bluuebunny/crossref_metadata_2025 huggingface dataset:
Highly excited vibrational states of molecules by thermal lensing spectroscopy and the local mode model. I. CHCl3, CHBr3, CH2Cl2, CH2Br2
Authors: Fang Howard L. , Swofford Robert L.
Published: February 2003

Visible absorption spectra (14 700–19 100 cm−1) measured with the thermal lensing spectrometer and near-infrared absorption (5 600–14 000 cm−1) measured with a conventional spectrophotometer are reported for CHCl3, CHBr3, CH2Cl2, and CH2Br2. Relatively strong absorption peaks are identified as overtones of the C–H stretching vibrations in these compounds. The overtone spectra are analyzed in terms of the local mode (LM) model, which treats the molecule as a set of loosely coupled anharmonic oscillators localized on individual C–H bonds. Relatively less intense peaks are observed and are assigned as combinations of a local mode C–H vibration and some lower frequency (normal) mode of the molecule. Fermi resonance is seen to occur in the spectra of CHCl3 and CHBr3, resulting in anomalously high intensities for the combination bands involved. Significant anharmonic local–normal coupling constants are seen for these same molecules. These coupling constants appear to indicate the presence of strong physical coupling of the two motions involved. The importance of these coupled motions in the intramolecular relaxation of vibrational energy is suggested.
Double ion chamber measurements of some photoionization cross sections and efficiencies
Authors: Borkenhagen Alan K. , Carr Robert W.
Published: February 2003

A double ion chamber has been used to measure photoionization cross sections and efficiencies of some polyatomic molecules at 123.6 nm with a Kr resonance lamp. An apparent increase of cross section with decreasing pressure, and an opposite, but smaller, pressure effect on the efficiency, was observed. Mathematical models for ion and electron distribution and motion in the chamber predict pressure effects similar to those observed in the case where ion pair formation occurs very close to the positive ion collector electrode.
Direct spectroscopic observation of proton exchange and Bjerrum defect migration in cubic ice
Authors: Ritzhaupt Gary , Devlin J. Paul
Published: February 2003

Evidence is presented that two steps in proton migration in ice can be clearly observed and followed spectroscopically for D2O isolated samples. The spectra and rates are evaluated at 145, 150, and 155 K. (AIP).
On the electron affinity of glyoxal and methyl glyoxal
Authors: Rawlings Diane C. , Davidson Ernest R.
Published: February 2003

a b initio calculations gives an estimated adiabatic electron affinity of 0.9 eV for glyoxal and 0.7 eV for methyl glyoxal. (AIP)
K, L, and M shell generalized oscillator strengths and ionization cross sections for fast electron collisions
Authors: Leapman R. D. , Rez P. , Mayers D. F.
Published: February 2003

Generalized oscillator strengths and ionization cross sections by fast electron impact are calculated for K, L, and M shells. A nonrelativistic Hartree–Slater program is used to generate the initial states as well as the continuum wave functions. Core edge shapes and their dependence on momentum transfer are computed within this atomic model up to some tens or hundreds of eV above threshold. Some comparisons are made with experimental measurements and though details near threshold are not predicted, the gross shape of the spectrum is in quite good agreement with these data. While we confirm that the hydrogenic model is a reasonable approximation for K edges, we expect our computations to be useful in obtaining more accurate total cross sections for L and M shell excitation.
Photoexcitation and ionization in molecular fluorine: Stieltjes–Tchebycheff calculations in the static-exchange approximation
Authors: Orel A. E. , Rescigno T. N. , McKoy B. V. , Langhoff P. W.
Published: February 2003

Theoretical investigation of outer (1πg, 1πu, 3σg) and inner (2σu, 2σg) valence-shell electronic photoexcitation and ionization cross sections in molecular fluorine are reported employing separated-channel static-exchange calculations and Stieltjes–Tchebycheff (S–T) moment-theory techniques. The discrete vertical electronic 1πg excitation series are found to be in good agreement with recent spectral assignments and previously reported theoretical studies, and those for 1πu, 3σg, 2σu and 2σg excitations are in general accord with position and intensity estimates based on quantum-defect analysis. Certain of the partial-channel photoionization cross sections in F2 are seen to exhibit resonancelike features similar to those reported recently in related S–T studies of photoionization in N2, CO, and O2. The resonances can be attributed to valencelike and pre-Rydberg diabatic states that cross the outer limbs of appropriate Rydberg series and corresponding ionic-state potential curves as functions of internuclear coordinate, giving rise to large continuum transition intensities at the ground-state equilibrium internuclear separation. In contrast to the situation in N2, CO, and O2, however, there is no evidence of a resonance like σ→σ* feature in the 3σg→kσu photoionization channel in F2. Rather, this resonance in F2 appears as a strong N→Vg transition below the 3σg ionization threshold, and the corresponding partial-channel photoionization cross section is seen to be structureless. Although experimental studies of partial-channel photoionization cross sections are apparently unavailable for comparison, the calculations reported here should provide reliable approximations to the dipole excitation/ionization spectra in F2, and are helpful in understanding and clarifying the dependences of photoionization spectra in light diatomic molecules on shell occupancy and equilibrium internuclear separation when compared with the results of previous studies of photoionization in N2, CO, and O2.
Polarizability calculations with the SCF method. II. The benzene molecule
Authors: Zamani-Khamiri O. , McIntyre E. F. , Hameka H. F.
Published: February 2003

