bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational exchange upon interconversion tunneling in (HF)2 and (HCCH)2
Authors: Fraser G. T.
Published: July 2002

Model calculations are presented to interpret the large H–F and C–H stretching vibrational dependencies of the interconversion tunneling splittings and the corresponding infrared vibrational-tunneling state selection rules in (HF)2 and (HCCH)2. The model consists of two potential curves in the tunneling coordinate, coupled by an interaction term that allows the vibrational excitation to be exchanged between the two monomer units, permitting tunneling to occur. The interaction term is approximated by resonant infrared transition–dipole coupling. The magnitudes of the calculated vibrational dependencies, their isotopic shifts, and the predicted selection rules are in agreement with previous experimental observations.
Time resolved luminescence spectroscopy of alkaline earth oxides after pulsed electron beam irradiation. II. Spectra and thresholds of MgO
Authors: Grant Janice L. , Cooper Ronald , Zeglinski Phillip , Boas John F.
Published: July 2002

Time-resolved luminescence spectroscopy has been used to study the emissions occurring in MgO in the time range 100 ns to 100 μs after irradiation with nanosecond duration pulses of electrons of energy between 0.2 and 3.0 MeV. An emission at ∼380 nm with a threshold energy of 0.28±0.01 MeV is attributed to processes resulting from the displacement of oxygen anions and the formation of F+ centers. An emission at ∼240 nm is attributed to processes resulting from the displacement of magnesium cations, with a threshold energy of 0.32±0.01 MeV. An emission at ∼470 nm at an electron energy of 3.0 MeV is attributed to processes resulting from the formation of aggregates of F+ centers, such as the F2+2 center. Displacement energies of 49±2 and 38±2 eV were estimated for oxygen anions and magnesium cations, respectively.
Fixed angle reactor model calculations for the D+H2(v=0,1)→HD(v′=0,1,2)+H reaction
Authors: Jansen op de Haar B. M. D. D. , Balint-Kurti G. G.
Published: July 2002

An approximate three-dimensional quantum mechanical method, the fixed angle reactor model (FARM), is reviewed and applied to the D+H2→HD+H reaction. Calculated integral and differential cross sections and rate constants are presented and compared with other calculations and with experimental data. Good agreement is found between calculated and experimental rate constants. The effects of tunneling are clearly apparent in the temperature dependence of the rate constant for the D+H2(v=1)→HD+H reaction.
Energy distribution of the CN products of the H+HCN, H+ClCN, and F+HCN reactions
Authors: Johnston Grace Wei , Bersohn Richard
Published: July 2002

Three reactions of the type X+YCN→XY+CN (X, Y=H, H; H, Cl; F, H) were studied by probing the internal state distribution of the CN product. The fraction of the available energy released as CN rotation is very small consistent with theoretical predictions that these reactions are collinear abstractions. At higher incident kinetic energies of the X atom a larger fraction of the available energy is released into rotation, suggesting that the reactive collision can take place at larger angles of attack. In the reverse reaction CN+HD→HCN(DCN) +D(H) using rapidly rotating CN as a reactant the [H]/[D] ratio is 1.4±0.1.
Uniform adiabatic invariance analysis of chemical reaction dynamics
Authors: Skodje Rex T.
Published: July 2002

It is shown that the usual primitive adiabatic theory of classical reaction dynamics is inconsistent when separatrix crossing occurs. In such cases, primitive theory yields errors in the reaction probabilities and other observables which do not scale to zero even when the time scale ratios become infinitely large, i.e., the adiabatic limit. This motivates a fundamental modification to the classical adiabatic theory of reactions to include the effects of separatrix crossing. The approach is explicitly formulated for direct heavy–light–heavy collinear reactions where two separatrix crossings must occur during the course of each reactive trajectory: once when the orbit untraps from the incoming reactant channel well and once again when it retraps in the final product channel well. The uniform adiabatic invariance analysis we propose reduces the classical reaction dynamics to the form of a simple measure preserving map. That is, the final conditions of the product trajectory are written as explicit analytic functions of the initial conditions. This eliminates the need to propagate any trajectories. The map is formulated in terms of the quantities from the adiabatic theory of reactions, i.e., vibrationally adiabatic potential curves, instantaneous frequency, etc., which are easily computed numerically. It is found that the imaginary frequency of the potential surface along the ridge separating reactants from products is a crucial parameter in the reaction dynamics. The uniform adiabatic analysis permits the calculation of vibrational inelasticity, complex lifetimes, the structure of reactivity bands, and other quantities inaccessible in usual adiabatic theory of reactions. Numerical result are presented for the I+HI reaction where it is found that the theory is quite accurate.
Rotational state distributions from vibrational autoionization of H2
Authors: Dehmer J. L. , Dehmer P. M. , Pratt S. T. , Tomkins F. S. , O’Halloran M. A.
Published: July 2002

