bluuebunny/crossref_metadata_2025 huggingface dataset:
NMR relaxation rate and the libron energy of solid hydrogen
Authors: Sugawara Kazushi , Woollam John A.
Published: February 2003

Taking the rotational relaxation of orthohydrogen (o-H2) in solid hydrogen into account, we have theoretically investigated the longitudinal NMR spin lattice relaxation rate T1−1 of o-H2. The rate T1−1 is characterized by an anomalous maximum, as a function of temperature, at temperatures close to the mean libron energy of o-H2. Application of the theory for o-H2 concentrations between 42% and 75% reveals a nearly concentration-independent mean libron energy equivalent to about 1 K. This qualitatively and quantitatively contradicts the conclusions of other theories, but agrees with recent experiments.
Reflection spectra of the first singlet transition of crystalline phenanthrene
Authors: Syassen K. , Philpott M. R.
Published: February 2003

Reflection spectra in the range of the first singlet transition are reported for the (001) face of a phenanthrene crystal at 1.5 K and at several higher temperatures. The spectra were taken at near normal incidence with light polarized parallel and perpendicular to the monoclinic b axis. The (001) face was obtained by cleaving a crystal grown from the melt. For the a-polarized transition the reflectivity of the 0–0 component is 30% and the stopband is approximately 5 cm−1 wide. The 0–0b transition has a much lower reflectivity, close to 16%, and a stopband almost 40 cm−1 wide. The dielectric function was calculated for the 0–0 region by fitting the spectrum to that of a series of Lorentzian oscillators and also by using the Kramers-Kronig transformation. The factor group splitting in reflection is measured to be 41 cm−1, while the same splitting from the derived absorption spectra is 46 cm−1. This difference is due to a red shift of the a-polarized absorption peak and a blue shift of the b-polarized absorption peak from the corresponding peaks in the reflection spectrum. The absorption linewidths are narrower than previously reported. The temperature dependence of the linewidths is discussed.
New vibronic levels of NH2 excited by a blue cw dye laser
Authors: Vervloet M. , Merienne-Lafore M. F.
Published: February 2003

Fluorescence of the free radical NH2, produced in the gas phase in a flow system and excited by a cw dye laser operating in the region 4600–5000 Å has been studied. Several new vibronic Π and Φ states have been identified in the region of the previously known (0, 14, 0) vibronic state near 21 000 cm−1. The analysis is based on combined use of laser fluorescence data and the optical emission spectrum. An astrophysical line, observed in comet spectra, has been assigned.
Theory of unimolecular reactions induced by monochromatic infrared radiation
Authors: Quack Martin
Published: February 2003

A theory of unimolecular reactions induced by coherent, monochromatic infrared radiation (URIMIR) in the absence of collisions is presented. It is shown that the set of first order linear differential equations for the amplitudes of molecular states (Schrödinger equation) can be reduced, under specified conditions, to a much smaller set of first order linear differential equations for the coarse grained populations of levels for polyatomic molecules (master equation). Four limiting situations are identified in which such linear rate equations provide a reasonable approximation. Rate coefficients are obtained as a function of spectroscopic parameters (energy levels and transition moments). Solutions for the master equations are given as a function of time and at steady-state. Simple limiting cases (Strong Field Limit, Weak Field Limit, Reaction Threshold Bottleneck, etc.) are identified and very simple rate expressions are obtained for these cases. A complete statistical mechanical theory of URIMIR is formulated and the computational approaches for the quantitative treatment for any molecule are summarized. Predictions are made concerning the dependence of the unimolecular rate constant and product (state) distributions upon radiation intensity. In particular at high intensities a less than proportional increase of the rate constant with intensity is predicted. The possibility of specific pumping of reaction paths with high energy thresholds is discarded. Comparison with thermal unimolecular reactions shows that collisionless URIMIR are quite different in all respects. Fundamentally these differences are traced to the fact that the underlying molecular distribution functions are different. They are Boltzmann distributions for thermal reactions and ’’microcanonical’’ [without exp  (−E/kT)] in URIMIR. Although real distribution functions are still different in general, the underlying distribution functions often dominate the dynamical behavior.
Electron trapping in irradiated single crystals of organic compounds
Authors: Box Harold C. , Budzinski Edwin E. , Freund Harold G.
Published: February 2003

Single crystals of arabinose were x-irradiated and maintained at 4.2°K. ESR spectra of trapped electrons were obtained. The trapping of electrons in crystalline environment of polar hydrogen atoms is reported.
Calorimetric studies of choline chloride, bromide, and iodide
Authors: Petrouleas V. , Lemmon R. M.
Published: February 2003

Crystalline choline chloride is known to exhibit sensitivity to ionizing radiation. Measurements are reported for two other choline salts choline bromide and iodide on a differential scanning calorimeter and are extended to low temperatures to elucidate the radiation sensitivity of these salts.(AIP)
Application of molecular orbital method to crystalline solids: Calculation of the electronic energy bands of diamond-type crystals
Authors: Nishida Masahiko
Published: February 2003

