bluuebunny/crossref_metadata_2025 huggingface dataset:
Multireference cluster expansion theory: MR–SAC theory
Authors: Nakatsuji Hiroshi
Published: July 2002

Multireference cluster expansion theory, called MR–SAC (multireference symmetry-adapted-cluster) theory, is presented. This theory is exact and unique, and yet does not include noncommutative algebra operator without imposing the completeness of the multireference space. The ansatz is simple enough for a general use in the study of potential energy surfaces of the ground and excited states of molecules. We have explained a recommended choice of the multireference operators on the basis of the analysis of an origin of the breakdown of the single reference theory. The method of solution of the MR–SAC theory is shown. The theory is given for the ground and excited states of closed-shell molecules, doublet and triplet states, and molecules with other symmetries. Test calculations are given for the ground and excited states of the CO molecule at the equilibrium and elongated distances.
An optimized random phase approximation for the dynamics of tunneling systems in condensed phases
Authors: Behrman E. C. , Jongeward G. A. , Wolynes P. G.
Published: July 2002

We derive an optimized random phase approximation from an expansion of the free energy in terms of the generating functional. We apply the theory to a quantum mechanical tunneling system coupled to a Gaussian bath. The theory reproduces both short-time behavior and long-time averages of (real time) correlation functions, and compares favorably with both nonoptimized RPA and the cumulant expansion.
The effect of high pressure on the electronic and infrared spectra of TCNE and its charge transfer complexes with hexamethylbenzene
Authors: Jurgensen C. W. , Peanasky M. J. , Drickamer H. G.
Published: July 2002

The visible and infrared spectra of the 1 : 1 and 2 : 1 solid-state complexes of hexamethylbenzene with tetracyanoethylene were investigated as a function of pressure up to 100 kbar. The degree of charge transfer is estimated as a function of pressure from the vibrational frequencies. A comparison with the visible spectra is used to check the internal consistency of the results. The observed pressure shift of the charge transfer peak is discussed in terms of the environmental and resonance contributions.
Vibrational excitation of ground-state p-difluorobenzene in collisions with He
Authors: Hall Gregory , Giese Clayton F. , Gentry W. Ronald
Published: July 2002

Crossed molecular beam experiments on state-resolved vibrational excitation of S0 p-difluorobenzene show only v=0→1,2 excitation of mode 30, the in-phase bend of the fluorine atoms perpendicular to the benzene plane. This high degree of mode 30 selectivity in transitions from the rotationally cold vibrational ground state suggests that efficient and mode-specific transfer of vibrational energy in collisions of polyatomic molecules need not depend strongly on intramolecular level mixing.
Measurement of the ν1 vibration-rotation spectrum of the thioformyl ion (HCS+) by velocity modulation laser spectroscopy
Authors: Rosenbaum Neil H. , Owrutsky Jeffrey C. , Tack Leslie M. , Saykally Richard J.
Published: July 2002

The measurement and analysis of the C–H stretching vibration (ν1) of HCS+ is reported. Eighty transitions [R(0)–R(44), P(1)–P(35)] were measured near 3.2 μm with a color center laser by the velocity modulation technique in a He/CO/H2S plasma. The ν1 band origin is at 3141.6823(5) cm−1, in excellent agreement with the ab initio prediction of Botschwina and Sebald.
Analysis of the laser photoelectron spectrum of CH−2
Authors: Bunker P. R. , Sears Trevor J.
Published: July 2002

We have simulated the photoelectron spectrum of CH−2 using the model described previously [Sears and Bunker, J. Chem. Phys. 79, 5265 (1983)]. The optimization of the fit of the simulated spectrum to the recently observed spectrum of Lineberger and co-workers [J. Chem. Phys. 81, 1048 (1984) and preceding paper] has enabled us to determine the rotation-bending energy levels of triplet CH2 over an energy range of more than 1 eV. It has also enabled us to determine that the rotational temperature of the CH−2 in the experiment is 220 K and that, for v2=1, the vibrational temperature is 680 K. For CH−2 we determine that ae=103° and that ν2=1230 cm−1. The singlet–triplet splitting in methylene is determined to be 3150±30 cm−1 (0.3905±0.004 eV, 9.01±0.09 kcal/mol) from the photoelectron spectrum, in excellent agreement with the more accurate value previously obtained from LMR spectroscopy [McKellar et al., J. Chem. Phys. 79, 5251 (1983)] of 3165±20 cm−1 (0.3924±0.0025 eV, 9.05±0.06 kcal/mol), and the electron affinity of triplet CH2 is determined to be 0.652±0.006 eV.
An improved time-dependent harmonic oscillator method for vibrationally inelastic collisions
Authors: DePristo Andrew E.
Published: July 2002

A quantal solution to vibrationally inelastic collisions is presented based upon a linear expansion of the interaction potential around the time-dependent classical positions of all translational and vibrational degrees of freedom. The full time-dependent wave function is a product of a Gaussian translational wave packet and a multidimensional harmonic oscillator wave function, both centered around the appropriate classical position variables. The computational requirements are small since the initial vibrational coordinates are the equilibrium values in the classical trajectory (i.e., phase space sampling does not occur). Different choices of the initial width of the translational wave packet and the initial classical translational momenta are possible, and two combinations are investigated. The first involves setting the initial classical momenta equal to the quantal expectation value, and varying the width to satisfy normalization of the transition probability matrix. The second involves adjusting the initial classical momenta to ensure detailed balancing for each set of transitions, i→f and f→i, and varying the width to satisfy normalization. This choice illustrates the origin of the empirical correction of using the arithmetic average momenta as the initial classical momenta in the forced oscillator approximation. Both methods are tested for the collinear collision systems CO2–(He, Ne), and are found to be accurate except for near-resonant vibration–vibration exchange at low initial kinetic energies.
Radiative and collisional decay of NBr(b 1Σ+, v′)
Authors: MacDonald M. A. , David S. J. , Coombe R. D.
Published: July 2002