Optical–optical double-resonance excitation together with electron spectroscopy was used to measure the H+2 rotational state distributions produced by vibrational autoionization of singlet np Rydberg states of H2 . In the two-color excitation scheme, one laser was used to excite the two-photon transition to the H2 E, F 1∑+g, v′=1, J′=1 state, and a second laser was used to probe single-photon transitions to the vibrationally autoionized np Rydberg series converging to the X 2∑+g, v+=1, N̄+=1 and N̄+=3 levels of the ion. The expected P(1)npσ, Q(1)npπ, R(1)np1, and R(1)np3 Rydberg series converging to v+ =1 were observed and assigned, as were several interlopers converging to higher vibrational levels of the ion. Rotationally resolved photoelectron spectra were determined for all of the autoionizing transitions by using a magnetic bottle electron spectrometer. Under the normal assumptions that p waves are ejected and that spin effects are negligible, vibrational autoionization of the upper levels of the P(1)npσ and Q(1)npπ transitions should produce only v+ =0, N̄+ =1, while vibrational autoionization of the upper levels of the R(1)np1 and R(1)np3 transitions should produce a mixture of v+ =0, N̄+ =1 and v+ =0, N̄+ =3. Significant deviations from these expectations were observed. For example, vibrational autoionization of the upper levels of the Q(1)npπ transitions produced substantial amounts of v+ =0, N̄+ =3, while vibrational autoionization of the upper levels of certain Q(1)npπ, R(1)np1, and interloper transitions produced nonnegligible amounts of v+ =0, N̄+ =5. This indicates that vibrational autoionization of npπ Rydberg states is accompanied by rotational state changes in the H+2 core to an unexpected degree, and that additional mechanisms for exchange of angular momentum within the excited complex must be considered. Possible contributing

mechanisms are critically assessed.
Chemical reaction in medium density fluid. Solvent density effects on the dimerization equilibrium of 2-methyl-2-nitrosopropane in carbon dioxide
Authors: Kimura Y. , Yoshimura Y. , Nakahara M.
Published: July 2002

The dimerization equilibrium of 2-methyl-2-nitrosopropane in carbon dioxide was studied in a wide range of solvent density from 200 to 1100 kg m−3 at 25.0 and 35.0 °C. We found an inversion of the density dependence of the equilibrium constant. The configurational volume change for the dimerization has a large positive value in the medium density region, although it is negative in the high density region as usually observed for the bond formation reactions in the liquid state. This anomaly is reproduced by the cavity distribution function calculated from the integral equation based on the HNC or PY approximation for the Lennard-Jones fluid model. The large positive value in the medium density region is interpreted in terms of desolvation effect due to the dimerization. The long-range correlation between molecules due to attractive forces is suggested to play an important role in determining the reaction volume in the medium density region.
On stability of molecular solids ‘‘under chemical pressure’’
Authors: Luty Tadeusz , Fouret René
Published: July 2002

The problem of stability of a molecular solid with local, randomly distributed perturbations of a chemical type (substitutional, reaction generated, ionized, excited molecules) has been addressed. It has been shown that local displacement fields (‘‘chemical pressure’’) produced by the perturbations influence stability of the solid in two ways. First, via indirect interactions between perturbations which are mediated by phonons of the host lattice and then, via modified (chemical pressure dependent) static single molecule susceptibility. The theory takes into account direct interaction between (the energy of) perturbations. A competition with indirect interaction leads to formulation of metastability as well as stability conditions. Finally, it has been shown how the conditions can be applied to predict photo-induced structural changes and transformations in mixed charge–transfer crystals.
Time resolved studies of vibrational relaxation dynamics of CO(v=1) on metal particle surfaces
Authors: Berkerle J. D. , Casassa M. P. , Cavanagh R. R. , Heilweil E. J. , Stephenson J. C.
Published: July 2002

The vibrational relaxation dynamics of CO chemisorbed on small Pt and Rh particles supported on SiO2 has been investigated by picosecond time-resolved infrared transient bleaching experiments. A vibrational T1 lifetime of ≊7 ps has been observed for several different samples, independent of polarization, pump intensity, and sample temperature from 100–400 K. A 1:3 isotopic dilution has no effect upon T1. This T1 lifetime is a factor of 10–50 times shorter than T1 reported for metal carbonyl cluster compounds in solution and on SiO2 supports. Two possible mechanisms are considered to account for the rapid T1 decay; redistribution of the energy throughout the broad CO vibrational band, and relaxation directly to electron–hole pairs in the metal particles.
Structure of the dipolar hard sphere fluid near a charged hard wall: Density profile and polarization
Authors: Russier V.
Published: July 2002

The density profile and the local polarization of the dipolar hard sphere fluid in contact with a hard wall, from which emerges an electric field, is investigated. We first consider a linearized version of the wall particle reference hypernetted chain approximation (RHNC), where the reference system is the dipolar fluid in the absence of the field. Then we make a nonlinear approximation with respect to the electric field. Particular attention is paid to the perturbations induced by the field in the bulk, the polarization, and the modification of the mean density (electrostriction).
The general model space effective Hamiltonian in order-for-order expansion
Authors: Meissner Leszek , Bartlett Rodney J.
Published: July 2002

The order-by-order perturbation expansion of the effective Hamiltonian for a multireference space is presented. The concept of the general model space (complete or incomplete) of Hose and Kaldor is used as well as their concept of a graphical representation of the perturbation expansion. The obtained result differs, however, from that of Hose and Kaldor. In the Hose and Kaldor derivation many types of disconnected diagrams of the effective Hamiltonian are declared reducible and omitted by in fact they do not completely cancel and these give an irreducible contribution to the effective Hamiltonian maxtrix causing the method to not be fully extensive. To ensure that all terms are included, we present modified diagram rules for the effective Hamiltonian.
Analytical approach to molecular liquids. I. Site–site interaction model using an extended mean-spherical approximation
Authors: Stell George , Zhou Yaoqi
Published: July 2002