The electronic energy band structures of diamond-type crystals (C, Si, Ge, and α-Sn) are calculated by use of extended Hückel theory, taking into account all interactions between atoms up to the sixth neighbors. Reasonable overall agreement is obtained with experiments or other calculations when the four parameters included are adjusted to fit the experimental energies at two points in the Brillouin zone, zone center and conduction band minimum. This suggests the possibility of a reliable utility of this method for the systematic study of electronic structures of Si, Ge, and α-Sn as well as diamond.
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o SCF calculations on molecular silicon dioxide
Authors: Pacansky J. , Hermann K.
Published: February 2003

A b initio SCF calculations are performed on molecular silicon monoxide and dioxide. These computations show that the electronic ground state for molecular SiO2 is 1Σ+g and has a D∞h geometry, and that molecular SiO2 is bound with respect to SiO(1Σ+) and O(1D). Vibrational frequencies are computed in order to aid in the identification of this enigmatic species.
Energies and relative cross sections of singlet–triplet transitions in methyl-substituted 1,3-butadienes by ion-impact spectroscopy
Authors: Moore John H. , Sato Yukinori , Staley Stuart W.
Published: February 2003

The energies of the 1 3Bu←? 1Ag and 1 3Ag←? 1Ag transitions of a number of methyl-substituted 1,3-butadienes have been determined by ion (He+ and H2+) -impact energy-loss spectroscopy. The observed values were relatively invariant at 3.2 and 4.9 eV, respectively, except in the case of 2,4-dimethyl-1,3-pentadiene where triplet states at 3.5 and 4.4 eV indicate a nonplanar conformation or conformations. The relative cross section for the 1 3Bu←? 1Ag transition in the He+-impact spectrum of trans-1,3-pentadiene was observed to increase slightly relative to that for the 1 3Ag←? 1Ag transition and substantially relative to that for the 1 1Bu←? 1Ag transition on decreasing the ion energy from 3.5 to 1.5 keV. Cross sections for all three transitions decreased on changing the projectile from 3.0 keV H+ to 3.0 keV H2+ and from the latter to 3.0 keV He+. Finally, the relative cross section for the 1 1Bu,2 1Ag←? 1Ag transitions is decreased only slightly (by up to a factor of 2) on adding up to four methyl groups whereas the 1 3Bu←? 1Ag and 1 3Ag←? 1Ag relative cross sections suffer a decrease of about a factor of 10 for He+ impact and a bit less for H2+ impact.
Inert gas atom–molecule collisions in a laser field: Vibrational excitations in homopolar molecular ions
Authors: Brandi H. S. , Koiller Belita , Lins de Barros H. G. P. , Miranda L. C. M.
Published: February 2003

We investigate the effect of an electromagnetic field in the vibrational excitations of diatomic molecules, considering its collisions with noble gas atoms. Considering colinear collisions we obtain an expression for the vibrational transition amplitudes, and we apply the formalism to the specific collision He+H+2 in CO2 laser field, in the high energy approximation. The general behavior of the multiphoton transition amplitude as a function of the relevant parameters of the system is discussed for the 0→1 vibrational excitation.
Thermodynamics of surface clusters—direct observation of Re2 on W(211)
Authors: Stolt Kaj , Wrigley John D. , Ehrlich Gert
Published: February 2003

Direct observations in the field ion microscope make it possible to determine the ratio of bound to dissociated dimers at equilibrium on a crystal surface. In principle, the thermodynamics of dissociation are accessible from the temperature dependence of this ratio. To establish the feasibility of such measurements, the kinetics governing changes in the dimer probabilities are worked out. This is done for isothermal processes, and for conditions typical of the quench from the temperature at which the equilibrium distribution is established to the temperature at which observations are made in the field ion microscope. Techniques are developed to correct for distortions of the equilibrium distribution during the quench. These rely upon measurements of the ratio of dimers in configuration 1 and 0 at low temperatures, combined with observations of PD, the fraction dissociated at high temperatures. Experiments conforming to this scheme have been carried out for rhenium dimers on W(211). They yield internal energies of −3.7±1.1 kcal/mole for rhenium dimers in configuration 1, and −2.3±1.2 kcal/mole for dimers in configuration 0, measured relative to the energy of dissociated pairs. Our results suggest important contributions from indirect interactions.
Brownian dynamics study of transitions in a polymer chain of bistable oscillators
Authors: Helfand Eugene
Published: February 2003

The potential of a bistable oscillator is taken as two quadratic wells separated by a quadratic barrier. The Brownian motion of a particle in such a field has been computer simulated for a variety of values of the barrier height and friction constant. The rate of passage over the barrier conforms well to Kramers’ theory. A chain in which each of the bonds is a bistable oscillator was then studied as a model of conformational transitions in a polymer. One question investigated was to what degree do the transitions occur cooperatively. The bond transition rates are found to compare fairly well with a Kramers-like theory, with no cooperativity. However, a direct examination of the correlation between transitions shows some enhancement of the transition rate of near neighbors immediately following the transition of a bond.
Rotational relaxation within the 1B2u(S1) state of benzene
Authors: Coveleskie R. A. , Parmenter C. S.
Published: February 2003