NBr(b 1Σ+, v′), v′≤10 is produced by photolysis of BrN3 at 193 nm. Using this method, radiative decay rates for v′=0, 2, and 5 of the b 1Σ+ state were measured and were found to be ∼2.5×104 s−1. The rate constant for NBr(b) quenching by collisions with BrN3 was determined to be 1.57×10−10 cm3 s−1 for v′=0 and slightly faster for v′>0. The nascent vibration distribution of NBr(b 1Σ+) produced by 193 nm photolysis of BrN3 was found to be inverted, with a maximum population in v′=2. The overall quantum yield for production of NBr(b 1Σ+) by the photodissociation is 0.12.
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o calculations on the positive ions of the alkaline-earth oxides, fluorides, and hydroxides
Authors: Partridge Harry , Langhoff Stephen R. , Bauschlicher Charles W.
Published: July 2002

Theoretical spectroscopic parameters are presented at the self-consistent-field and singles-plus-doubles configuration-interaction (SDCI) levels for the ground states of the positive ions of the alkaline-earth fluorides, oxides, and hydroxides. The alkaline-earth fluoride and hydroxide ions are found to have X 1Σ+ ground states corresponding to a M+2X−1 structure. The alkaline-earth oxide positive ions, on the other hand, undergo a change in ground state symmetry between CaO+(X 2Π) and SrO+(X 2Σ+), whereas the analogous alkali oxides undergo the change one row higher between NaO and KO. The greater stability of the 2Π states for the alkaline-earth oxide positive ions is due to the increased electrostatic

field created by the larger metal ion charge and reduced bond lengths. Calculations on the 3Π states of BeF+, MgF+, BeOH+, and MgOH+ indicate that these states, corresponding to the M+1X structure, lie much higher in energy. The bond distances of the positive ions are uniformly 0.07±0.02 Å shorter than the analogous electronic states of the neutral. By correcting for the relatively small errors in the theoretical bond distances for the neutrals, bond distances accurate to about 0.005 Å are obtained for the ions where experimental values are available for the neutral species. Since extensive one-particle basis sets are employed and differential correlation effects are minimized by dissociating directly to ions, theoretical dissociation energies and ionization potentials (I.P.) accurate to about 0.15 eV can be obtained. The dissociation energies (D0) in eV for the ground states at the SDCI level are found to be: BeO+(4.04), MgO+(2.31), CaO+(3.29), SrO+(3.28), BeF+(6.25), MgF+(4.46), CaF+(5.67), SrF+(5.71), BeOH+(6.00), MgOH+(3.62), CaOH+(4.73), and SrOH+(4.67). For all of the systems studied, the bonding is dominated by electrostatic interactions. However, the molecules containing Be show some covalent character owing to the large I.P. of Be+. We have found that the constrained space orbital variation analysis, which decomposes the bonding into intrafragment polarization and interfragment donations, is an excellent method for delineating the degree of ionic bonding in a molecular system.
Two-pulse two-laser photodissociations of molecular ions: Collisional relaxation of bromo-3-(trifluoromethyl)-benzene cation
Authors: Barfknecht Andrew T. , Brauman John I.
Published: July 2002

Two-pulse techniques using high power lasers have been developed for the study of collisional deactivation of vibrationally excited molecular ions in the gas phase. Using these techniques, absolute rate constants have been determined for quenching of excited bromo-3-(trifluoromethyl)-benzene cation in a variety of bath gases. These rates are compared to the predictions of two models for collisional energy transfer in ion–molecule collisions. These studies also show that both IR photodissociation after internal conversion of a visible photon and visible photodissociation after IR multiple photon absorption can be operative mechanisms for the ‘‘IR-enhanced’’ visible photodissociation of molecular ions.
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o calculations of 29Si NMR chemical shifts for some gas phase and solid state silicon fluorides and oxides
Authors: Tossell John A. , Lazzeretti Paolo
Published: July 2002

A b initio coupled Hartree–Fock perturbation theory (CHFPT) employing near Hartree–Fock basis sets has been used to calculate the 29Si NMR chemical shifts of SiF4(Td), SiF−5 (D3h), SiF−26 (Oh), and SiO−44 (in both Td and C3v geometries). The increase in the shielding constant, σ, from SiF4 to SiF−26 is calculated to be 114 ppm, compared to an experimental value of about 75 ppm, while the decrease in σ between SiF4 to SiO−44 is calculated to be 43 ppm, compared to an experimental value of about 40 ppm. These results indicate that CHFPT applied to properly chosen molecular cluster models for solids can accurately reproduce experimental trends in solid state NMR chemical shifts. Calculations on SiF4 and SiO−44 for different internuclear distances also establish that the chemical shift is very weakly dependent on distance, suggesting that observed correlations between chemical shift and average Si–O distance in silicates arise indirectly, through the relationship between degree of polymerization and the average Si–O bond distance. By contrast, a C3v distorted SiO−44 tetrahedron (one bond 1.584 Å long and three bonds 1.651 Å long) gives an anisotropy in the chemical shift tensor of 54 ppm, with the larger value of σ occurring along the direction of the short Si–O bond, in reasonable agreement with available experiment. The calculations also support the use of the simple superimposed free atom approximation for the diamagnetic contribution to σ. For the Td species the paramagnetic contribution is shown to be dominated by excitations from the t2 σ orbitals to the σ* orbitals, as suggested previously on qualitative grounds. It may therefore be possible to interpret trends in σ by focusing on the energies and compositions of the t2 σ orbitals.
Theory of mode specific vibrational predissociation: The HF dimer
Authors: Halberstadt N. , Bréchignac Ph. , Beswick J. A. , Shapiro M.
Published: July 2002