The observation that the intramolecular direct-correlation-function matrix is very closely approximated by its large-r form for rigid diatomics suggests an approximation such that the site–site Ornstein–Zernike equation becomes analytically solvable for several molecular models of interest. Alternatively, approximation of the particle–particle direct correlation function by its asymptotic form directly yields the same approximation, in particle–particle (in contrast to site–site) language. Applications of this approximation, which can be regarded as an extended mean spherical approximation when viewed in particle–particle terms, are made to rigid hard diatomics, dipolar diatomics, and a tetrahedral molecular model. The close but subtle connections between our approximation procedure and earlier approximations are discussed.
A trajectory surface-hopping study of H+2+He collisions with identification of the product electronic state in dissociation processes
Authors: Sizun Muriel , Gislason Eric A.
Published: July 2002

A trajectory surface-hopping study of collisions of H+2 (v)+He for v=0, 3, 6, and 10 has been carried out on the two lowest potential-energy surfaces at relative collision energies of 3.1, 5, and 10 eV. The diatomics-in-molecules (DIM) surfaces of Whitton and Kuntz, suitably modified at large internuclear distances, were used in the calculations. The probability for hopping between the two surfaces was calculated using the Demkov formalism. Both total cross sections and velocity vector distributions are reported. The reactive cross sections to give HeH++H were not affected by the accessibility of the excited potential surface. By comparison, the results for collision-induced dissociation (CID) to give He+H++H were quite revealing. In an earlier paper we have shown that it is possible to distinguish the two (nearly) degenerate product states in CID. The present work shows that between 33% and 45% of the CID products appear in the excited electronic state. The H+ velocity distributions are quite different in the two product states. The results obtained here are compared with other theoretical and experimental work on this system.
Propargylene: A C3H2 isomer with unusual bonding
Authors: Maier Günther , Reisenauer Hans Peter , Schwab Wolfgang , Čársky Petr , Špirko Vladimír , Hess B. Andes , Schaad Lawrence J.
Published: July 2002

Propargylene was identified in a matrix as a product of photolysis of cyclopropenylidene and diazopropyne. The molecule is a triplet. The optimum geometry predicted by ab initio calculations corresponds to a structure HC≡C–C̈H. The transition structure in the interconversion HC≡C–C̈H⇄HĊ=C=ĊH⇄HC̈–C≡CH is very low in energy and close to the energy of the vibrational ground state. Owing to this nonrigidity, computed infrared (IR) frequencies based on a harmonic treatment do not match the experimental spectrum. When this nonrigidity is taken into account by using a nonharmonic approximation calculated UMP2/6-31G** IR spectra are in good agreement with the observed spectra of HCCCH and DCCCD.
Infrared and microwave study of angular–radial coupling effects in Ar–HCN
Authors: Fraser G. T. , Pine A. S.
Published: July 2002

Microwave and infrared spectra of Ar–HCN have been obtained using an electric-resonance optothermal spectrometer. The microwave measurements extend to higher J the previous results of Leopold et al. and Klots et al., allowing the determination of higher-order centrifugal distortion constants for this quasilinear, highly nonrigid complex. A Padé approximant fit to the microwave data indicates a significant rotation-induced asymptotic increase in the zero-point center-of-mass separation between the Ar and the HCN, above that expected from pure radial distortion. This results from the large coupling between the angular and radial degrees in the intermolecular potential forcing the centrifugal alignment of the HCN. Infrared spectra are reported for the C–H streching fundamental ν1 and the combination band ν1+ν15, where ν5 is the van der Waals bending vibration. The band-origin difference between these two bands gives ν5=7.8 cm−1, in rough agreement with the 10 cm−1 harmonic value predicted from the microwave-determined nuclear quadrupole coupling constant. The complexation-induced red shift of the C–H stretching vibration is 2.69 cm−1 and the vibrational predissociation linewidths Γ are <10 MHz (FWHM). The vibrationally excited complex predissociates before striking the bolometer detector, implying that the predissociation lifetime τ<1 ms.
The effects of attractive and repulsive interaction on two-dimensional reaction-limited aggregation
Authors: Meakin Paul , Muthukumar M.
Published: July 2002

The effects of both attractive and repulsive interactions on reaction-limited cluster–cluster aggregation have been explored using a two-dimensional polydisperse off-lattice model. Interactions with a magnitude on the order of kBT(kB is the Boltzmann constant and T is temperature) have an important effect in both the aggregate structure and the kinetics of aggregation. Attractive interactions lead to more compact structures and repulsive interactions lead to less compact structures with a lower effective dimensionality. The effects of power law particle–particle interactions with interaction energies of the form E=A/r Ε (r is distance) on aggregate structure, aggregation kinetics and the evolution of the cluster size distribution were explored for Ε=2, 4, and 6. A mean field variational theory indicates that fractal dimensionality is unchanged if Ε≥2D0 (where D0 is the fractal dimensionality in the absence of interactions) and that D=D0 (Ε+2)/2(D0+1) for Ε in the range 1≤Ε≤2D0. The simulation results are in accord with this theoretical analysis.
The adsorption and thermal decomposition of formic acid on Si(100) and Si(111) surfaces
Authors: Tanaka S. , Onchi M. , Nishijima M.
Published: July 2002