Relaxation from nonequilibrium rotational distributions to a Boltzmann-like distribution within the zero point level of S1 benzene vapor is studied using ground state benzene and argon as collision partners. The initial distributions are established by excitation with a narrow-band (0.3 cm−1) laser tuned to various positions within the rotational contour of the 601 absorption band. The course of rotational relaxation is then followed by observations of the dependence of rotational contours in fluorescence bands upon gas pressure. Arguments are given to suggest that relaxation approximates a single-step process, with collisions taking the initial nonequilibrium distribution directly to a broad distribution of J′K′ levels reminiscent of the Boltzmann distribution. The rate constant for this relaxation is about 9×107 torr−1 sec−1 for benzene as a collision partner, which is about eight times the gas kinetic value. This constant is observed for each of two different initial distributions. It is likely that switching of electronic energy in collisions between S1 benzene and rotationally equilibrated S0 benzene makes a substantial contribution to this relaxation. The rate constant for relaxation in argon collisions is much smaller and depends on the initial distribution, being 1.5×107 and 3.5×107 torr−1 sec−1 for the two cases measured. Some applications of these data to rotational effects in radiationless transitions are discussed.
Excitation of the b 1Σ+g state of O2 by low energy electrons
Authors: Lawton S. A. , Phelps A. V.
Published: February 2003

Rate coefficients for excitation of the b 1Σ+g state of O2 by low energy electrons have been measured using a drift tube technique. The time dependence of the absolute intensity of the 762 nm band emission was measured for O2 densities between 1016 and 2×1018 molecules/cm3. When corrected for electron attachment, ionization, and metastable diffusion, the number of b 1Σ+g molecules produced per centimeter of electron drift and per O2 molecule calculated from the 762 nm emission varied from 1.3×10−18 cm2 at E/N=5×10−17V cm2 to 2.1×10−16 cm2 at E/N=2×10−15V cm2. These values of electric field to oxygen density ratio E/N correspond to mean electron energies of 0.75 and 6 eV, respectively. Measured decay constants for the 762 nm radiation yield a value for the product of the diffusion coefficient and the O2 density of (5.0±0.3) ×108 cm−1 sec−1 and a quenching coefficient for the b 1Σ+g state of (3.9±0.2) ×10−17 cm3 sec−1. Comparison of measured excitation coefficients with values calculated using a recommended set of electron collision cross sections for O2 show that the cross sections for direct excitation of the b 1Σ+g state are accurate near threshold and suggest that essentially all of the O2 molecules excited to levels at and above 1.63 eV result in the formation of molecules in the b 1Σ+g state.
The calculation of pair polarizabilities through continuum electrostatic theory
Authors: Oxtoby David W.
Published: February 2003

Recently, Oxtoby and Gelbart applied continuum electrostatic theory on an atomic scale to calculate collisional polarizabilities. In this paper we show how such a description can be derived from first order quantum mechanical perturbation theory, and obtain explicit expressions for the local susceptibilities of atoms and molecules. We apply the theory to a calculation of the pair polarizability of interacting hydrogen atoms, for which the model can be solved in closed form. We also show that the dipole moment induced by an arbitrary (weak) external field may be obtained from the wave function in a uniform field.
Theoretical study of CO chemisorption on nickel and copper surfaces
Authors: Yu H. L.
Published: February 2003

A comparative study of the chemisorption of CO on Ni(001) and Cu(001) surfaces has been performed. The study is based on an analysis of the electronic structure of (M)5CO clusters obtained from self-consistent field Xα scattered-wave calculations. The electron orbital formed principally by the 5σ orbital on CO was found to be mainly responsible for the bonding of the CO molecule to the metal surface for both the CO/Ni and CO/Cu systems. The different occupation of the antibonding 7a1 orbital in the two clusters is believed to be the major reason for the large difference observed in the measured heats of adsorption of CO on Ni and Cu surfaces. It was found also that metal atoms transfer electronic charge to the antibonding π*-like orbital of CO. A possible correlation between the amount of the charge transfered and the relative ease of dissociation of CO molecules on metal surfaces is discussed.
Kinetics of fast light-induced redox processes in micellar systems: Intramicellar electron transfer
Authors: Maestri Mauro , Infelta Pierre P. , Grätzel Michael
Published: February 2003

A phenomenological model is developed describing the kinetics of fast intramicellar redox reactions induced by light. It takes into account the influence of compartmentation as well as statistical distribution of reactants which lead to kinetic equations that differ from the rate laws in homogeneous kinetics. The theoretical predictions are experimentally tested by laser photolysis studies with solutions containing tetradecyltrioxyethylene sulfate as the micelle forming agent, N-methyl-phenothiazine (MPTH) as an electron donor, and tris(2,2-bipyridine) -ruthenium(II) [Ru(bipy)2+3], as an electron acceptor. The intramicellar electron transfer from MPTH to excited state Ru(bipy)2+3 occurs with a specific rate of 3×106 s−1. This reaction is followed by a back electron transfer occuring with a specific rate of 6×106 s−1. Possible ways to intercept the back reaction and to achieve light energy storage effects are indicated.
Brillouin scattering and segmental motion of a polymeric liquid. I
Authors: Lin Y. -H. , Wang C. H.
Published: February 2003