We evaluate the total third order electric polarizabilities of the benzene molecule by means of the extended Hückel method and we calculate the π electron contribution to the third order electric polarizabilities by means of the Pariser–Parr–Pople (PPP) method. The extended Hückel method leads to a value of γ=0.8578×10−36 esu and our most precise PPP treatment gives γπ=0.9782×10−36 esu. By combining the various results we obtain a theoretical value γ=1.1332×10−36 esu which is consistent with the experimental results. We also report calculations of the linear static and dynamic polarizabilities.
A theoretical study of the potential energy surface for OH+H2
Authors: Walch Stephen P. , Dunning Thom. H.
Published: February 2003

Barrier heights and transition state geometries have been calculated for the reaction OH+H2→H2O+H using large scale POL-CI wave functions (based on GVB wave functions using basis sets of up to triple zeta valence plus double zeta polarization quality). The saddle point geometry is found to be coplanar and to resemble OH+H2 as expected because of the large exoergicity (∼16 kcal/mole) of the reaction. The OH distance of the OH moiety is essentially the same as for the OH molecule, while the HH distance of the H2 moiety is 0.10 Å (∼14%) longer than for H2. The distance from the O to the near hydrogen of the H2 moiety is 0.35 Å (∼36%) longer than for the H2O molecule. The HOH angle is 98° and the H2 moiety is tilted from collinearity with the O atom by 15° toward the H of the OH moiety. The calculated barrier height using a [4s3p2d/3s2p] basis set is 6.2 kcal/mole. Transition state theory calculations (including a Wigner tunneling correction) using the theoretically computed surface predict rate constants which are in excellent agreement with experiment over the temperature range of 300–2000 °K.
Polaritons in molecular crystals
Authors: Luty T. , Uszynski I.
Published: February 2003

The simultaneous propagation of lattice phonons and photons in molecular crystals is discussed. The equation of propagation of the modes involving molecular displacements and an electric field is derived based on the concept of dynamic multipoles and taking into account retardation effects. The equation is analyzed mainly in a long wave limit in order to study the optical properties of lattice phonons. The elastic and piezoelectric constants are then obtained in terms of molecular parameters of the model. Finally, as an example, the polariton spectrum of crystalline HCN is analyzed.
Isotope effect in multiphoton ultraviolet photolysis of CO
Authors: Bokor J. , Zavelovich J. , Rhodes C. K.
Published: February 2003

Two-photon dissociation of carbon monoxide near 193 nm with a tunable ArF* laser to yield excited C(2 1D) atoms is reported. The atoms are detected by subsequent absorption of a third laser photon to C(3 1P°) followed by emission of 248 nm fluorescence to C(2 1S). The process is found to be isotopically sensitive; the C(2 1D) yield from 13C 16O and 12C 18O is a factor of 6 greater than from 12C 16O. The isotope effect is attributable to an enhancement in the two-photon matrix element due to the shift in the near resonant a 2Π, v=2 intermediate state. C2 Swan band emission is also observed, arising from association of the free carbon atoms, and exhibits a corresponding isotope effect. Finally, collisional processes are observed involving excited carbon atoms and electrons, using two-photon ionization of a small quantity of added xenon atoms as a new technique for producing a controlled density of free electrons.
Correlation effects in the isomeric cyanides: HNC↔HCN, LiNC↔gLiCN, and BNC↔gBCN
Authors: Redmon Lynn T. , Purvis George D. , Bartlett Rodney J.
Published: February 2003

Correlated values of the isomerization energy and barrier for the HNC→HCN reaction are obtained from many-body perturbation theory, including the effects of quadruple excitations. Extended basis sets of better-than-triple-zeta plus double-polarization quality are used, as well as basis sets including counterpoise and bond-centered orbitals. The best of these basis sets is sufficient to account for 84% of the valence correlation energy of HCN. These studies predict an isomerization energy for the HNC→HCN rearrangement to be −15±2 kcal/mole, in disagreement with a recent experimental value of −10.3±1. kcal/mole. Correlated isomerization energies of LiCN→LiNC and BCN→BNC are obtained in bases of double-zeta plus polarization quality. In all cases, correlation stabilizes the cyanide isomer more than the isocyanide. Trends in the series R–NC⇄RCN for R=H, Li, and B are discussed.
High-temperature Raman spectroscopic studies of indium halide vapors and molten salts: InX, InX2, InX3, and InAlX4 (X=Cl, Br)
Authors: Radloff Patricia L. , Papatheodorou G. N.
Published: February 2003