Quantum mechanical close coupling calculations on (HF)2 vibrational predissociation are presented. The model considers vibrational excitation of the proton donor monomer. The other momomer is frozen at its equilibrium position. The linewidth (lifetime) and final state rotational distribution of the initially vibrationally excited HF were calculated using the artificial channel method. The potential surface was taken from Cournoyer and Jorgensen. The calculated linewidth is in good agreement with recent experimental measurements. This result is drastically different from what is obtained in the collinear model, merely reflecting the important influence of the rotational–vibrational coupling in the highly anisotropic systems. The final rotational distribution of the initially excited HF monomer is highly inverted. The possibility of a rotational in conjunction with a conventional vibrational HF laser is raised.
Alignment of excited states following multiphoton excitation
Authors: Kessler W. J. , Poliakoff E. D.
Published: July 2002

We report the measurement of the fluorescence anisotropy from excited neutral NO*(A 2∑+) molecules produced by two-photon excitation, thus providing a measure of their alignment. The degree of fluorescence anisotropy depended on the excitation pathway, and the results for the fluorescence angular distributions following excitation via the O12 rotational branch were more anisotropic than the superimposed R12,Q22 branch results. Results are given for other rotational branches as well and they should serve as useful aids in photoelectron asymmetry studies following multiphoton ionization, where the alignment of the neutral target state directly influences the photoelectron asymmetries in the ionization step.
Laser induced fluorescence of HCCO (DCCO) radical formed in O+C2H2 (C2D2) reaction
Authors: Inoue Gen , Suzuki Makoto
Published: July 2002

Laser induced fluorescence of HCCO (DCCO) radical have been obtained in the reaction of O(3P)+C2H2(C2D2) for the first time (X 2A′–2A″). The laser excitation spectra have been obtained over the wavelength region from 310 to 370 nm. The collision-free lifetimes for HCCO and DCCO are 0.14 and 3.8 μs, respectively. From the excitation and dispersed fluorescence spectra, some vibrational energies have been obtained in cm−1 unit: 1762 (1727), 150–140 (n.a.), and 2334 (1901) for the ground state HCCO (DCCO), and 1183 (1167), 866 (607), and 2862 (2075) for the excited state HCCO (DCCO). The band origin ν00 is 28 296.7 cm−1.
Fluorescence polarization from 1B1 pyrimidine: Evidence for intramolecular vibration–rotation energy transfer in a highly excited molecule
Authors: Nathanson Gilbert M. , McClelland Gary M.
Published: July 2002

We have measured the polarization of ensemble-averaged fluorescence from the 000 and 5 vibronic regions of 1B1 pyrimidine, in order to determine the extent of intramolecular vibration–rotation energy transfer at high energies. The polarization of the 000 is 15.54%±0.35%, and decreases smoothly to 6.93%±0.22% at Evib=3700 cm−1. From comparisons with model calculations, we conclude that highly excited pyrimidine undergoes nearly statistical rotational motion during its fluorescence lifetime. Additional experiments in a supersonic expansion show that the rotational temperature dependence of the polarization is weak.
Dielectric relaxation of 4:1 v/v methanol–ethanol to 90 kbar
Authors: Mishima Osamu , Whalley Edward
Published: July 2002

The effect of pressure on dielectric relaxation in a 4:1 v/v methanol–ethanol mixture has been measured in the range −44 –+22 °C to a maximum pressure of 90 kbar using a tungsten–carbide anvil apparatus. Graphs of the complex capacitance in the complex plane are not semicircles, but are approximately skewed arcs. The frequencies of maximum loss were fitted to a general quadratic in the deviations of the pressure and the reciprocal temperature from a reference pressure and temperature near the middle of the field, and apparent activation enthalpies and volumes were calculated. The model in which molecules are represented by spheres of radius a immersed in a medium of viscosity η predicts that the quantity  fmη/T, where  fm is the frequency of maximum loss and T the temperature, is independent of temperature and pressure. Its value changes relatively little with temperature but changes by three orders of magnitude in the range 0–70 kbar at 21 °C. Our theories of the transport properties of fluids at very high pressures are therefore quite inadequate. As much of the interior of the earth is made of such fluids, our small understanding of them limits our understanding of the earth’s interior.
H+2 —A full three-body variational treatment
Authors: Kohl D. A. , Shipsey E. J.
Published: July 2002