The effect of segmental motion on the density–density correlation function of a viscous polymer liquid has been analyzed using a generalized relaxation equation developed by Zwanzig and Mori. It is shown that for polymer liquids of high viscosity, Brillouin scattering is closely associated with the structural relaxation associated with the motion chain segments. A single relaxation time theory is shown to yield good agreement with the experimental results on polypropylene glycol. The torsional motion involving a small number of monomer units is shown to be responsible for the dispersion and attenuation of the hypersonic wave. The fact that the Brillouin scattering spectrum of a polymer liquid is insensitive to the change of molecular weight is discussed. We have shown that temporal modulation of the spatial second moment of the intermolecular or intersegmental interaction energy is responsible for the relaxation process involved in Brillouin scattering.
Semiclassical theory of predissociation: Uniform semiclassical wavefunction approach
Authors: Eu Byung Chan , Zaritsky Nava
Published: February 2003

Predissociation and preassociation phenomena are theoretically investigated by means of uniform semiclassical wavefunctions for all cases of energy in two-state models. Three different sets of transition probability amplitudes are obtained for three different cases of energies, i.e., E≳Vmin; Vmax<E<Vmin; and E<Vmax, where Vmin and Vmax are the minimum and maximum of the upper and lower adiabatic potentials. The theory is different from the semiclassical theory previously used by other authors in semiclassical theory of predissociation, but leads to formally similar transition probability amplitudes for the case of E≳Vmin. For other cases of energy comparison is not possible owing to the lack of other theories.
Multiphoton resonance ionization bands in I2
Authors: Lehmann Kevin K. , Smolarek John , Goodman Lionel
Published: February 2003

Multiphoton resonance ionization (MPRI) bands of I2 vapor have been examined in the 360–600 nm region. A band system with T0 =48 426±4 cm−1 and ωe=254±1 cm−1 is assigned to a two-photon resonant intermediate state involved in either three- or four-photon ionization. Polarization measurements establish that the transition is either 1g or 2g←0g. Comparison to the Dalby system (2d), 5146 cm−1 to higher energy allows a more detailed interpretation of the excited states of both systems: 2g (2Π3/2g) nsσg and 1g (2Π1/2g) nsσg, respectively. Excitation into the visible B←X absorption band between 500 and 600 nm leads to a dense set of MPRI lines. An intense system in this region with T0 =35 762±4 cm−1 and ωe=104±1 cm−1 is assigned to a two-photon resonant intermediate state with polarization measurements establishing it as 0g. It is further interpreted as arising from the two-electron promotion 0g→σu, πu→σu. A striking feature of the MPRI spectra of I2 is the near absence of lines corresponding to the normal optical bands and also the absence of many possible multiphoton transitions allowed by selection rules. The cross section of the B→X transition does occur prominently in the spectrum multiplicatively superimposed on the higher resonances, however.
Nitric oxide chemiexcitation occurring in the reaction between metastable nitrogen atoms and oxygen molecules
Authors: Kennealy J. P. , Del Greco F. P. , Caledonia G. E. , Green B. D.
Published: February 2003

The infrared emission spectrum of the NO Δv=1 bands, chemiexcited in the reaction between metastable atomic nitrogen and molecular oxygen, has been studied at temperatures of 90–180 K and pressures near 5×10−6 atm. It is concluded that the observed radiation corresponds to an NO vibrational distribution created solely by the chemical reaction, unaffected by any relaxation process(es). Relative rate constants for production of NO into individual vibrational levels are found to be very nearly constant for levels v=1–7 and decrease monotonically for levels v=8–12. The average degree of excitation is about 4.5 vibrational quanta per NO molecule, and, assuming that N(2P) atoms play a negligible role, it is determined that 27% of the N(2D)+O2 exothermicity is converted to vibrational excitation of the nitric oxide product. Both the nature of the distribution and the results of a surprisal analysis suggest that those reactions which produce NO in levels higher than v=7 may also be producing O(1D).
The gravity effect and the mobility of electrons in critical neopentane
Authors: Huang Sam S.-S. , Freeman Gordon R.
Published: February 2003

The rapid change of density with temperature, the high compressibility of the vapor and liquid near the critical region, and the force of gravity make it difficult to avoid a density gradient in the ’’critical fluid’’. Electron mobilities were measured simultaneously in the juxtaposed vapor and liquid so that density changes could be monitored. The mobility in neopentane at the critical density and ∼0.1 K above Tc is 45 cm2/V s; the estimated mobility in the critical fluid is 42 cm2/V s. The effect of electric field strength on the mobility changes sign at the reduced density and temperature n/nc=1.2 and T/Tc=0.995. The reversal is attributed to the Ramsauer–Townsend scattering minimum occurring at an energy slightly above thermal at this density.
On the influence of nonrandom sequential coupling on radiationless relaxation processes
Authors: Muthukumar M. , Rice Stuart A.
Published: February 2003

We have considered radiationless decay in a simple skeleton spectrum of states between which there are systematic functional and random coupling matrix elements; the former are treated as a perturbation of the latter. As particular examples we have analyzed mixing of constant coupling and random coupling and mixing of Lorentzian coupling and random coupling in a spectrum which can model photodissociation and/or vibrational relaxation, and have calculated the total dissociation probability and lifetime of the initially excited state. For small times the constant coupling–random coupling case leads to a linear combination of t and t2 terms characteristic of nonsequential and sequential decays, respectively. The Lorentzian coupling–random coupling case can be thought of as a model for vibrational relaxation in solution. The calculated time evolution of the population of the initially excited vibrational mode exhibits two time constants for small times and is rather complex for larger times. The shorter time constant corresponds to the redistribution of energy among modes which are nearly resonant with the initially excited mode and the longer time constant corresponds to the relaxation to lower lying modes by the solvent. This is in good qualitative agreement with the experimental results of raiser et al.
Absorption and emission spectra of Na atoms trapped in rare-gas matrices
Authors: Balling L. C. , Havey M. D. , Dawson J. F.
Published: February 2003