Raman spectra were obtained at temperatures up to 1200 K for vapors over liquid indium halides with In:X (X=Cl, Br) ratios of 1:1, 1:2, and 1:3 and for vapors over liquid InAlX4. Spectra were also measured for molten InX2 and InAlX4 salts. The spectra of InX vapors consisted of O, Q, and S vibrational–rotational contours with maxima at ΔνQ(InCl) =305 cm−1 and ΔνQ(InBr) =213 cm−1. Pure rotational O and S band contours were also resolved at frequencies close to the frequency of the laser excitation line. Vibrational–rotational contours for the InX molecules were simulated using known molecular constants and accounting for contributions of both trace and anisotropic scattering. Preresonance enhancement of the Raman intensities and changes in the ’’linear’’ depolarization ratio with laser frequency were also observed for the InX molecules. The spectra of vapors over InX3 were characteristic of a dimer–monomer indium (III) halide equilibrium. Raman spectra of vapors over InX2 and InAlX4 liquids show the formation of vapor complexes and vapor dissociation according to the reactions: A comparison of the Raman spectra of liquid InX2 and InAlX4 with the corresponding spectra of the vapors indicates that the same molecular species are present in both phases.
Evidence for two-dimensional liquid–vapor coexistence: A Monte Carlo computer simulation
Authors: Abraham Farid F.
Published: February 2003

The reality of liquid–vapor coexistence for a two-dimensional fluid is demonstrated using the Monte Carlo simulation method. (AIP)
Triplet–triplet absorption of pyrazine related to the n+π* (B3u)–n−π* (B2g) splitting
Authors: Inoue A. , Webster D. , Lim E. C.
Published: February 2003

The triplet triplet absorption of pyrazine in the region 230–1900 nm is reported. (AIP)
Comment on ’’Electron molecule scattering in momentum space’’
Authors: Ritchie Burke
Published: February 2003

Certain points that need clarification in the author’s paper on the momentum-space description of electron–molecule scattering are discussed here. (AIP)
Comparative studies of various methods in time domain spectroscopy
Authors: Chahine R. , Bose T. K.
Published: February 2003

Various aspects of time domain and frequency domain analysis of the dielectric response function have been discussed in detail. Time domain results of several alcohols have been found to be in reasonably good agreement with the frequency domain results. The accuracy of the experimental results in the high frequency region is considerably improved by the application of successive synchronized sampling for drift reduction.
Proton spin T1 relaxation dispersion in liquid H2O by slow proton-exchange
Authors: Graf V. , Noack F. , Béné G. J.
Published: February 2003

We have measured the longitudinal proton spin relaxation time T1 in pure water as a function of temperature (10, 37, 80°C) and Larmor frequency (50 Hz⩽ν0⩽50 kHz) by means of field-cycling techniques. T1 becomes frequency dependent below ν0≈5 kHz due to the slow proton-exchange between different oxygen environments (16O, 17O, 18O), which modulates the magnetic 1H–17O interaction. The proton-exchange time τe was found to be Arrhenius-like with activation energy 13.4 kJ/mol and pre-exponential 1.5×10−6 s (37 °C:τe=2.6×10−4 s). This is in fairly good agreement with results obtained previously in the literature by more indirect NMR methods. The new access to proton-exchange reduces the experiemtnal error limits and allows measurements at extremely small frequencies, where the familiar T1ρ technique no longer works.
The role of thermal energy in the laser induced dissociation of complex molecules
Authors: Cox D. M. , Horsley J. A.
Published: February 2003

The role of thermal energy in the laser-induced unimolecular decomposition of a complex molecule, UO2 (hfacac)2⋅THF, has been investigated experimentally and theoretically. In the temperature range 70–150 °C, the laser-induced decomposition yield was observed to vary exponentially with the initial temperature of the molecules at low yields, changing to a linear variation at high yields. The laser-induced decomposition process has been represented by a model that simulates the pumping of an ensemble of molecules, initially spread over a broad Boltzmann distribution, through a series of dissociative levels. This model is able to reproduce the experimental observations assuming a ’’loose’’ transition state that leads to a value for the A factor in the range 1016–1018 sec−1.
Deformation mechanism of polymer spherulite by linear isothermal viscoelastic theory
Authors: Matsuo Masaru
Published: February 2003

A mathematical representation for the deformation mechanism of polymer spherulite is proposed through a linear isothermal viscoelastic theory to investigate the relaxation mechanism of strains and stresses within the spherulite. This description may be represented by the extension of the method proposed by Chen and Wang concerning an elastic problem. Displacements within the spherulite are obtained as solutions of equations of motion omitting an inertia term by assuming two potential functions of complex quantities. Viscoelastic stiffnesses of lamellae in the equations are obtained by applying the correspondence principle to a composite model, in which the mechanical behavior of amorphous phase and grain boundary region is assumed to be dependent upon a three-element viscoelastic model of Maxwell type. As to the results, both the relaxations of strains and stresses within the spherulite are found to be dependent upon the relaxation of viscoelastic constants in bulk.
Mechanical behavior of crystal lattice strains detected by x-ray diffraction technique in a polymer spherulite to external excitation
Authors: Matsuo Masaru
Published: February 2003

A estimation of crystal lattice strain detected by dynamic x-ray diffraction technique is proposed by using a linear isothermal viscoelastic theory. In the actual calculation, the results with regard to the polar angle dependences of the apparent dynamic crystal lattice strain as well as the crystal lattice strain normalized by sample strain, are obtained and are tested by the experimental ones. In addition, from the theoretical calculation, the distribution of the in-phase component of crystal lattice strain is affected by the magnitude with regard to the real part of mechanical quantity such as viscoelastic Young’s modulus, while that of the out-of-phase component is affected by the magnitude with regard to the imaginary part of the mechanical quantity.
Quasi-cw inverse Raman spectroscopy of the ν1 fundamental of 13CH4
Authors: McDowell Robin S. , Patterson Chris W. , Owyoung Adelbert
Published: February 2003