The adsorption of formic acid (HCOOH) on the Si(100)c(4×2), Si(100)(2×1), Si(100)9° vicinal and Si(111)(7×7) surfaces has been studied by means of high resolution electron energy loss spectroscopy. Formic acid is partially dissociated to form the unidentate formate species (HCOO) and H adatoms on these surfaces at both 90 and 300 K. The formate species is bonded to the Si surface with a covalent bond formed between one of its oxygen atoms and the dangling bond of the surface Si atom. A condensed multilayer of molecular formic acid is formed after the Si surface is saturated with the formate species at 90–150 K. The thermal decomposition mechanism has been studied in detail. The decomposition temperatures of the formate species on Si(100) and Si(111) are ∼600 and 550 K, respectively. It appears that the surface steps do not play an important role in the Si–HCOOH interaction. As the formate-covered surface is exposed to atomic hydrogen, methylendiolate (H2COO) is formed on the Si surface.
The effect of site-specific isotopic substitutions on transport coefficients of liquid methanol
Authors: Weingärtner H. , Holz M. , Sacco A. , Trotta M.
Published: July 2002

A new approach is suggested for studying the nature of molecular transport in simple liquids, which makes use of site-specific isotopic substitutions. Its application represents the first systematic experimental study of a theoretically predicted correlation between transport coefficients in liquids and molecular moments of inertia. For this purpose, we have determined the viscosities and self-diffusion coefficients at 25 °C of normal methanol and seven isotopically labeled methanol species: CH3OD, CD3OH, CD3OD, CH2DOH, CHD2OH, CHD2OD, and 13CH3OH. Except for 12C/13C substitution, the observed isotope effects are significantly larger than predicted by a square root of mass dependence, but are well correlated with the square roots of the moments of inertia. The results give strong evidence that translation–rotation coupling influences the transport processes in methanol, thus confirming earlier interpretations of isotope effects upon the transport in water.
Laser-induced fluorescence spectroscopy of NCS in a free jet expansion
Authors: Northrup F. J. , Sears Trevor J.
Published: July 2002

Laser-induced fluorescence excitation spectra have been recorded for many vibronic transitions of the A(2Π)–X(2Π) and B(2Σ+)–X(2Π) systems of NCS under supersonic free jet expansion conditions. New assignments have been made for many of these bands and several of the assignments from previous work have been revised. Vibronic energies have been determined for levels involving excitation of all three vibrational modes in the ground electronic state and both excited electronic states. A detailed rotational and vibrational analysis has been carried out for levels involving excitations of the two stretching modes and the many rotational and potential function constants have been determined for the A and X states. The variation of the spin–orbit constant with vibrational level has also been investigated. Results of this paper provide the ground work for a detailed analysis of the Renner–Teller effect in the bending vibrational mode of this radical for both the X2(Π) and A(2Π) states to appear in a forthcoming paper. They also provide a spectroscopic database for future dynamical and kinetics studies of processes involving this radical.
Elastic and dynamic properties of polymer electrolytes: A Brillouin scattering study of poly(propylene glycol)–NaCF3SO3 complexes
Authors: Sandahl J. , Schantz S. , Börjesson L. , Torell L. M. , Stevens J. R.
Published: July 2002

In the present study of ion conducting polymers we demonstrate how light scattering can be used to obtain information about the elastic properties and the local intrachain mobility, which are essential characteristics of a high performance polymeric solid electrolyte. Brillouin scattering has been applied in studies of sodium triflate (NaCF3SO3)–poly(propylene glycol) (PPG) complexes of concentrations O:M=5:1, 8:1, and 16:1, where O:M is the backbone oxygen to sodium ratio. From the spectra sound velocity and absorption data were obtained. The acoustic data reveal structural relaxation processes associated with local translational (and orientational) intrachain motions of the polymer segments. The characteristic relaxations times τ were determined and found to be strongly dependent on the concentration of the dopant salt. This is because the dissociated ions act both as crosslinks betwen polymer chain segments and as ‘‘free’’ ions available for conduction. Therefore, as the concentration of dissociated ions increases the intrachain mobility is slowed down (τ increases) due to increased crosslinking. The effect is demonstrated in the hypersonic velocity dispersion curve and in the absorption peak which both shift to higher temperatures with increasing salt concentration. The shift correlates with the increase in the glass transition temperature as salt is added to the polymer. It also correlates, at least at low salt concentrations, with the concentration of dissocated ions and with the conductivity. It can be concluded that the motions of free ions responsible for charge transport are strongly coupled to the intrachain motions of the polymer.
Weakly bound NeHF
Authors: ONeil S. V. , Nesbitt D. J. , Rosmus P. , Werner H.-J. , Clary D. C.
Published: July 2002