The eigenstates of the hydrogen molecule ion have been determined by a variational calculation without resorting to the Born–Oppenheimer or adiabatic approximations. A Slater-type orbital basis in terms of perimetric coordinates was used and the eigenvalues were obtained by diagonalizing a 300×300 matrix. Computational techniques and numerical stability tests are described. Because of the relatively small basis and the large number of bound states, most of the eigenvalues were sensitive to the basis set. With a basis which favored the upper states, 18 bound states were obtained in a single diagonalization vs 20 bound states in the adiabatic approximation. With the best basis for the ground state, the eigenvalues of the six lowest levels were 1 to 3×10−6 a.u. lower than adiabatic calculations and the differences appear to increase with increasing vibrational quantum number.
A molecular theory for freezing: Comparison of theories, and results for hard spheres
Authors: Haymet A. D. J. , Oxtoby David W.
Published: July 2002

We review the modern density functional theory of freezing, and compare different versions of it. New results for the freezing of hard spheres are presented using as input a very accurate, empirical structure factor. The results of these calculations are in good agreement with the results of computer simulations. The correct solid phase density is also displayed.
Temperature and pressure dependence of the reaction of He+ ions with H2
Authors: Böhringer H. , Arnold F.
Published: July 2002

The reaction of He+ ions with H2 has been studied in the temperature range from 18 to 408 K using a selected-ion drift-tube apparatus. The reaction rate coefficients are very small and do not exceed 2×10−13 cm3 s−1 at low pressure in the whole temperature range. The rate coefficients are pressure dependent. This indicates that a two-body and a three-body collision process contributes to the observed reaction rate. The three-body reaction rate coefficient is steadily increasing with decreasing temperature, approximately k3=1.6×10−30(100/T)1.27. To explain the three-body reaction process it has to be assumed that an intermediate complex He+–H2 with a lifetime of just a few vibrational periods exists.
Quantitative photoexcitation study of SiH4 in vacuum ultraviolet
Authors: Suto Masako , Lee L. C.
Published: July 2002

The photoabsorption and fluorescence cross sections of SiH4 were measured in the 106–160 nm region using synchrotron radiation as a light source. Two states with long vibrational progressions in the 106–123 nm region were observed and assigned to the 5s and 6s Rydberg series converging to the first ionization state (2t2)−1. The wavelength thresholds for producing fluorescences from the SiH ( A 2Δ→X 2Π) and Si (4s 1P0→3P2 1D2) transitions were observed at 131.5 and 120.0 nm, respectively. The quantum yields for both fluorescence systems were measured. The photodissociation processes for producing the excited fragments of SiH* ( A ) and Si* are discussed.
Proton magnetic resonance study of the H2–Ne potential energy surface
Authors: Wagner Robert S. , Armstrong Robin L. , Lemaire Claude , McCourt Frederick R.
Published: July 2002

Measurements of the proton spin-lattice relaxation time (T1) carried out at 18.775 MHz in two H2–Ne mixtures are presented. Values for T1/ρ)∞lin, accurate to within 2% are deduced for ten temperatures between 99 and 296 K. These data are compared with values obtained from close-coupled scattering calculations for a Hartree–Fock plus damped dispersion (HFD) model potential energy surface, allowing a refined anisotropic contribution to be determined.
Electron transport in low density alkane gases: Effects of chain length and flexibility
Authors: Floriano M. Antonio , Gee Norman , Freeman Gordon R.
Published: July 2002

The Ramsauer–Townsend (RT) minimum in the electron momentum transfer cross section (σm) occurs at the same energy (ξRT=0.12±0.01 eV) in all n-alkanes from C2H6 to n-C10H22. The lack of dependence on chain length indicates that the electron being scattered interacts with a chain segment that contains only two or three carbon atoms. The value of ξRT increases with increasing sphericity of the molecules, being 0.17 eV for i-butane, 0.22 eV neo-pentane, and 0.25 eV for methane. At high fields the electron drift velocity attains a ‘‘saturation’’ value (vsat) in the C1 to C5 hydrocarbons: 100 km/s in CH4, 54 km/s in C2H6, 51 km/s in C3H8, 49 km/s in n-C4H10, and 43 km/s in n-C5H12. Increasing the molecular sphericity increases vsat: 55 km/s in i-C4H10 and 57 km/s in neo-C5H12, to be compared with the values 49 and 43 in the n-isomers, and 100 km/s in CH4. The value of σm averaged over the thermal velocity distribution, σav, is an order of magnitude smaller than that expected from the simple polarization interaction, and is similar in magnitude to the physical cross section of the molecule. The scattering cross sections of the deuterated methanes CHxD4−x, x=0–3, are the same as those of CH4, in the low density gases.
Chemiluminescent reactions of the IVA elements: Dihalide formation
Authors: Rosano W. J. , Parson J. M.
Published: July 2002

Earlier work on chemiluminescent beam-gas reactions of Si, Ge, and Sn with F2 [W. J. Rosano and J. M. Parson, J. Chem. Phys. 79, 2696 (1983)] is extended to reactions with Br2, I2, ClF3, SF4, and SF6. In all of these new systems electronic emission is attributable exclusively to IVA dihalide products. Reaction of Sn with the diatomics most likely procedes by a two-step mechanism involving a highly vibrationally excited Sn monohalide intermediate. The polyatomic reactants, on the other hand, yield IVA difluoride products in a single collision. Radiation of the dihalides from the 3B1 state to the ground 1A1 state is consistent with the thermochemistry of these mechanisms. The dependence of the emission on the IVA atom velocity shows that energy barriers to reaction are small or nonexistent.
Analytical calculation of MCSCF dipole-moment derivatives
Authors: Helgaker Trygve U. , Jensen Hans Jo/rgen Aa. , Jo/rgensen Poul
Published: July 2002