Pulsed dye-laser excitation has been used to reinvestigate the structure of the optical absorption and emission spectra of Na atoms trapped in Ar, Kr, and Xe matrices at 10 °K. Two absorption bands have been found to be associated with the same lattice site, and optical pumping between levels of the bistable state of the Na–Ar system for this site has been observed. The intensity of luminescence produced by laser excitation in each absorption band has been measured as a function of emission wavelength. Observation of the laser-induced luminescence has permitted the unambiguous determination of the wavelengths of the peaks overlapping absorption triplets. The lifetimes characterizing the decay of the luminescence have been measured as a function of emission wavelength.
Automatic calculation of the energy levels of one-electron diatomic molecules
Authors: Bailey Paul B.
Published: February 2003

A method is described for the calculation of energy levels of one-electron diatomic molecules which is completely automatic, does not have to have an initial guess supplied, and cannot accidentally compute a wrong energy level even when they are very close together.
Resonance Raman excitation profiles and depolarization dispersion curves, and their use in the analysis of vibronically coupled excited states
Authors: Henneker W. H. , Penne A. P. , Siebrand F W. , Zgiersk M. Z.
Published: February 2003

The relationship between vibronic coupling and resonance Raman scattering is studied on a model system consisting of two excited electronic states coupled by a non-totally-symmetric mode of vibration. Scattering cross sections and depolarization ratios are calculated as a function of the wavelengths of the incident and scattered light, giving rise to Raman excitation profiles and Raman spectra, respectively. The model system allows exact solution for arbitrary values of the electronic energy gap between the coupled states, of their vibrational frequencies, and of linear and nonlinear adiabatic coupling parameters. The results are reported in the form of three-dimensional graphs for combinations of parameter values typical for weak, strong, and intermediate vibronic coupling. They are compared with previous results, based on simpler models and/or more approximate solution methods. In addition, they are related to the results obtained for the molecular dimer [J. Chem. Phys. 63, 5475 (1975)], which is the prototype of an exactly solvable vibronic coupling model. It is shown that resonance Raman spectroscopy generally allows one to probe more deeply into vibronically coupled states than conventional absorption–emission spectroscopy and thus provides a more powerful tool for their spectroscopic analysis. This is particularly significant in the region intermediate between weak and strong vibronic coupling where interference between Franck–Condon and vibronic coupling effects or between adiabatic and nonadiabatic coupling (in the adiabatic Born–Oppenheimer representation) may produce highly irregular spectra. The basic advantage of resonance Raman scattering over absorption spectroscopy is that one can obtain a higher effective resolution of excited state spectra, namely, by studying each vibrational mode separately, by comparing the excitation profiles of fundamentals and overtones, and by investigating the dispersion of the depolarization ratio.
Charge transfer reactions of ground O+(4S) and excited O+(2D) state ions with neutral molecules
Authors: Moran T. F. , Wilcox J. B.
Published: February 2003

Total charge transfer cross sections have been measured for the reactions of 0.6 to 3.0 keV O+(4S) and O+(2D) ions with Ar, H2, N2, O2, CO, NO, and CO2. Time-of-flight techniques have been used to measure the fast neutral O products from charge transfer reactions in which reactant O+ ion beams have been produced by controlled electron impact ionization of oxygen. Reactions have been examined as a function of the electron energy used to produce the reactant ions and reaction cross sections determined for ground O+(4S) and excited O+(2D) state ions. The cross sections for charge transfer reactions involving the excited O+(2D) ions are significantly larger than the corresponding reactions of ground state ions in each of the above systems with the exception of CO2 where the revserse situation occurs due to small energy defects and favorable vibrational overlaps for reactions of O+(4S) ions.
Information theoretic analysis of multiphoton excitation and collisional deactivation in polyatomic molecules
Authors: Jensen C. C. , Steinfeld J. I. , Levine R. D.
Published: February 2003

The procedure of maximal entropy is applied to characterize the three distributions over energy states which are of direct interest for studies of multiple photon excitation of polyatomic molecules. These distributions are (a) the original population of the different energy states given the mean number of photons absorbed, (b) the distribution over the final energy states after a single collision, given a well-defined initial energy state and the mean energy transfer per collision, and (c) the time evolution of the population of the different energy states due to collisional deactivation, given the mean energy transfer per collision. Good agreement with experimentally determined values of 〈ΔE〉, the average amount of energy removed in a collision, are obtained for deactivation of sec-butyl, cyclohexane, 3-hexyl, and β-naphthylamine by structureless collision partners such as He or H2. The vibrational relaxation surprisal parameter is found to be λ1?0.1 for all these systems. This is much closer to a statistical, or strong-collision limit than vibrational deactivation of diatomics by atoms, for which λ1?1.0. Deactivation most likely proceeds through a sequence of maximal-entropy distributions. These results are used to interpret experimental data on the infrared multiphoton reactions of deuterated vinyl chloride.
Low temperature vibrational spectroscopy. II. Evidence for order–disorder phase transitions due to weak C–H⋅⋅⋅Cl hydrogen bonding in tetramethylammonium hexachloroplatinate (IV), -tellurate (IV), and -stannate (IV) and the related perdeuterated compounds
Authors: Berg Rolf Willestofte
Published: February 2003