We have used ab initio methods to characterize the Ne–HF van der Waals complex. The interaction energy was determined using size consistent, correlated CEPA wave functions expanded in a Gaussian basis chosen to represent both intraatomic effects and the low order multipole moments and polarizabilities of Ne and HF. The calculated well depths are −65 cm−1 for linear Ne–HF and −39 cm−1 for linear Ne–FH, with an intervening saddle point at −27 cm−1. The induction contribution to the energy is significantly greater for Ne–HF than for Ne–FH, but dispersion remains the dominant attraction over the region of interest. Converged variational and close-coupling calculations using the ab initio potential surface reveal three bound levels of the Ne–HF stretch mode, and several metastable levels correlating asymptotically with rotationally excited HF( j=1). Though nearly degenerate, the lifetimes of the two metastable Π (body frame Λ=±1) bending levels differ markedly because of different rotational coupling strengths to the Σ (body frame Λ=0) bending state, which undergoes rapid rotational predissociation. From the calculated line positions, widths, and intensities we have synthesized far infrared and infrared spectra of Ne–HF and Ne–DF.
Vibrationally (and rotationally) inelastic scattering characteristics for the He+I*2 system
Authors: Krajnovich Douglas J. , Butz Kirk W. , Du Hong , Parmenter Charles S.
Published: July 2002

An analysis is provided for the state-resolved vibrationally inelastic scattering cross sections σ(Δυ) for He interacting with I2 B0+u molecules in either υ′=15, 25, or 35. The collision energy for these crossed molecular beam data is 720 cm−1 (89 meV), whereas the local I*2 vibrational quantum size varies from about 100 to 60 cm−1. The σ(Δυ) encompass scattering events with Δυ ranging to ±3 for υ′=15 and to ±7 for υ′=35. The sets of σ(Δυ) for each initial υ′ scale with an exponential energy gap law, and the scaling is identical for all initial υ′ levels. Additionally, σ(Δυ) values for conjugate T→V and V→T transitions (i.e., pairs of Δυ=±n for UP vs DOWN transitions) are nearly equal so that the single scaling law σ(Δυ)∝exp(−‖ΔEvib ‖/110 cm−1) describes the entire set of data. The scaling for the He target beam is identical to that for D2 but different from H2 indicating that the pattern of vibrational energy flow is determined mainly by the mass of the target gas and collision energy as opposed to subtle details of the interaction potential. 1D and 3D classical trajectory calculations replicate the principal characteristics of the scattering, particularly the common exponential scaling and UP–DOWN symmetry of conjugate σ(Δυ), but fail to account quantitatively for processes with large Δυ. The vibrational flow pattern is not markedly influenced by big variations in the rotational energy content of the initial υ′ level. The competition between rotationally and vibrationally inelastic scattering is about the same for each initial υ′. The rotational cross section is only about 2.5× larger than σ(Δυ=−1), the largest vibrational cross section. The total vibrational cross section, however, actually equals or exceeds that for pure rotationally inelastic scattering for all initial υ′ levels. Comparisons are made with the vibrational and rotational energy transfer characteristics observed in 300 K bulb experiments.
Dipole-bound anionic state of nitromethane. A
b i
n
i
t
i
o coupled cluster study with first-order correlation orbitals
Authors: Adamowicz Ludwik
Published: July 2002

The theoretical procedure based on the coupled cluster method and the first-order correlation orbital for studying dipole-bound states of polyatomic molecules is presented. The calculation of the lowest dipole-bound state for the nitromethane molecule indicates that the molecular dipole of this molecule, which is experimentally known to be 3.46 D, can attract an extra electron with the energy of 0.000 127 hartree and form a bound state. This state should be considered as a short-living intermediate leading to the formation of a stable valence-bound anion.
Nature of the transformations of ice I and low-density amorphous ice to high-density amorphous ice
Authors: Floriano M. A. , Handa Y. P. , Klug D. D. , Whalley Edward
Published: July 2002

The changes of enthalpy at the irreversible transformation of high-density amorphous ice to low-density amorphous ice and of low-density amorphous ice to ice Ih have been measured in a Tian–Calvet heat-flow calorimeter. The equilibrium pressures of the two transformations have been estimated from the measured enthalpy and volume changes, neglecting the unknown, but small, entropy changes, as about 5 and 2 kbar, respectively. The course of the transformations has been followed in a piston-cylinder apparatus having an internal diameter of either 50 or 12 mm. The transformations are extraordinarily sharp when they are done slowly, the transformation pressure being constant to ±50 bar in 5 or 10 kbar in several transformations, although sometimes a sample transformed in two steps that were a few hundred bars apart. It seems that the activation volumes of the transformations are large and negative, and so the mechanism of the transformations is highly cooperative.
Direct measurement of the HCl dimer tunneling rate and Cl isotope dependence by far-infrared laser sideband spectroscopy of planar supersonic jets
Authors: Blake Geoffrey A. , Bumgarner R. E.
Published: July 2002

The large amplitude tunneling motion of the HCl dimer has been directly studied with a tunable far-infrared laser sideband/two-dimensional free jet expansion spectrometer at hyperfine resolution. Rotationless tunneling rates for the three common chlorine isotopic forms are v(35–35) =463 979.2(1) MHz, v(35–37)=463 357.7(1) MHz, and v(37–37)=462 733.7(3) MHz. Both the rotational constants and hyperfine parameters indicate that the vibrationally averaged structure shows little variation within a given tunneling state, with both HCl bond angles giving an average projection on the a-axis of 47° in all states with resolved hyperfine patterns.
Solute–solvent interactions in infinitely dilute supercritical mixtures: A molecular dynamics investigation
Authors: Petsche Irena B. , Debenedetti Pablo G.
Published: July 2002