The analytical calculation of molecular dipole-moment derivatives for MCSCF wave functions is described. The formalism is based on exponential unitary transformation of the wave function and symmetric orthonormalization of the molecular orbitals. The response equations are solved using an iterative, direct technique to allow for large configuration expansions. Translational and rotational symmetries of the dipole moment are used to minimize computational costs. Sample calculations involving several thousand configurations are presented for H2O and ONF.
Diffusion of an adsorbed particle: Temperature dependence
Authors: Wahnström Göran
Published: July 2002

This paper completes a theoretical investigation of the motion of an atom adsorbed on a face centered cubic (fcc) (001) surface. We consider here different temperatures and the diffusion constant D is found to follow an Arrhenius curve. Quite similar results for D are obtained if one uses the absolute rate theory for the escape rate together with the assumption of single uncorrelated jumps. The static potential is treated properly by including the local distortion of the lattice around the adatom and by taking into account the thermal fluctuations of the substrate atoms. The distortion of the lattice has a significant effect on D, particularly at low temperatures. With all the various physical effects included the calculated values for the diffusion constant and for the velocity correlation function are found to be consistent with available molecular dynamics data on the same system.
Dipolar SASS NMR spectroscopy: Separation of heteronuclear dipolar powder patterns in rotating solids
Authors: Terao T. , Miura H. , Saika A.
Published: July 2002

Two-dimensional sample-spinning NMR techniques for separately obtaining heteronuclear dipolar powder patterns of chemically distinct nuclei are proposed, which involve switching the spinning axis on and off the magic angle. Procedures for the computation of dipolar spectra obtained by this method are described, and various features of such spectra are discussed using simulated spectra. Practical applications of this method are demonstrated with 13C spectra of calcium formate, β-quinol-methanol clathrate, and urea-trans-4-octene clathrate, providing information on the absolute sign and magnitude of the indirect dipolar couplings, 13C–1H distances, and dynamics of trapped molecules in clathrates.
Single molecule dynamics of three body water
Authors: Wojcik Mark , Clementi Enrico
Published: July 2002

The effects of three body forces on the single molecule dynamical properties of liquid water are studied by molecular dynamics using ab initio two body and three body potential models. The molecule’s motion within the cage of neighbors becomes faster while the overall molecular diffusion and reorientation becomes slower. The latter brings the simulated NMR relaxation time into good agreement with experiment.
Laboratory studies of gas phase sodium diffusion
Authors: Ager Joel W. , Howard Carleton J.
Published: July 2002

The gas phase diffusion coefficients of Na in Ne, N2, and CO2 were measured in a flow tube with an oven source and resonant fluoresence detection of Na. DNa,Ne=209±21 cm2 Torr s−1 at 281 K, DNa,N2=129±13 cm2 Torr s−1 at 281 K, and DNa,CO2=134±13 cm2 Torr s−1 at 281 K. DNa,He was measured over the temperature range 211– 424 K. DNa,He(T)=(385±40) (T/300)(1.72±0.18) cm2 Torr s−1. The experimental results are compared with previous studies. The sticking coefficient for Na on the walls of the flow tube was determined to be approximately equal to 1 by an analysis of the diffusion coefficient data. The results for DNa,He, DNa,Ne, and DNa,Ar are compared with the predictions of Chapman–Enskog calculations using Na–noble gas potentials from recent spectroscopic measurements.
On the pyrazine and pyrazine–pyrimidine dimers
Authors: Wanna J. , Bernstein E. R.
Published: July 2002

Spectra of the pyrazine-d4, pyrazine-h4–pyrazine-d4, and pyrazine-d4–pyrimidine dimer are obtained and analyzed with the help of Lennard-Jones–hydrogen-bonding (LJ–HB) potential energy calculations. The pyrazine isotopic hetero and homo dimers possess nearly identical spectra with the exception that the perpendicular dimer features are displaced to the red by ∼11 cm−1. Exchange or exciton interactions in this system are vanishingly small (less than 1 cm−1). The geometries suggested by the isotopically substituted pyrazine dimer spectra are the same as those found for the pyrazine-h4 homo dimer: a parallel planar hydrogen bonded and a perpendicular dimer. The pyrazine-d4– and pyrazine-h4–pyrimidine dimer spectra are quite complicated. These spectra can be assigned as arising from one parallel stacked head-to-tail displaced dimer, one parallel planar dimer, and three perpendicular dimers based on comparisons with spectra of the pyrazine and pyrimidine dimers.
Vibrational force field for t
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Authors: Takeuchi Hideo , Harada Issei
Published: July 2002

The in-plane force field of trans polyene chains, which had previously been derived from vibrational frequency data on short polyenes and poyacetylene, gave frequencies lower than the experimental ones when it was applied to polyene chains of intermediate lengths. However, a better agreement between the calculated and experimental frequencies is obtainable solely by reducing interaction force constants between C–C stretching coordinates according to the chain length dependence of bond–bond polarizability.
Generalized hydrodynamics and specific heat spectroscopy of the glass transition
Authors: Oxtoby David W.
Published: July 2002

Recently Birge and Nagel have developed a new experimental technique for studying the frequency dependent thermal response of supercooled liquids approaching the glass transition. We use projection operator methods to analyze their experiments and to show the relation between the frequency and wave vector dependent thermal conductivity and their frequency dependent specific heat. We show that even in liquids with a single internal mode, two relaxation times appear and the spectrum is non-Lorentzian. We relate frequency and time domain thermal response measurements to one another using our formalism.
Precise representation of the J=3° energy levels of the m
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p3 configurations in Si i, Ge i, and Sn i using a perturbed coupled-channel model
Authors: Ginter Dorothy S. , Ginter Marshall L. , Brown Charles M.
Published: July 2002