The low frequency infrared and Raman spectra of normal and per-deuterated ((CH3)4N)2[MCl6] (M=Pt, Te, or Sn) have been measured at temperatures down to ∼100 K and evidence for phase transitions was found. The spectra have been carefully assigned and it was shown that bands due to forbidden methyl torsions and other noncubic features play a role, especially in spectra at low temperatures. Possible site symmetries of the [PtCl6]2− ion, which cannot have strictly Oh symmetry in either phase, have been deduced. The spectra of a mixed Pt : Te compound showed that the hexachlorometallate anions vibrate approximately independent of each other. The results have been compared with von der Ohe’s recent extensive low temperature Raman study on protonated compounds with M=U, Sn, and Zr, and his conclusions are discussed. It is shown that crystals of this kind can be characterized by methyl–chlorine interaction and it is suggested that the phase transitions are caused by an ordering of rotationally disordered methyl groups via the formation of weak C–H⋅⋅⋅Cl hydrogen bonds at low temperatures. The transition temperatures and hence the interactions are shown to depend on both the kind of hydrogen isotope and metal present in the crystals.
Observation of the collapse of a polymer chain in poor solvent by small angle neutron scattering
Authors: Nierlich M. , Cotton J. P. , Farnoux B.
Published: February 2003

The variation of the radius of gyration R of a polymer chain in an extremely dilute solution is measured as a function of the temperature T below the Flory temperature ϑ. The data are deduced from small angle neutron scattering experiments. In the temperature range studied two domains appear: In the first one (the ϑ region) R is rather independent of the reduced temperature τ[τ= (T−ϑ)/ϑ]. In the second one the collapse of the chain is observed and the τ dependence of R is R∼‖τ‖−0.32±0.05. The temperature variation of the second virial coefficient is also given. These results are discussed with regards to a possible demixion of the solution.
The EPR spectrum of SF3 trapped in SF3BF4 crystals
Authors: Morton J. R. , Preston K. F. , Strach S. J.
Published: February 2003

An anisotropic EPR spectrum observed in γ-irradiated single crystals of SF+3BF−4 is ascribed to the radical SF3. Each of the two magnetically distinct sites exhibited hyperfine interactions with four 19F nuclei, one of which is associated with a near-neighbor lattice nucleus. The angular dependence of the spectrum has been explored in the principal crystallographic planes and the principal values and directions of the g and hyperfine tensors determined. These results taken in conjunction with crystallographic data for the undamaged crystal lead to the conclusions that (i) SF3 is a planar radical, having only two equivalent fluorines; (ii) its electronic ground state is 2A1 in C2v; and (iii) its SOMO consists primarily of a combination of in-plane S (3p) and F(2p) atomic orbitals.
Formation of small metal clusters by ion bombardment of single crystal surfaces
Authors: Garrison Barbara J. , Winograd Nicholas , Harrison Don E.
Published: February 2003

The mechanism for the formation of small metal clusters ejected from an ion bombarded metal surface is examined in detail. The analysis is performed by classical trajectory methods which determine the positions and momenta of all particles in a model microcrystallite as a function of time. The calculation utilizes pair potentials for Cu derived from elastic constants of the solid and is performed for 600 eV Ar+ ion at normal incidence to the crystal. The results show that cluster species do not leave the surface as intact parts of the solid but form in a region above the surface. A trajectory for Cu5 formation is traced in detail showing a typical mechanism which is valid for Cun formation where n?7.
Hermitian formulation of diatomics-in-molecules theory
Authors: Eaker Charles W.
Published: February 2003

The Hermitian formulation of the diatomics-in-molecules (DIM) theory is capable of reproducing the results of the non-Hermitian DIM procedure developed by Tully and Truesdale [J. Chem. Phys. 65, 1002 (1976)]. This is shown for the collinear H3 and H1 systems for which the two methods are in excellent agreement. The conclusion of this work is that it is neither necessary nor desirable to invoke a non-Hermitian formulation of DIM.
Fluorescence lifetime studies of NO2. II. Dependence of the perturbed 2B2 state lifetimes on excitation energy
Authors: Donnelly V. M. , Kaufman F.
Published: February 2003

Radiative lifetimes and quenching rates of the perturbed 2B2 state were measured, using pulsed Nd–YAG laser excitation between 473 and 659 nm. The decay of broad band (3000 to 9000 cm−1) fluorescence is nearly exponential at low pressures (P?0.1 mtorr) for excitation wavelengths less than 562 nm. Lifetimes increase smoothly from 88 μsec at λE=473 nm to 120 μsec at 562 nm. Low pressure decays at 659 nm are nonexponential with at least two components of 124 and 200 μsec. The excitation energy dependence of the radiative lifetime and the change from exponential decays at higher energies to nonexponential decays at lower energies is explained by assuming that the 2B2 state couples strongly to high vibrational levels of the ground state.
Energy partitioning in the collision-free multiphoton dissociation of molecules: Energy of ? CF2 from CF2HCl, CF2Br2, and CF2Cl2
Authors: Stephenson John C. , King David S.
Published: February 2003