A Doppler-limited Raman spectrum of the symmetric stretching fundamental (ν1) of 13CH4 has been recorded using high-sensitivity ’’quasi-cw’’ inverse Raman spectroscopy. The band is very different in appearance from ν1 of 12CH4, due mainly to a much smaller value of ΔB for the heavier species, which causes many of the transitions to overlap near the band origin. Line assignments have been made for J?11. The spectroscopic constants have been determined from the frequencies of 25 resolved transitions and a computer synthesis of the largely unresolved central region. The observed 12C–13C isotope shift, 1.04±0.02 cm−2, does not agree completely with predictions based on any available set of anharmonicity constants, but it does indicate that a simplified calculation based on a Dennison-type approximation for the anharmonicity constants of isotopic species is not valid. The large difference in ΔB between the two species is attributed to the effect of Coriolis interaction with the nearby combination level ν2+ν4.
Chemical bonding and electronic structure of B2O3, H3BO3, and BN: An ESCA, Auger, SIMS, and SXS study
Authors: Joyner David J. , Hercules David M.
Published: February 2003

We have measured the ESCA, Auger, and SIMS spectra of B2O3 (boron trioxide) and the Auger and SIMS spectra of H3BO3 or B(OH)3 (boric acid). The ESCA data of B2O3 are used with published soft-x-ray-emission (SXS) data to determine the electronic structure of boron trioxide and to make deductions about chemical bonding. This was similarly done for boron nitride using published ESCA and SXS results of other workers. Both of these compounds were found to be partly covalent and partly ionic, and bonding was described in terms of σ bonds in B2O3, and σ and π bonds in BN. Thus, the electronic structure of boron nitride is similar to graphite. A detailed picture of the electronic structure of B2O3 enabled us to understand its KLL Auger spectrum. Two broad peaks were interpreted as KLL band-structure features while fine structure was found to be due to extrinsic plasmons. A similar interpretation was presented for H3BO3. The SIMS spectra of B2O3 and H3BO3 reflected similar structural units in the two compounds; covalent bonding; and weaker B–O bonds in H3BO3.
Quantum effects in rotationally inelastic molecular scattering: K+N2 and K+CO collisions on simple model surfaces
Authors: Schinke R.
Published: February 2003

Rigid-rotor calculations within the infinite-order-sudden approximation have been performed for K+N2 and K+CO collisions in the energy range 0.75 eV?E?2.0 eV and for scattering angles 50°?Θ?150°. Simple model potentials have been chosen to reproduce roughly the experimental findings of Schepper, Ross, and Beck [Z. Phys. A 290, 131 (1979)]. In the asymmetric case two distinct rotational rainbows in the final rotational distribution at each scattering angle are observed, which coincide in collisions with homonuclear molecules. Additional oscillations on the ’’bright’’ side of the main rainbow maxima caused by interference of a maximum of four stationary phase points in the integration over the orientation angle are not yet resolved experimentally. The dependence of these rotational rainbow structures on collision energy, scattering angle and potential parameters is investigated. The findings for homonuclearmolecules are partly described by a simple ’’inversion’’ formula derived from an exponential potential in the case of backward scattering.
Analytic theory of extrema. III. Results for master equations with application to unimolecular decomposition
Authors: Seshadri V. , West Bruce J. , Lindenberg Katja
Published: February 2003

We obtain exact expressions for the mean first passage time to reach a given state in systems described by a master equation. Our results are simple and lend themselves to direct physical interpretation and to a straightforward asymptotic analysis. An asymptotic series is obtained for the mean first passage time that is rapidly convergent in many cases and that is easy to implement. We apply our formulas to the problem of unimolecular dissociation and calculate the deviation of the dissociation rate from that obtained using statistical theories.
The determination of mass transport coefficients and vibrational relaxation rates of species formed in laser photolysis experiments
Authors: Bialkowski Stephen E. , King David S. , Stephenson John C.
Published: February 2003

A simple analytical solution of the equations which govern the formation, collisional relaxation, and mass transport rates of species produced in radially symmetric laser-induced processes is given. These equations are specifically applied to the CO2 laser-induced dissociations of CF2HCl and C2F3Cl dilute in argon. The concentration of the vibrational ground state of the CF2 radical product was probed as a function of time and pressure both during and after the photolyzing laser pulse by the laser-excited fluorescence technique. From these measurements, the vibrational relaxation rate of ? CF2 in argon was determined to be kVT=2.0×10−15 cm3 sec−1 and its diffusion coefficient was found to be D=90 cm2 Torr sec−1 in argon.
Energy partitioning in CO2 laser induced multiphoton dissociations: Energy of X̃/CF2 and X̃/CFCl from CF2CFCl
Authors: Stephenson John C. , Bialkowski Stephen E. , King David S.
Published: February 2003