A molecular dynamics investigation of the environment surrounding an infinitely dilute Xe-like Lennard-Jones atom in a supercritical Ne-like Lennard-Jones fluid shows enhanced solute–solvent interactions (clustering) in the vicinity of the solvent’s critical point. Under near-critical conditions, the solute is at all times surrounded by an environment which is greatly enriched in solvent with respect to bulk conditions, but the identity of the solvent molecules in the cluster changes continuously. The enhancement of solute–solvent interactions in the Xe-in-Ne near-critical system is in contrast with the behavior exhibited by the symmetric Ne-in-Xe near-critical system (i.e., infinitely dilute Ne in near-critical Xe). In this case, the environment surrounding Ne atoms tends to be solvent lean with respect to bulk conditions. As Ne’s critical point is approached, Ne–Xe interactions become progressively irrelevant with respect to Ne–Ne interactions, giving rise to pronounced density fluctuations around the Xe atoms. Differences between Xe-in-Ne and Ne-in-Xe near-critical systems are confirmed by cluster statistics and stability analysis.
Modified rotationally adiabatic model for rotational autoionization of dipole-bound molecular anions
Authors: Simons Jack
Published: July 2002

Modifications are introduced to a rotationally adiabatic model developed and successfully applied by Clary to describe rotational autoionization of dipole-bound molecular anions. These modifications address branching ratios for forming neutrals in various final rotational states. They also produce rate-enhancement corrections to the original model in the neighborhoods of channel openings. Example calculations are presented and the essential ingredients of the corrections are tabulated in a manner that permits straightforward application to any linear or pseudolinear (i.e., symmetric top with ΔK=0) anion.
Ultrafast vibrational predissociation of hydrogen bonds: Mode selective infrared photochemistry in liquids
Authors: Graener H. , Ye T. Q. , Laubereau A.
Published: July 2002

Using intense ultrashort excitation pulses in the infrared (IR) tuned to the CH or oligomeric OH absorption bands of ethanol dissolved in CCl4, vibrational predissociation and partial reassociation of hydrogen bonds are observed on the picosecond time scale. Induced absorption at the OH stretching frequency 3500 cm−1 of proton donor end groups serves as a spectroscopic probe of the broken H bridges and is studied as a function of time by the help of delayed, tunable interrogation pulses. Different values for the effective dissociation rates and total quantum yields provide strong evidence for mode specifity of the IR photodissociation in the liquid at ambient temperature. The experimental results on vibrational population decay and energy transfer rates suggest a V–V transfer mechanism for CH to OH modes for the vibrational predissociation after CH excitation.
Pulsed-nozzle Fourier-transform microwave spectroscopy of laser-vaporized metal oxides: Rotational spectra and electric dipole moments of YO, LaO, ZrO, and HfO
Authors: Suenram R. D. , Lovas F. J. , Fraser G. T. , Matsumura K.
Published: July 2002

The rotational spectra of YO, LaO, ZrO, and HfO have been measured using a Fourier-transform microwave spectrometer in combination with a laser-ablation source. Here, a Q-switched Nd:YAG laser (532 nm) was used to vaporize the metal oxides from a target source rod located in the throat of a pulsed-molecular-beam valve. A description of the instrument is given. The electric dipole moments of the four species have been measured and compared to ab initio results, where available. The experimental values are μYO =4.524(7), μLaO =3.207(11), μZrO =2.551(11), and μHfO =3.431(5) D. Of special note are the extremely large nuclear quadrupole coupling constants, eQq, determined for the 177HfO and 179HfO isotopic species, with values of −5952.649(35) MHz and −6726.981(39) MHz, respectively. This is the first determination of nuclear quadrupole coupling constants for a molecule containing the Hf atom.
On the effects of solvent and intermolecular fluctuations in proton transfer reactions
Authors: Morillo M. , Cukier R. I.
Published: July 2002

We present a theory of proton transfer reactions which incorporate the modulation of the proton’s potential surface by intermolecular vibrations and the effect of coupling to solvent degrees of freedom. The proton tunnels between states corresponding to it being localized in the wells of a double minimum potential. The resulting tunnel splitting depends on the intermolecular separation. The solvent response to the proton’s charge is modeled as that of a damped oscillator, allowing the introduction of friction effects in the solvent dynamics. The rate of transfer is evaluated by perturbation theory in the level splitting. We find that typically the intermolecular and solvent contributions enhance the rate relative to the values obtained in their absence. This effect is evident at low temperature where friction can enhance the rate by increasing opportunity for solvent tunneling. At high temperature the intermolecular motion enhances the rate by sampling over a distribution of tunnel splittings.
Zeeman spectroscopy and deperturbation of the low-lying states of NiH
Authors: Gray Jeffrey A. , Li Mingguang , Field Robert W.
Published: July 2002