We have developed a simple laser-excited fluorescence method to determine the translational energy of the nascent products of infrared multiphoton dissociations, and have applied this method to the measurement of the average translational energy ET of the ? 1A1 CF2 radicals formed from the collision-free dissociation of CF2HCl, CF2Br2, and CF2Cl2 by CO2 TEA laser pulses. The initially formed CF2 (v,J,K) is distributed in many vibrational (v) and rotational (J,K) states, and we have obtained ET (v,J,K) specifically for different values of these internal quantum numbers. ET of the CF2 is different for each parent molecule, and is independent of the intensity or wavelength of the CO2 laser for the range of values investigated. For the CF2 produced from CF2HCl, ET was the same for fragments formed with no vibrational energy and for those formed in the ν2=5 level with 3320 cm−1 of vibrational excitation, and ET was also the same for the products formed with little rotational excitation (ER?40 cm−1) and for those born with substantially higher rotational excitation (ER=240 cm−1). Assuming a Boltzmann velocity distribution in the photoproducts, we deduce an average kinetic energy ET=6.9±2, 1.5±0.5, and 1.7±0.5 kcal/mol for the nascent CF2 fragments formed from CF2HCl, CF2Cl2, and CF2Br2, respectively. The method we used previously to measure the collision-free distribution of vibrational energy Ev in the CF2 formed from CF2Br2 and CF2Cl2 was applied to the CF2 from CF2HCl. The distribution was found to follow the relation P (Ev) =exp(−Ev/kTv), where P (Ev) is the probability that a nascent CF2 fragment contains vibrational energy Ev, k is Boltzmann’s constant, and Tv, the single parameter necessary to characterize the distribution, is the vibrational temperature. For the CF2 formed from CF2HCl, Tv =1160±100 K. These measurements complete our determination of the complete distribution of energy (electronic, vibrational, rotational, and translational) in the CF2 product of these reactions.
High resolution spectroscopy of the OsO4 stretching fundamental at 961 cm−1
Authors: McDowell Robin S. , Radziemski Leon J. , Flicker Herbert , Galbraith Harold W. , Kennedy R. Craig , Nereson Norris G. , Krohn Burton J. , Aldridge Jack P. , King Joe D. , Fox Kenneth
Published: February 2003

The ν3 bands of 187Os16O4, 189Os16O4, and 192Os16O4 have been recorded using both a Michelson interferometer (resolution 0.06 cm−1) and a tunable semiconductor diode laser (resolution limited by the Doppler width, ∼0.0007 cm−1). The rotational fine structure differs from that of most other spherical-top molecules, for only rotational levels of A symmetry exist. A total of 112 individual vibration–rotation lines in the P and R branches of the three isotopic species were calibrated against stimulated emission lines from a high-voltage CO2 gain cell, and were used to determine three scalar and two tensor spectroscopic constants for each species; an additional scalar constant was obtained from an analysis of the Q branch of 192OsO4. The strength of P (11) A2 0 was measured for 192OsO4 and yields a vibrational transition moment for ν3 of 0.17±0.02 D. Transitions of all isotopic species that are expected to fall near CO2 laser lines in the region 949–972 cm−1 are tabulated as an aid in the interpreation of saturation spectroscopy experiments. The general quadratic symmetry and valence force constants of OsO4 were redetermined, using the isotope shifts in ν3 as the additional constraints for the F2 symmetry block.
The covalent and ionic states of the xenon halides
Authors: Hay P. Jeffrey , Dunning Thom. H.
Published: February 2003

The electronic states and spectroscopic properties of the xenon halides, XeX (where X=F, C1, Br, I), are characterized on the basis of ab initio configuration interaction calculations. States dissociating to Xe(1S)+X(2P) and Xe+(2P)+X−(1S) are treated, and the effects of spin-orbit coupling are introduced in a semiempirical atoms-in-molecules model. The intense laser emissions in XeF (353 nm), XeC1 (308 nm), and XeBr (283 nm) and the strong fluorescence in XeI (253 nm) arise from a transition from the lowest Ω=1/2 state to the lowest covalent 1/2 state (III1/2→I1/2 in our notation). The calculated lifetimes for these transitions are all 11–12 nsec. Four other major emissions with ΔΩ=0 are predicted for each XeX species.
Tricritical phenomena in ternary and quaternary fluid mixtures
Authors: Fox Jeffrey R.
Published: February 2003

The phase behavior associated with tricritical phenomena in two classical fluid mixture models is described. The characteristic features of the isothermal composition–space phase diagram near a tricritical point are examined in the order–parameter expansion model previously investigated by Griffiths. These characteristic features are then related to the global phase behavior and the underlying interparticle interactions of the four-component lattice-gas model solved in a mean-field approximation and to the known phase behavior of relevant experimental systems.
Ion–polar molecule collisions. Conservation of angular momentum in the average dipole orientation theory. The AADO theory
Authors: Su Timothy , Su Elkie C. F. , Bowers Michael T.
Published: February 2003