We have measured the vibrational (v), rotational (J,K), and translational energy, (ET), of the X̃ CF2 and X̃ CFCl fragments formed in the CO2 laser induced multiphoton dissociation of CF2CFCl (chlorotrifluoroethylene): CF2CFCl→CF2 (v,J,K)+CFCl(v,J,K)+ET(v,J,K), which was the only detectable reaction path for CF2CFCl. More vibrational energy (Ev) appears in CF2 than in CFCl. Direct spectroscopic measurements of populations in levels 0<ν2<7 show that Ev is distributed statistically in the bending mode (ν2) of CF2 according to P (Ev) =exp(−Ev/kTv), where P (Ev) is the probability of a CF2 product being formed with a particular amount of energy in ν2, and the vibrational temperature which characterizes the nascent distribution is Tv(ν2) =1860±250 °K. A vibrational relaxation method was used to accurately determine fo, the fraction of CF2 and CFCl molecules initially formed in the ground vibrational level. The measurements of fo showed that the energy in the stretching modes (ν1 and ν3) of CF2 is not characterized by this Tv(ν2); if the energy in ν1 and ν3 is also thermal, it must be characterized by a lower temperature: Tv(ν1 and ν3) ≃1100 °K. For the CFCl product, direct spectroscopic measurement of the relative populations in ν2=1 and ν2=0 are consistent with Tv (ν2) =1550±300 °K. However, the measured fo for CFCl was consistent with a thermal distribution characterized by a lower vibrational temperature. Values of fo for CF2 were measured as a function of laser fluence for the condition where the reactant was extremely dilute (XCF2CFCl<10−5) in a high pressure (119 Torr) of Ar buffer gas. These measurements showed that the fraction of CF2 product molecules formed in vibrationally excited states decreased from 76% to 53% as the fluence decreased by a factor of 5.5, from 30 J cm−2 to 5.7 J cm−2. This decrease in Ev reflects a change in the ratio of laser excitation rate to vibrational deactivation rate for the CF2CFCl reactant. Under conditions where collisions are unimportant, the initial rotational energy in the CF2 was probed and found to be consistent with a thermal distribution characterized by a rotational temperature TR=1550±150 °K. The translational energy ET was the same for CF2 fragments formed with no vibrational energy and for those formed in the ν2=5 level with Ev=3320 cm−1, and ET was also the same for products formed with little rotational excitation (ER≃40 cm−1) and for those born with substantially higher rotational energy (ER=240 cm−1). The kinetic energy of the products is less than that observed in the photodissociation of CF2HC1.
Host dependence of the rare-earth ion energy separation 4f N–4f N−1 n
l
Authors: Morrison Clyde A.
Published: February 2003

The interaction between two 4f electrons in a rare-earth ion by means of the polarization of a ligand in a solid is discussed. It is shown that this interaction gives rise to a host dependent shift of the energy separations between the 4f N and 4f N−1 nl configurations. The results of the theory of these energy shifts are compared to experimental data for Nd3+, Er3+, and Tm3+ in the hosts YF3, LuF3, LiYF4, and LaF3. Also a shift in the Slater integrals and the spin–orbit parameter is predicted by the same interaction.
Photodissociation of aryl and aryl–alkyl halides at 193 nm: Fragment translational energy distributions
Authors: Freedman A. , Yang S. C. , Kawasaki M. , Bersohn R.
Published: February 2003

Molecular beams of a number of aryl and aryl–alkyl halide molecules were photodissociated using an excimer laser at 193 nm and fragment translational energy distributions measured. Previous work had shown that dissociation of aryl halide molecules takes place by a spin–orbit dependent crossing from an intermediate (π, π*) delocalized state to a triplet (σ, σ*) state localized on the C–X bond (X=Cl,Br,I). The vibrational energy of the intermediate state remains in the aromatic framework; thus the fragment translational energy distribution reflects only the electronic but not the vibrational energy of the intermediate state. This fact is exploited in identifying the electronic nature of this state. Present work indicates that both phenyl and naphthyl halide molecules dissociate at 193 nm to produce vibrationally hot aryl radicals. The actual dissociation pathway depends on the competition between intersystem crossing and internal conversion which are functions of both ring size and halogen substituents. Iodobenzene dissociates from S3, S2, and S1 whereas chlorobenzene internally converts to S1 entirely before dissociating. Bromobenzene also dissociates from all three singlet states whereas 2-bromonaphthalene dissociates from S1 even though it is excited to S4 at 193 nm. Fluorination (C6F5I and C6F5Br) drastically reduced the kinetic energy of the fragments which seems to indicate dissociation from a highly distorted aryl ring configuration which may be related to the well-known benzene structural isomers. At lower photon energies it had been found that the flux of fragments of aryl iodides was anisotropic with respect to the E vector of the dissociating light but the flux from aryl bromides was isotropic. The same results are found at 193 nm with a somewhat different interpretation. In all cases, C6H5CH2X and C6H5C2H4X produced translational energy distributions peaking at zero energy which implies that intersystem crossing is so slow that the molecule internally converts from S3 to S0 with subsequent statistical unimolecular decomposition.
Bounds for Coulomb energies
Authors: Gadre Shridhar R. , Bartolotti Lee J. , Handy Nicholas C.
Published: February 2003