Results given demonstrate the ability of an energy-dependent coupled-channel model which includes a perturbation function to represent precisely (σ≤0.30 cm−1) all mpnd J=3° energy levels observed experimentally below the lowest ionization threshold for Si i, Ge i, and Sn i. In this model all energy dependence is placed into the closed-coupled μα parameters. The energy-independent Uiα elements of the frame transformation matrix are found to be close to those describing the transformation between pure LS- and jj-coupled states. Inclusion of corrections for perturbations which depend on the energy separation of a perturber level from the levels in the channels permits the quantitative inclusion of the msmp3 J=3° levels. To the authors’ knowledge these examples are the first to demonstrate the ability of coupled-channel models to reproduce entire, perturbed, strongly coupled multichannel- and multilimit-based excited electronic structures observed below the lowest ionization limit to within the average experimental uncertainties of the energy levels.
Pure rotational spectrum of SnH4 in the vibrational ground state observed by infrared-radio frequency double resonance
Authors: Ohshima Yasuhiro , Matsumoto Yoshiyasu , Takami Michio , Kuchitsu Kozo
Published: July 2002

Pure rotational Q-branch transitions caused by the centrifugal-distortion-induced dipole moment were observed in the vibrational ground state of SnH4 by infrared-radio frequency double resonance using a tunable diode laser. Forty-nine double resonance signals were observed for 15 rotational transitions with 6≤J≤20 for seven isotopic species of Sn. The isotope effects on the transition frequencies were found to be negligible to the experimental accuracy of a few tens of kilohertz. The tensor centrifugal distortion constants were determined as follows: Dt =58 398.6±7.0 Hz, H4t =−4.297±0.046 Hz, and H6t =2.340±0.016 Hz.
Assignment of the lattice modes in TCNQ single crystals
Authors: White K. M. , Brose K.-H. , Eckhardt C. J.
Published: July 2002

A complete assignment of the six librational modes of the TCNQ0 crystal has been made employing both polarized Raman spectroscopic measurements and lattice dynamical calculations. Agreement between theory and experiment is good and modes at 40.5, 74.5, and 96 cm−1 are assigned as Ag active and those at 63, 75.5, and 104 cm−1 are Bg active. The lattice modes are found to be clearly separated from the molecular modes. The study emphasizes the importance of crystal optics and quality in the measurement of the polarized Raman spectra of biaxial crystals.
Recognition of the violet system of S2Cl in the pyrolysis of S2Cl2
Authors: Chiu C. L. , Pan S. C. , Ni C. K.
Published: July 2002

A total of 37 absorption bands, lying between 3785 and 4805 Å, have been observed during the pyrolysis of S2Cl2 at 750 K. These bands have been tentatively assigned to transitions between the X̃ A″ state of S2Cl to an unidentified electronically excited state with vibrational frequencies ω′1 =480 cm−1 and ω2 =407 cm−1. From this assignment it follows that the ground state vibrational frequencies are ω″1 =659 cm−1, ω″2 =600 cm−1, and ω″3 =336 cm−1. A simple molecular orbital diagram has been constructed from the frontier orbitals of S2 and Cl. The predicted changes in the bond strengths and bond angles between the ground state and the excited state are consistent with experimental assignments.
Electron bound states near a charged dielectric sphere
Authors: Ballester J. L. , Antoniewicz P. R.
Published: July 2002

We have calculated the binding energies for electron states which are localized just outside of the surface of a (liquid) dielectric sphere in which a positive ion impurity is dissolved. The model considers four quantities: the static and optical dielectric constants of the liquid, the conduction band minimum in the liquid, and the radius of the sphere. We find that surface states are possible for water and ammonia but for argon the ion is not sufficiently screened to permit surface states.
Vibrational excitation in collisions between two diatomic molecules using an operator algebra
Authors: Recamier Jose A. , Micha David A. , Gazdy Bela
Published: July 2002

We introduce an operator algebra to study vibrational–translational energy transfer and apply it to collinear collisions between two diatomic molecules. The system is modeled by two linearly driven parametric oscillators with a bilinear time-dependent coupling between them. We describe the time evolution of the linearly driven parametric oscillators accounting for part of the coupling with a sequence of transformations that reduces the coupling at each step, and use perturbation theory to account for the remainder. Results of a basis set expansion are compared with those of the algebraic approach for the collisions N2+O2, N2+CO, and H2+H2. The algebraic approach requires solving a substantially smaller number of coupled differential equations, and gives very good agreement for all systems, for several transitions and relative collision energies.
Gas-phase hydrolysis of SOF2 and SOF4
Authors: Van Brunt R. J. , Sauers I.
Published: July 2002

The rates for gas-phase hydrolysis of SOF2 (thionylfluoride) and SOF4 (thionyl tetrafluoride) have been measured at a temperature of 298 K. The second order rate constant for SOF2 hydrolysis in SF6 buffer gas was found to have the value (1.2±0.3)×10−23 cm3/s which agrees with previous estimates of Sauers et al., but is three orders of magnitude lower than the value obtained by Rüegsegger et al. at 340 K. The rate constant for SOF4 hydrolysis has not previously been measured and its value in both SF6 and N2 buffer gases was found here to be (1.0±0.3)×10−21 cm3/s.
Spin uncoupling in the 6s Rydberg states of methyl iodide. Rotational subband structure in one-photon absorption
Authors: Dagata J. A. , Felps W. S. , McGlynn S. P.
Published: July 2002

