are in good agreement with ab initio predictions.
Adsorption of potassium and oxygen on graphite: A theoretical study
Authors: Lamoen D. , Persson B. N. J.
Published: July 2002

We have performed electronic structure calculations of the interaction of potassium and oxygen with graphite (GR), individually and as coadsorbates. We use up to three graphite planes to represent the graphite surface, but we show that the main physics is correctly described by a single graphite layer. At low coverage the potassium–graphite bond is largely ionic, and the variation of the K–GR bond energy with the lateral position of the K atom in the graphite unit cell is very small. We study the interaction between atomic oxygen and graphite. We find that O binds strongest at the bridge site, but the barrier for diffusion is rather small. The frequency for the perpendicular O–graphite vibrational mode is remarkably low (53 meV), reflecting the relative slow variation of the O–graphite interaction energy with the separation z between the O atom and the graphite surface. We consider the adsorption of O2 on a clean graphite surface and on a graphite surface with a low concentration of potassium. On the clean surface the O2–graphite interaction is found to be repulsive (the weak attractive van der Waals interaction is not included in our theoretical method), in accordance with the extremely low sticking coefficient observed for O2 on clean graphite. When potassium is adsorbed on the graphite surface, O2 chemisorbs at the potassium sites which is consistent with the large sticking coefficient observed for O2 on a potassium covered surface. The energy barrier towards dissociation of O2 on the clean graphite surface is estimated to be similar to that of gas phase O2. For O2 on K/graphite we find that O2 chemisorbs “side-on” K, and that the barrier for dissociation is much smaller than in the gas phase or on the clean graphite surface.
Comparison of spin-orbit configuration interaction methods employing relativistic effective core potentials for the calculation of zero-field splittings of heavy atoms with a 2P
o ground state
Authors: Buenker Robert J. , Alekseyev Aleksey B. , Liebermann Heinz-Peter , Lingott Rainer , Hirsch Gerhard
Published: July 2002

Computational strategies for the treatment of relativistic effects including spin-orbit coupling at a highly correlated level are compared for a number of heavy atoms: indium, iodine, thallium, and astatine. Initial tests with perturbation theory emphasize the importance of high-energy singly excited configurations which possess large spin-orbit matrix elements with the ground state. A contracted basis consisting of L–S CI eigenfunctions (LSC–SO–CI) is found to give an accurate representation of both spin-perturbed 2Po components as long as key np→pi* singly excited configurations are included. Comparison is made with a more extensive treatment in which all selected configurations of various L–S symmetries form the basis for the multireference–spin-orbit–configuration interaction (MR–SO–CI). Good agreement is obtained with experimental SO splittings for the In, I, and At atoms at a variety of levels of treatment, indicating that the L–S contracted SO–CI approach can be implemented quite effectively with relativistic effective core potentials (RECPs) for both very electronegative atoms and also for lighter electropositive elements up through the fifth row of the periodic table. The thallium atom SO splitting is more difficult to obtain accurately because of greater differences between its valence p1/2 and p3/2 spinors than in the other cases studied, but good results are also possible with the contracted SO–CI approach in this instance, provided proper care is given to the inclusion of key singly excited L–S states. The relationship between all-electron two-component SO–CI treatments and those employing RECPs is also analyzed, and it is concluded that triply excited configurations relative to the 2Po ground state are far less important than previously reported.
Quantum dynamics study of H2+CN → HCN+H reaction in full dimensions
Authors: Zhu Wei , Zhang John Z. H. , Zhang Y. C. , Zhang Y. B. , Zhan L. X. , Zhang S. L. , Zhang D. H.
Published: July 2002

Time-dependent (TD) quantum dynamics calculation for the title reaction has been carried out in full mathematical (six) dimensions on a new potential energy surface (denoted TSH3). Our numerical calculation shows that as far as total reaction probabilities and cross sections are concerned, the CN vibration behaves like a spectator bond when both reagents are at ground vibrational state. The vibrational excitation of CN slightly decreases the reaction probability and cross section while vibrational excitation of H2 considerably enhances the reaction probability and cross section. The reaction probability is enhanced by excitations of H2 rotation and more so of CN rotation. Overall, the reaction proceeds by a direct abstraction path without contribution from the insertion process. Comparison of our calculated rate constant with experimental measurements indicates that the effective barrier of the TSH3 PES for the title reaction is perhaps too high by about 0.3 kcal/mol.
Singly vibrationally enhanced infrared four wave mixing spectroscopy
Authors: LaBuda Mitchell J. , Wright John C.
Published: July 2002