It is shown by comparison calculations on many neutral atoms that excellent representations of the Coulomb energy J and the exchange energy K are AN2/3ℱρ4/3 dτ and Bℱρ4/3dτ, respectively (A=0.8994, B=−0.7969). It is also proved that J?1.0918N2/3ℱρ4/3dτ for all atoms and molecules, and that J (N2/3ℱρ4/3dτ)−1→0.856 as N→∞. Lower bound representations for J are also given. Implications of these results for local density functional theory are discussed.
Hard-cube model analysis of gas-surface energy accommodation
Authors: Grimmelmann Erik K. , Tully John C. , Cardillo Mark J.
Published: February 2003

Two simple hard-cube models of gas-surface collisions are re-examined in light of recent argon–tungsten atomic beam scattering experiments. Both models provide a good description of the average energy exchange. The inclusion of a square well attraction to the hard-wall potential results in an accurate two parameter fit to the data. The derived well depth is in agreement with previous measurements of the heat of desorption.
Spin-quantization and spin–orbit coupling effects on the line shapes of triplet states. II. The ‘‘small’’ exciton problem
Authors: Lemaistre J. P. , Zewail A. H.
Published: February 2003

This paper presents a detailed study of the effect of spin-quantization and spin–orbit coupling on the transition energies of triplet state dimers or small excitons. We consider both translationally equivalent (AA) and inequivalent (AB) dimers. For the AA and AB systems, we calculate transition frequency shifts induced by the spin–orbital coupling and by the spin–spin interactions between the plus (+) and minus (−) states of the dimer. As a result of these combined effects the selective coupling between the ± states of the singlet and the ± states of the triplet AA dimer system is not operative in the AB system. Furthermore, the role of the gas-to-crystal shifts and the intermolecular spin–spin interactions is to change the observed transition frequencies and hence cause a dispersion in the frequencies of the ± states. The relationship between such a dispersion in the AA and the same AB system is directly related to molecular parameters such as the strength of spin–orbital coupling. These results are applied to three experimental findings obtained for different dimer systems—phenazine, naphthalene, and tetrachlorobenzene dimers isolated in isotopically mixed crystals at T<2 °K. The phenazine results are reported here and the other data on naphthalene and tetrachlorobenzene were obtained from the literature. Agreement between theory and the recent experiments is encouragingly good.
Density distribution in a fluid monolayer at a solid surface
Authors: Prasad S. D. , Toxvaerd S.
Published: February 2003

The density distribution in a two-dimensional system of particles in a periodic field is determined by solving the BBGKY integrodifferential equation in the one-particle distribution function and the result is compared with the corresponding molecular dynamics obtained function. The calculations are performed for a system of Lennard-Jones particles in the field from a surface of fcc crystal. The solution to the BBGKY equation is found to be in excellent agreement with the molecular dynamics result.
IR multiple photon dissociation of fluorinated ethanes and ethylenes: HF vibrational energy distributions
Authors: Quick C. R. , Wittig C.
Published: February 2003

The IR multiple photon dissociation of fluorinated ethanes and ethylenes produces vibrationally excited HF via collisionless molecular elimination. The HF† fluorescence spectra have been measured and analyzed in order to determine the relative vibrational level populations produced by the dissociation processes. These results are compared to those obtained by others who used alternate methods of excitation. The measured vibrational level distributions cannot be adequately represented by single temperature Boltzmann distributions or by a statistical partitioning of the available energy. It is estimated that less than 30% of the fixed energy appears as vibrational excitation of the HF fragment.
Molecular dynamics simulations of depolarized Rayleigh scattering from liquid argon at the triple point
Authors: Ladd A. J. C. , Litovitz T. A. , Clarke J. H. R. , Woodcock L. V.
Published: February 2003

The depolarized Rayleigh spectrum of liquid argon at the triple point has been calculated using molecular dynamics simulations. In these calculations, it is necessary to truncate the pair polarizability anisotropy at some finite value, and it has been found that the intensity of the spectrum at liquid densities is an oscillatory function of this truncation. Despite the resulting difficulty in estimating the intensity of the dipole-induced-dipole (DID) spectrum, it is apparently several times larger than the experimental result. The truncation of the anisotropy also affects the time correlation functions beyond about 0.1 psec, and thereby the simulated band shapes. An approximate model of depolarized light scattering is used to help analyze the dependence of the integrated intensity of the DID spectrum on the truncation of the anisotropy.
Dipole autocorrelation functions for rare-gas mixtures
Authors: Weiss Shmuel , Strauss Herbert L.
Published: February 2003

Dipole autocorrelation functions are calculated classically and assuming binary interactions only, for rare-gas mixtures. For a Lennard-Jones potential and an exponential dipole with a reduced range ρ/σ=0.09, a normalized correlation function which is quite close to the experimental one is obtained. The calculated normalized correlation functions are found to be almost independent of the reduced potential well depth, ε/kT, over a wide range of ε/kT and for ρ/σ=0.09–0.11. Omission of the attractive part of the potential has only a small effect on the calculated correlation functions.
Microscopic theory of condensed phase chemical reactions. I. Pair phase space kinetic equation
Authors: Cukier Robert I. , Kapral Raymond , Mehaffey J. R. , Shin K. J.
Published: February 2003