Molecular constants of electronic states, all origins as well as some vibronic bands, derived from the lowest-energy Rydberg configuration of methyl iodide, are reported. Values of the internal angular momentum parameter ζeff, which is a measure of the strength of Coriolis and vibronic coupling, indicate that the spin of the Rydberg electron is effectively uncoupled from the molecular axis in the pure electronic states and that dynamic interactions between electronic, rotational, and vibrational motions completely quench the internal angular momentum in some vibronic levels. It is concluded that the parameters ζeff, as obtained from one-photon absorption spectra, are approximately those of the alternate levels, 2E3/2 and 2E1/2, of the CH3I+ ion-core ground configuration X̃.
Orientation effects in the scattering of Na(3 2P3/2) and K(4 2P3/2) with rare gas targets at thermal energies
Authors: Düren R. , Hasselbrink E.
Published: July 2002

The σ−/σ+ asymmetry in the scattering of excited Na(3 2P3/2) and K(4 2P3/2) atoms with oriented electronic angular momentum from rare gas targets has been systematically studied. The orientation is achieved by exciting the atoms with circularly polarized laser light, which is directed perpendicularly into the scattering plane. The origin of the asymmetry is discussed on the basis of the close coupling formalism. This reveals the centrifugal part of the interaction to be the only central reason. The asymmetry is observed for all of the different target gases studied (Ne, Ar, Kr, Xe). Quantitatively the observed features change strongly for the different combinations. Model calculations are presented which show the different fine-structure splitting of the alkali atom and the polarizability of the rare gas atoms to be responsible for the variation.
Coriolis-assisted vibrational energy transfer in D2CO/D2CO and HDCO/HDCO collisions: Experiment and theory
Authors: Haub J. G. , Orr B. J.
Published: July 2002

The technique of time-resolved infrared–ultraviolet double resonance is used to characterize the rates and propensity rules for mode-to-mode vibrational (V–V) energy transfer in D2CO/D2CO and HDCO/HDCO collisions. Such processes are found to be exceptionally efficient when collision-induced transfer is between the ν6 and ν4 modes of D2CO or between the ν5 and ν6 modes of HDCO: in the case of D2CO prepared in a specific ν6 rovibrational state by the 10R32 line of a CO2 laser, the rate of V–V transfer to specific states of the ν4 rovibrational manifold is approximately three times greater than the hard-sphere gas-kinetic collisional rate. This efficiency is much higher than for typical V–V transfer processes and approaches that of pure rotational relaxation, with the result that rotationally specific V–V transfer channels can be identified. The essential mechanism depends on the strong Coriolis coupling between the modes of D2CO or HDCO involved, as demonstrated by a semiclassical theoretical treatment which

considers only the electric dipole/dipole portion of the intermolecular potential. The combined effect of Coriolis and asymmetric-rotor perturbations causes mixing of rovibrational basis states and induces nonvanishing matrix elements of the permanent electric dipole moment between the vibrational modes of interest. These effects are most pronounced at moderate values of the rotational quantum number Ka (∼4), because quantum-mechanical interferences tend to annihilate the transition moment induced by Coriolis coupling alone at higher values of Ka. The theory also assumes that particularly efficient V–V transfer channels arise from very small energy differences between initial and final states of the state-selected molecule, owing to the abundance of collision-partner molecules then available to yield a zero overall energy defect for the pair of colliding molecules. The predictions of the simple long-range theory adopted yield order-of-magnitude agreement with the experimental results; possible deficiencies of the theory are discussed. Also discussed are the wider implications of the results, with regard to collision-induced V–V transfer between discrete rovibrational levels of small polyatomic molecules in general, to intramolecular vibrational redistribution in congested rovibrational and rovibronic manifolds, and to mechanisms of infrared multiple-photon excitation.
Fourier transform detection of laser-induced fluorescence from the CCN free radical
Authors: Brazier C. R. , O’Brien L. C. , Bernath P. F.
Published: July 2002

The technique of high-resolution Fourier transform spectroscopy has been used to detect the laser-induced fluorescence spectrum of the CCN free radical. Emission from a single vibronic level in the Ã 2Δ state to the X̃ 2Π state yielded rotational constants for several vibronic levels of the ground state. From the analysis of the 0v21 levels the size of the Renner–Teller interaction was determined, yielding ε=0.410.
Nonphotochemical holeburning in a protein matrix: Chlorophyllide in apomyoglobin
Authors: Boxer Steven G. , Gottfried David S. , Lockhart David J. , Middendorf Thomas R.
Published: July 2002

Nonphotochemical holeburning spectroscopy has been used to study a chlorophyllide chromophore substituted for heme in apomyoglobin. The observed holewidths were compared with those found for the same chromophore embedded in the polymeric glass polystyrene. Hole formation occurs with comparable efficiency in the protein and in the polymeric matrix. A T(1.26±0.07) temperature dependence was measured for the pure-dephasing contribution to the homogeneous linewidth for the chromophore in the protein between 1.35 and 2.5 K. This temperature dependence is very similar to that observed for many chromophores is amorphous host matrices. This suggests that the characteristic properties of the matrix responsible for pure dephasing in glasses and in the protein are very similar.
Highly excited vibrational states by adiabatic vs self-consistent-field methods
Authors: Certain Phillip R. , Moiseyev Nimrod
Published: July 2002