High resolution laser spectroscopy of the NiH molecule in a magnetic field has revealed strong homogeneous and heterogeneous perturbations among all of the low-lying electronic states. Fully resolved Zeeman splitting patterns from transitions between NiH magnetic sublevels were recorded with the technique of Zeeman optical–optical double resonance (ZOODR) spectroscopy. Using only the zero-field rotational energy levels as input to an electronic structure model, we have calculated Zeeman splittings (g values) for 19 rotational levels, and the predicted splittings are in very good agreement with observed Zeeman spectra. A group of 10 NiH molecular electronic states is seen to form a supermultiplet of levels originating from the Ni+ (3d9)2D atomic multiplet. We describe an effective Hamiltonian matrix that contains explicit terms coupling low-lying states through spin–orbit, vibrational, and rotational interactions. Supermultiplet eigenvectors graphically illustrate the profound mixing hidden beneath the apparent regularity of term value plots for the low-lying states of NiH. The success of the supermultiplet model for this simplest case (a single hole in a highly contracted 3d subshell), namely the successful prediction of strongly J dependent g values, makes us confident that this model will be applicable to other transition metal monohydrides.
Pseudolocal modes of guest molecules in mixed molecular crystals: Photon echo experiments and computer simulations
Authors: Olson R. W. , Meth Jeffrey S. , Marshall C. D. , Newell Vincent J. , Fayer M. D.
Published: July 2002

The temperature-dependent optical dephasing of anthracene, 9-methylanthracene, and 2-methylanthracene monomers in phenanthrene host crystals has been measured using photon echo experiments. Despite large linear electron–acoustic phonon coupling, all three systems dephase because of coupling to pseudolocal modes (local motions of the guest molecule). Computer simulations of the three systems calculate the pseudolocal mode eigenvalues and eigenvectors. In contrast to previous discussions in the literature which describe pseudolocal modes as librations, the predicted eigenvalues are in reasonable agreement with the measured pseudolocal mode energies. The predicted eigenvectors are combinations of translational motion along the long molecular axis and rotational motion about the out-of-plane axis of the guest. Differences in site energies for various locations and orientations of the methyl group are calculated.
A study of the neon–methane interaction potential as determined from low energy molecular beam experiments
Authors: Phillips Timothy R. , Secrest Don
Published: July 2002

An exhaustive search of the parameter space for the Ne–CH4 interaction potential is made in an attempt to determine a potential which will reproduce the results of molecular beam scattering experiments. The experiments were performed at low energy, well below the threshold for vibrational excitation. It is shown that the effects of the long-range part of the potential were not seen in the experiment. It is further shown that the anisotropy is well represented as far as the experiments are concerned by a fourth rank expansion. In the light of this and earlier studies, it is concluded that a rigid top model for methane is not capable of describing the experimental results even though the collision energy is well below the threshold for vibrational excitation. This is in marked contrast to atom–diatomic molecule scattering.
Photoexcitation of M(CH3)2 (M=Zn, Cd, Hg) compounds in the 106–270 nm region
Authors: Ibuki Toshio , Hiraya Atsunari , Shobatake Kosuke
Published: July 2002

The photoabsorption cross sections and fluorescence excitation spectra of group IIb dimethylmetals were measured using synchrotron radiation at 106–270 nm. The observed absorption bands were accounted for in terms of Rydberg transitions. The fluorescence spectra of MCH3 (Ã 2E→X̃ 2A1) transitions were observed and the radiative lifetimes determined were 47±2 and 62±2 ns for M=Zn and Cd, respectively. The excited M(np 3P01) atoms were detected in the photolyses of dimethylcadmium and dimethylmercury, but not in dimethylzinc. The formations of the electronically excited MCH3 (Ã 2E) and M(np 3P01) states are characteristic of the Rydberg excitations of the C–H in methyl and M–C bonding electrons, respectively.
Surface photochemistry 15: On the role of substrate excitation
Authors: Hatch S. R. , Zhu X.-Y. , White J. M. , Campion Alan
Published: July 2002

The rates of photodissociation and photodesorption of dioxygen adsorbed on Ag (110) under ultraviolet irradiation have been measured as a function of polarization at normal incidence. No dependence of the rate was found on the angle between the O2 bond axis and the electric vector of the incident light. This result supports a mechanism for surface photochemistry in which substrate excitation is the dominant process.
Structure of nonaqueous electrolyte solutions by small-angle neutron scattering, hypernetted chain, and Brownian dynamics
Authors: Kunz W. , Calmettes P. , Turq P.
Published: July 2002

Small-angle neutron scattering from solutions of n-tetrapentylammonium bromide in deuterated acetonitrile was studied at room temperature for concentrations between 0.17 and 1.01 M. The hypernetted chain (HNC) integral equation with solvent-averaged pair potentials was used to describe both the scattering spectra and the osmotic coefficient of the nondeuterated solutions. It is shown that the so-called Friedman–Gurney potential can be replaced by a simple soft sphere potential in order to reproduce the scattering data. The results give a new insight into the structure and the distribution of ions. Within statistical uncertainties Brownian dynamics simulations for the same pair potentials lead to correlation functions similar to those given by HNC.
A low-energy quasiclassical trajectory study of O(3P)+OH(2Π) →O2(3Σ−g)+H(2S). I. Cross sections and reaction dynamics
Authors: Davidsson Jan , Nyman Gunnar
Published: July 2002

Quasiclassical trajectory calculations for the title reaction have been carried out using the recent DMBE III (double many body expansion) potential-energy surface by Varandas et al. (1988). The dynamics of complex formation were studied in detail and two different types of complexes, referred to as close and distant complexes, were observed. Corresponding cross sections and reaction cross sections are reported as a function of diatom rotational quantum number and atom–diatom relative translational energy in the range 0.25–2.0 kcal/mol. The cross sections decrease with increasing translational energy. The rotational-state dependence of the reaction cross section is found to be complicated. One interesting observation is that the cross section for the rotational ground state is much smaller than for other rotational states. The features in the cross sections are related to the shape of the potential-energy surface and the reaction dynamics.
Quantum theory of the electronic absorption line shape of a solvated molecule
Authors: Loring Roger F.
Published: July 2002