A repeated ring kinetic theory describing the reversible chemical reaction A+B?C+D is obtained. We derive a kinetic equation for the equilibrium time correlation function of the solute pair phase space density. The solute–solvent and solvent–solvent interactions are described by a hard sphere potential. The solute–solute interactions consists of both hard and soft potential terms. This choice permits the use of a pseudo-Liouville operator formalism to obtain the kinetic theory. This greatly simplifies the derivation by comparison with a Liouville operator formalism for continuous potentials. The solute is dilute such that the kinetic theory describes isolated solute pairs interacting with the solvent. The static memory function of the kinetic equation is treated exactly. It describes solute propagation induced by direct interaction, Enskog collisions with the solvent and by the potential of mean force operating between the solute pair. The dynamic memory function is obtained at the repeated ring level of approximation. It describes solute propagation by dynamically correlated collisions (collective effects). Specialization of the reactive theory to exclusively hard sphere interactions is given. Also, nonreactive kinetic equations with elastic hard and/or soft solute–solute interactions are presented. In a companion paper we begin an analysis of the configuration space projection of these phase space kinetic equations and the associated transport coefficients of diffusion and reaction.
A compact formulation of polar tensors on bond moment hypothesis and calculation of bond moments and their derivatives of COF2
Authors: Dash P. C. , Mishra K. C. , Mohanty B. S.
Published: February 2003

A compact formulation for calculation of polar tensors on the basis of bond moment hypothesis has been developed. This has been used to calculate the bond moments and their derivatives of COF2 from integrated infrared intensity data. The calculated dipole moment is −0.93 D. The observed value is −0.95 D. A unique sign combination has been obtained.
The absorbing boundary method. III. Tunneling decay and scattering resonances
Authors: Bacskay George , Nordholm Sture
Published: February 2003

The recently developed absorbing boundary method (ABM) is applied to the calculation of tunneling decay rates and corresponding shape resonances in the scattering cross section. The analysis is carried out in terms of the contribution to the density of states from the resonant region of the spatial domain. One-dimensional test calculations have been carried out using the ABM and several related continuum state methods. While the ABM produces practically useful predictions for the location and shape of the resonance lines, it cannot in its present forms match the accuracy of the best continuum state methods. We have compared results obtained by the ABM (SMA and ISMA), the R-matrix method, the recently developed CGFEM and Bloch corrected R-matrix method, the stabilization method of Hazi and Taylor, and a simple pseudo bound state method.
Ultra thin films of anthracene on fused quartz and sapphire
Authors: Seki H. , Itoh U.
Published: February 2003

Ultra thin films of anthracene ranging in thickness from a half to several monolayers have been deposited on fuse quartz and sapphire substrates cooled down to liquid He temperatures in vacuums of 10−9 Torr. These films have been investigated by transmission spectroscopy of the first singlet–singlet absorption. The films have been examined in the incremental steps of annealing up to about 240 °K at which point noticeable sublimation begins. The films as deposited are disordered but still partially oriented. During the annealing process there is a red shift of the spectral peaks and the formation of some ’’stable ground state dimers’’ is seen. After a complete anneal the film spectra can be quantitatively related to the bulk single crystal spectra of anthracene indicating that the film consists of microcrystallites. The orientation of the microcrystallites have been found to differ for fused quartz and sapphire substrates.
Polar bonds and polarizability in conformational energy calculations: Application of a polarization model to alkyl halides
Authors: Boyd Richard H. , Kesner Laya
Published: February 2003

Conformational energy models traditionally attempt to simulate molecular properties by means of a total molecular energy assembled from empirically determined transferable energy functions that include bond stretching, bending, and torsion as well as nonbonded interactions. In extending the model to polar molecules through the addition of bond-centered dipoles or atom-centered charges, questions arise concerning inductive effects on assumed bond moments and the effect of intervening atoms on the electrostatic interaction between polar groups. We investigate here the use of a polarization–mutual induction model in representing these effects. Each bond is assumed to carry an intrinsic dipole moment and a polarization center. The induced moment at each polarization center is computed from the electric field arising from all sources (the other permanent and induced moments in the molecule and an external field). The model is parameterized for calculations on haloalkanes by determining intrinsic C–X bond monents, C–X, C–H, and C–C bond polarizabilities as well as stretching and bending constants involving halo atoms from experimental data. Other C, H parameters are taken from a previous hydrocarbon force field. It is found that molecular polarizabilities and dipole moments are reproduced very well including moments in molecules where inductive effects render a fixed bond moment scheme inaccurate. Conformational energies are generally satisfactorily accounted for. Electrostatic contributions to the latter are found to be important in determining conformer stabilities.
Vibrational Raman scattering from excited triplet state chrysene by time-resolved resonance Raman spectroscopy
Authors: Atkinson G. H. , Dosser L. R.
Published: February 2003

Raman scattering from the excited 3Bu+ state of chrysene dissolved in n-hexage was observed. The time-dependent band structure is determined. (AIP)
Electron-impact excitation cross sections for N II
Authors: Ganas P. S.
Published: February 2003