We describe a new nonlinear, nonparametric process that provides a direct measure of vibrational enhancement in four wave mixing. This process has 14 coherent pathways that create the final coherence. In the limit of no pure dephasing and a single vibrational resonance, the 14 pathways combine into a single process that is vibrationally enhanced. The frequency and concentration dependence expected for this process matches that observed in experiments with the methyl- and methylene C–H stretch vibrations in hexane samples. The third-order nonlinear susceptibility for the C–H vibrations was measured by the interferometric method of Levenson and Bloembergen. The vibrational nonlinearity was observed as a difference between the nonresonant electronic contributions and the contribution that depended on resonance with the vibrational absorption transition. The measurements show that the vibrational enhancement is 50 times larger than the nonresonant electronic contribution and it shows that spectroscopic methods based on resonant nuclear nonlinear polarizations are observable above the electronic nonlinear polarization background.
Orthogonal polynomial approach to fluids with internal degrees of freedom: The case of polar, polarizable molecules
Authors: Lado F. , Lomba E. , Lombardero M.
Published: July 2002

The molecules of real liquids have internal degrees of freedom that may couple with the external coordinates of position and orientation so that they affect and are affected by the microscopic liquid structure. For cases where the internal coordinates possess a Boltzmann-like distribution, a procedure was recently proposed [Phys. Rev. E 55, 426 (1997)] whereby the internal coordinates are incorporated into the conventional integral equation formulation of classical liquid state theory with no approximations beyond some reliable closure relation familiar from simple liquids. The basis of the procedure is expansions in special orthogonal polynomials of the internal coordinates. Here we use this technique to obtain the structural, thermodynamic, and electrostatic properties of a classical liquid of polar polarizable molecules, with classical Drude oscillators modeling the internal variable of fluctuating polarization. Sample results obtained using several approximate closures are compared with simulation data.
Auger electron photoion coincidence technique combined with synchrotron radiation for the study of the ion desorption mechanism in the region of resonant transitions of condensed H2O
Authors: Mase Kazuhiko , Nagasono Mitsuru , Tanaka Shinichiro , Urisu Tsuneo , Ikenaga Eiji , Sekitani Tetsuji , Tanaka Kenichiro
Published: July 2002

The Auger electron photoion coincidence (AEPICO) technique has been applied for the study of H+ desorption induced by resonant excitations of O 1s of condensed H2O. The peak positions of the AEPICO yield spectrum at the 4a1←O 1s resonance (hν=533.4 eV) are found to correspond to spectator-Auger transitions leaving (O 2s)−2(4a1)1, (O 2s)−1(O 2p)−1(4a1)1, and (O 2p)−2(4a1)1 states. The H+ AEPICO yield is greatly enhanced at 4a1←O 1s while it is suppressed at 3p←O 1s (hν=537 eV) as compared with that at the O 1s ionization (hν=560 eV). On the basis of these results, the ultrafast ion desorption mechanism is suggested to be favorable for the H+ desorption at 4a1←O 1s, that is, the repulsive potential energy surface of the (O 1s)−1(4a1)1 state is responsible for the H+ desorption. For H+ desorption at 3p←O 1s, a spectator-Auger stimulated ion desorption mechanism is concluded to be probable. The suppression of the H+ AEPICO yield is ascribed to the reduction of the hole–hole repulsion due to the shield effect of the 3p electron. These results demonstrate the power of the AEPICO technique to clarify the mechanism of ion desorption induced by core–electron excitations.
Nascent rovibrational distributions of CO(d 3Δi,e 3Σ−,a′ 3Σ+) produced in the dissociative recombination of CO2+ with electrons
Authors: Tsuji Masaharu , Nakamura Masafumi , Nishimura Yukio , Obase Hiroshi
Published: July 2002

The d 3Δi–a 3Πr, e 3Σ−–a 3Πr, and a′ 3Σ+–a 3Πr transitions of CO resulting from the dissociative recombination of CO2+(X̃ 2Πg:0,0,0) with electrons have been observed from the He afterglow reaction of CO2. The formation rate constants of CO(d), CO(e), and CO(a′) were estimated to be 1.6×10−7, 3.3×10−9, and 2.4×10−7 cm3 s−1, respectively. The vibrational and rotational distributions of CO(d:v′=0–6,e:v′=2,3,a′=3–11) were determined. Most of available excess energies (91%∼98%) were deposited into the vibrational energy of CO(d,e,a′) and the relative translational energies of the products, indicating that CO(d,e,a′) were produced by direct curve crossings between the entrance e−/CO2+(X̃ 2Πg:0,0,0) potential and repulsive CO(d,e,a′)+O(3P) potentials with linear geometries. The vibrational distributions of CO(d) and CO(a′) slightly shifted to lower states than those in photodissociation at a similar excitation energy. A simple statistical model was unable to explain the observed vibrational distributions obtained by dissociative recombination.