A quantum statistical mechanical theory of the electronic absorption line shape of a polarizable solute molecule at infinite dilution in a polarizable solvent is developed. Solute and solvent molecules are modeled as electronic two-level systems whose interactions have the transition dipole–transition dipole form. The theory predicts solvent-induced line broadening, as well as a line shift. The inhomogeneously broadened line shape corresponding to a single vibronic transition of the solute molecule is predicted to be asymmetric. The line shape falls off more slowly as the frequency is tuned from the solute absorption peak away from the solvent absorption band, than it does as the frequency is tuned toward the solvent absorption band. The results suggest that the line shape is sensitive to the local fluid structure.
Nonadiabatic effects in photodissociation dynamics: A quantum mechanical study of ICN photodissociation in the A continuum
Authors: Guo Hua , Schatz George C.
Published: July 2002

The photodissociation dynamics of ICN in the A continuum has been studied using a quantum coupled-channel method. The two-state empirical potential energy surfaces derived by Goldfield et al. are used in our calculations, and the CN bond is assumed frozen throughout the dissociation (rigid-rotor approximation). The excited state dynamics involving nonadiabatic transitions between the I and I* channels is treated in a diabatic representation. The CN rotational distributions have been obtained for several wavelengths and zero total angular momentum by calculating the appropriate Franck–Condon integral. Bimodal structure in CN rotational distributions is obtained in the quantum calculations, but the width of the peaks is narrower than in semiclassical results for the same surfaces. Discrepancies between quantum and semiclassical results are also found in the total cross-section and in the I*/I branching ratio. Comparison of our quantum results with experiment indicates that the potentials derived from the semi-classical calculations are not accurate enough to describe the ICN photodissociation dynamics. Further modifications are needed for the potential energy surfaces.
An interpretation of the multiple fluid–fluid transitions in liquid supported amphiphile monolayers
Authors: Shin Seokmin , Wang Zhen-Ghang , Rice Stuart A.
Published: July 2002

We describe an extension of the generalized regular solution model of a liquid supported monolayer of long chain amphiphile molecules in which chain–chain interactions outside the surface layer are included as well as the effects of chain flexibility in the surface and in the space above the surface layer. Self-consistent equations for the distribution of molecular configurations and the surface density and pressure are derived by using a model free energy expression. Two first order fluid–fluid phase transitions are predicted for three different models for the chain–chain interactions outside the surface. These transitions are identified as the observed gas-to-‘‘liquid-expanded’’ and liquid-expanded-to-‘‘liquid-condensed’’ phase transitions in a monolayer, the first to be associated with the condensation of the in-surface portions of the amphiphile chains and the second to be associated with the condensation of the out of surface portions of the amphiphile chains. This interpretation comes from the observation of the different distributions of the molecular configurations in the coexistence regions for the two transitions and is reinforced by the analysis of the sensitivity of the equation of state to the parameters used in the model. The theory leads to a simple and qualitative but accurate physical picture of the successive fluid-fluid transitions of the monolayer.
Electronic spectroscopy of the ArOH and ArOD complexes
Authors: Fawzy Wafaa M. , Heaven M. C.
Published: July 2002

Laser induced fluorescence spectra of the ArOH bands associated with electronic excitation of the OH A–X(v′=0−v″=0) transition were reported previously. An extensive search in the vicinity of the OH/OD A (2 Σ+ )−X(2 Π) system (v′=0−v″=0 and v′=1−v″=0) led to the observation of 36 vibronic bands which were identified as belonging to ArOH or ArOD complexes. All of the bands were found to originate from the 2 Π(3/2), v″=0 ground electronic state of the OH/OD radical. Two distinguishable vibronic structures were identified. A lower energy vibrational progression was assigned to the van der Waals stretch. This band system was designated as the ‘‘A’’ bands. A higher energy set of bands exhibited a different vibrational structure which did not fit a simple vibrational mode progression. These bands were labeled as the ‘‘U’’ system. Spectra showing partially resolved rotational structure were recorded for all of the observed ArOH/ArOD bands. The A and U systems were also distinguished by differences in their rotational structure and effects of H/D substitution on the vibrational and rotational parameters. Preliminary rotational analysis gave ground state constants (B″0 ) of 0.100 cm−1 for ArOH and 0.098 cm−1 for ArOD. (Be constants for the excited state of the A system were determined to be 0.178 cm−1 for ArOH and 0.168 cm−1 for ArOD. These rotational constants show that the Ar–OH/Ar–OD van der Waals bond has been considerably shortened upon electronic excitation. This suggests that partial charge transfer might contribute to the stability of ArOH and ArOD complexes in the excited electronic state. Observed spectra and results of rovibronic analysis are discussed.
The gyration radius distribution of two-dimensional polymer chains in a good solvent
Authors: Victor J. M. , Lhuillier D.
Published: July 2002

The probability distribution for the gyration radius of a two-dimensional self-avoiding walk is determined numerically by a Monte Carlo simulation on a square lattice, using the ‘‘slithering snake’’ algorithm of Wall and Mandel. The results are in good agreement with recent scaling predictions.