Generalized oscillator strengths and integrated cross sections from threshold to 1 keV are calculated in the Born approximation for the electron impact excitation of N II. (AIP)
Alternating linewidth effects in the ESR spectrum of 5-6-dihydro-6-methyluracil
Authors: Close David M. , Lloyd Roger V. , Bernhard William A.
Published: February 2003

Variable temperature ESR spectra of radicals formed by x-irradiation of single crystals of DH6MU are reported. (AIP)
Pattern selection at the bifurcation point
Authors: Pismen L. M.
Published: February 2003

The bifurcation analysis of inhomogeneous solutions to nonlinear reaction–diffusion equations in the infinite plane is carried out without imposing any a priori symmetry properties. A continuum of solutions bifurcating at the infinitely degenerate eigenvalue is constructed and these solutions are consequently sorted out by the stability analysis. The symmetric honeycomb pattern composed of a triplet of interrelated plane waves emerges as a preferred solution from second-order bifurcation equations, which is further confirmed by the conservation of the hexagonal symmetry in higher orders as well as by the character of fastest growing unstable modes. A condition involving third derivatives of kinetic functions decides whether a ’’crystalline’’ pattern based on a single hexagonal combination or an ’’amorphous’’, or turbulent, composite pattern tends to develop. The transition to an inhomogeneous state, which is normally a first-order, or hard phase transition, becomes a second-order, or soft one at the critical point, where stable small-amplitude patterns bifurcate supercritically under the same condition that is required for the formation of the honeycomb structure beyond the criticality.
Mössbauer effect investigation of the dynamics of ferrocene in liquid and glassy o-terphenyl
Authors: Vasquez Adalberto , Flinn Paul A.
Published: February 2003

Measurements of the Mössbauer spectra for 57Fe in ferrocene dissolved in o-terphenyl have been used to investigate the dynamics of the system, both in the glass and in the liquid. A rapid drop in the resonant fraction is observed above the glass transition temperature; the behavior is similar to that observed in other glass forming systems. The diffusion rate in the liquid region up to 16 K above the glass transition is too low to produce significant broadening of the resonance. The glass transition temperature of 273 K for o-terphenyl is consistent with that determined by other means.
Energy of heavy positive ions related to eigenvalue sum and chemical potential
Authors: March N. H.
Published: February 2003

It is shown that for heavy positive ions, the relation that the total energy is 3/2 times the sum of the eigenvalues is modified by a contribution from the chemical potential. This additional term vanishes in the limit of a neutral atom.
Another orthorhombic crystal modification of n-hexatriacontane and its vibrational spectra
Authors: Kobayashi Masamichi , Kobayashi Tohru , Itoh Yūzō , Chatani Yōzō , Tadokoro Hiroyuki
Published: February 2003

n-Hexatriacontane has been found to crystallize into a new type of orthorhombic form with the unit cell dimensions of a=7.40 Å, b=5.58 Å, and c=84.6 Å, quite different from the ordinary orthorhombic lattice. On the basis of the crystallographic as well as the spectroscopic evidence, it has been concluded that one repeat distance along the c direction contains two monomolecular layers, each assuming essentially the same structure as that of the ordinary (011) monoclinic modification. Contrary to the monoclinic form, the successive layers turn alternately through 180° about an axis normal to the ab plane. The possible space group has been found to be Pbcn-D142h or Pbca-D152h. The crystal structure presented here corresponds to an extreme case in polytypism found in crystal growth of the monoclinic lattice. The infrared absorptions due to the terminal methyl groups and the low-frequency Raman spectra of the present orthorhombic form differ from those of the monoclinic one, reflecting difference in the interfacial structure between these two modifications.
Polymer dynamics including side group motion: Free draining limit
Authors: Adler Ronald S. , Freed Karl F.
Published: February 2003

We present an exactly soluble model of polymer chain dynamics in which the effects of side group interactions are explicitly treated. Calculations for the relaxation spectrum, intrinsic viscosity, [η (ω)], and dielectric dispersion for both longitudinal and perpendicular dipole moments are presented in the free draining limit. Perpendicular dipoles are described by attaching a dipole moment to each side group. The relaxation spectrum displays two branches corresponding to local and global chain motions, the latter being highly depressed. The low frequency behavior of [η (ω)] and of longitudinal dipoles is shown to be well represented by an effective Rouse model, whereas [η (ω)] displays a plateau, [η]∞, at higher frequencies which is (i) not associated with the gap in the relaxation spectrum, (ii) independent of molecular weight for long enough chains, and (iii) correlated to the degree of side group motional freedom in accordance with experiment. For short chains the plateau region [η]∞ vanishes, and the model displays a molecular weight dependence. The relaxation of perperdicular dipoles is demonstrated to be independent of molecular weight for long enough chains. For shorter chains, however, global chain motions (e.g., overall rotation) significantly affect the behavior of the dielectric loss. Our model is shown to reproduce the phenomenological model of Dubois–Violette et al. and of Stockmayer et al. which has been used to rationalize the perpendicular dipole dielectric data. Furthermore, we show how our model and generalizations thereof can be derived from a true molecular model with realistic bonding and nonbonding interactions using the Zwanzig–Mori projection operatior formalism.