Highly excited states for model Hamiltonians of two strongly coupled modes are calculated by adiabatic and by the self-consistent-field methods. The energy spectra obtained by the two separation methods are similar and deviate from the accurate spectrum in the stochastic energy regime where the system is far from separability. However, the natural expansion analysis of the adiabatic wave functions in the stochastic energy region shows that the adiabatic approach did not totally suppress the stochastic information about the eigenstates, and the adiabatic natural expansion coefficients are in very good agreement with the accurate ones.
The dynamics of dissociative chemisorption of H2 on a Si(111) surface
Authors: Rice Betsy M. , NoorBatcha I. , Thompson Donald L. , Raff Lionel M.
Published: July 2002

The dissociative chemisorption and scattering of H2 on an unreconstructed Si(111) surface has been investigated using classical trajectories on a potential-energy surface previously used to study H2 recombination/desorption from Si(111) [J. Chem. Phys. 85, 3081 (1986)]. The results show H2 scattering from Si(111) to be elastic and predominately specular. The scattered translational energy distributions are broadened but there is virtually no change in the average values. There is very little energy transfer to or from the internal H2 modes. Lower limits to sticking probabilities are obtained as a function of the temperature. Adsorption of H2 is found to always involve dissociation and subsequent chemisorption of both hydrogen atoms. The reaction exothermicity for chemisorption is primarily deposited into vibrational motion of the newly formed Si–H bonds. Energy transfer out of these bonds into the phonon modes of the lattice is a rapid, first-order process whose rate decreases with increasing surface temperature. Hydrogen atom mobilities on the surface computed from the velocity autocorrelation function are found to decrease rapidly with increasing residence time on the surface due to energy transfer to the lattice. Thermal diffusion of hydrogen atoms on the Si(111) surface is a slow process with a high activation energy.
Towards HF SCF value of electron affinity of SF6
Authors: Klobukowski M. , Barandiarán Z. , Seijo L. , Huzinaga S.
Published: July 2002

Adiabatic electron affinity (aEA) of SF6 was calculated on the SCF level using large basis sets on both sulfur and fluorine. The calculated value of aEA was 0.3 eV. d-type polarization function on fluorine was shown to affect the value of aEA more than the 3d orbital on sulfur. Inclusion of f-type function on sulfur was discussed.
Electronic energy transfer at semiconductor interfaces. I. Energy transfer from two-dimensional molecular films to Si(111)
Authors: Alivisatos A. P. , Arndt M. F. , Efrima S. , Waldeck D. H. , Harris C. B.
Published: July 2002

The fluorescence decays from submonolayers of pyrene separated from Si(111) by Xe spacer layers are measured as a function of spacer thickness (17–200 Å), pyrene coverage, and emission wavelength. The results are explained in terms of two decay channels: energy transfer and trapping among the molecules in the two-dimensional pyrene overlayer, and excitation of electrons from the valence to the conduction band in the Si(111) by the dipole near field of the electronically excited pyrene molecule. The intralayer energy transfer is modeled using the Kohlrausch equation N(t)=N0 exp(−t/τ)α, in which α is related to the distribution of pyrene molecules in energy. Energy transfer from the molecule to the semiconductor is modeled using the classical image dipole theory. The classical model is used to calculate the energy transfer rates from a dipole to Si and GaAs as a function of dipole–semiconductor separation, and as a function of dipole emission wavelength.
Angular distributions of photofragments generated in the two-photon dissociation of nitrogen dioxide and carbon disulfide
Authors: Kawasaki M. , Sato H. , Kikuchi T. , Fukuroda A. , Kobayashi S. , Arikawa T.
Published: July 2002

Two-photon dissociation of nitrogen dioxide and carbon disulfide at 308 nm has been studied by means of multiphoton ionization of photofragments. The angular distributions of fragments are well represented by a form of 1+β2P2(cos θ)+β4P4(cos θ). Because the intermediate state of NO2 (2B2) generated by the first one-photon process is dissociative, the coefficient β4 for the fourth degree Legendre polynomial is appreciable. The coefficient β4 for CS2 is, however, not appreciable because the intermediate CS2 (1B2) state has a long fluorescence lifetime. The angular distributions not only reveal the symmetry and lifetimes of the two-photon excited states of NO2 and CS2 but also information on the two-photon excitation process of these molecules.
Resonance rotational hyper-Raman scattering intensities of symmetric top molecules
Authors: Ziegler L. D. , Chung Y. C. , Zhang Y. P.
Published: July 2002

Resonance rotational hyper-Raman scattering cross sections excited by identical linearly polarized incident photons are derived for symmetric tops. In the absence of pure dephasing, the rotational structure of a RHR vibrational transition is described, in general, by five scattering invariants which are derived in an irreducible tensor framework. The pattern of relative rotational RHR intensities is a sensitive function of the two-photon resonant vibronic state dephasing constant. High quality rotational RHR spectra of NH3 are obtained using blue incident radiation at half the X̃→Ã transition energy. Fits to the observed patterns of relative rotational RHR intensities determine lifetimes of 38, 60, and 100 fs for the v′2 =1,2,3 Ã state vibronic bands respectively. Although seven rotational branches (N,O,P,Q,R,S,T) are possible in HR, only three (P,Q,R) are observed in these RHR spectra of ammonia due to the magnitude and relative signs of vibronic matrix elements in the scattering cross section expressions.