The average dipole orientation (ADO) theory for capture collisions between ions and dipolar molecules is modified to include conservation of angular momentum approximately; termed AADO theory. The AADO theory is compared to experiment for proton transfer reactions from various positive ions to NH3, HCN, and CH3CN substrates and for several negative ions to HCN. The agreement between theory and experiment is quite good in all cases. The AADO rate constants are larger than the ADO rate constants in all cases. For the systems studied the average increases in AADO rate constants over ADO values were 12% (NH3), 20% (HCN), and 31% (CH3CN). For most polar molecules, AADO predictions should be about 10%–30% larger than ADO values. The theory has been parameterized in the form kAADO= (2πq/μ1/2)[α1/2+CμD (2/πkT)1/2+(ZμDI1/2/α1/4)], where the first two terms are simply kADO and the last term comprises the correction due to angular momentum conservation. The constant C is the ADO dipole locking constant. The constant Z is a function of temperature only and a table of Z values for a wide temperature range is given. For molecules with moments of inertia I?1×10−39g cm2 a somewhat different parameterized equation must be used. As part of the development of the AADO theory, a comparison is made between ADO theory and a recent treatment of Barker and Ridge. A number of recommendations are made for modifying the Barker–Ridge approach. Implementation of these modifications brings essential agreement between the Barker–Ridge approach and the ADO theory.
Equation of state of a hard-disk fluid. I. The virial expansion
Authors: Kratky Karl W.
Published: February 2003

The existing values of the sixth and seventh virial coefficients of hard disks have been improved upon. The subsequent three coefficients (B8,B9,B10) have been estimated using various extrapolation techniques. Furthermore, the equation of state of the fluid has been evaluated by means of various theories. The results obtained by these methods are very consistent with each other.
Branching ratios for TlBr photodissociation with 2660 Å radiation
Authors: White J. C. , Zdasiuk G. A.
Published: February 2003

Radiation from a Nd:YAG laser was used to study the selective photodissociation of TlBr into the Tl metastable state, 6p2P3/2, as a potential candidate for stimulated Roman up-converters.(AIP)
Gradient expansion result for surface tension of binary liquid mixtures
Authors: Bhatia A. B. , March N. H. , Sutton J.
Published: February 2003

The free energy density for a fluid mixture in the gradient expansion form of Fleming, Yang, and Gibbs is shown to lead to a formula for the surface tension which reduces to the phenomenologically derived result of Bhatia and March under some additional approximations. The status of this latter formula is thereby clarified.
Calculated transitions in Li2 isotopes at argon laser frequencies
Authors: Lightman Allan J. , Mathur B. P.
Published: February 2003

Recent spectroscopic studies of lithium dimers have been conducted with high precision. Based on these results we have determined the isotopic shift in the term energy for the B1Πu–X1Σ+g transition. Also, discrepancies with other work have come to light and are noted.
Relaxation processes of 9-fluorenone and benzophenone in o-terphenyl solution
Authors: Crossley J. , Tay S. P. , Walker M. S. , Walker S.
Published: February 2003

Dielectric absorption studies have been made on both 9-fluorenone and benzophenone in o-terphenyl solution at several temperatures above the melting point of o-terphenyl in the frequency region 1 to 6 GHz. These relaxation data may be explained in terms of molecular relaxation of individual solute molecules. This is to be contrasted with the behavior of these systems as supercooled liquids where the absorption may be accounted for in terms of an α process which involves cooperative motion. It seems likely that,between the glass transition temperature and the melting point, the motion of regions of the solvent govern the relaxation process of the solute. Above the melting point, however, local solute–solvent motion as involved in the customary dielectric relaxation process of a solute at low concentration in a nonpolar solvent predominates.
Dissociation of CH4 by 1 MeV H+, He+, and O+ projectiles
Authors: Edwards A. K. , Graves J. E. , Wood R. M. , Steuer M. F.
Published: February 2003

Methane is bombarded by 1 MeV H+, He+, and O+ projectiles. Kinetic-energy spectra are presented for fragment ions with integer mass-to-charge ratios of 1,2,3,4,6, and 12. The H+ and H+2 kinetic-energy spectra show well-defined structure which is indicative of two-body breakup of the CH4. In addition, the H+ spectra exhibit high-energy tails which may be attributed to multibody breakup. This feature is greatly enhanced in the H+ energy distribution produced with O+ projectiles. Bombardment with O+ projectiles also produces carbon fragments Cq+(q=1,2,3), C4, and/or H+3.
Uniform theory of electron kinetics in nonpolar liquids
Authors: Rice Stephen A. , Baird James K.
Published: February 2003

Application of the Smoluchowski–Debye equation results in a uniform treatment of the kinetics of either of the bulk reactions e− +M+→M or e−+A→A−, and the corresponding photoionization reaction, either M⇄hνM++e− or A−⇄hνA+e−. For simplicity of terminology, the bulk reactions are referred to as class I and the photoionizations as class II. Included in the theory are the following effects: (1) solvent screening of the interaction potential between e− and either of the acceptors M+ or A, (2) spatial variation of electron transport coefficients (mobility and diffusion tensor) due to the internal electric field associated with the interaction potential, (3) electron tunneling, and (4) chemical activation processes. By use of Green’s function methods, we arrive at the result n1(r)exp[−ψ (r)]=NsP (r), where n1(r) is the pair density at a relative separation r in a reaction of class I, and P (r) is the escape probability for a pair initially at a separation r in the corresponding reaction of class II. The quantity Ns is the electron concentration in the class I reaction (Ns=NM+ or NA).