bluuebunny/crossref_metadata_2025 huggingface dataset:
Computer simulation of the main gel–fluid phase transition of lipid bilayers
Authors: Mouritsen O. G. , Boothroyd A. , Harris R. , Jan N. , Lookman T. , MacDonald L. , Pink D. A. , Zuckermann M. J.
Published: February 2003

Monte Carlo techniques have been applied to a study of two related quasi-two-dimensional microscopic interaction models which describe the phase behavior of phospholipid bilayers. The two models are Ising-like lattice models in which (a) the acyl chains of the phospholipids interact via anisotropic van der Waals forces and (b) the rotational isomerism of the chains is accounted for by two and ten selected conformational states, respectively. Monte Carlo experiments are performed on both models so as to determine whether the static thermodynamic properties of lipid bilayers are most accurately represented by a simple two state gel–fluid concept or whether a more complicated melting process involving intermediate states takes place. To this purpose, the temperature dependence of several static thermodynamic properties has been calculated for both models. This includes the chain cross-sectional area, the internal and free energies, the coherence length, the lateral compressibility, and the specific heat. Particular care has been devoted to the transition region, since no analytical results are available in this region for either model. The comparison between the Monte Carlo results for the two models demonstrates that, whereas the two-state model has a first-order transition with jumplike behavior in the transition region the ten-state model exhibits a first-order transition associated with a closed hysteresis loop. Next, the Monte Carlo results for cross-sectional areas per lipid chain, coherence lengths and lateral compressibilities are discussed in the context of experimental results for dipalmitoyl phosphatidylcholine (DPPC). A detailed comparison is made with the results of molecular field calculations throughout the paper. Finally, a Monte Carlo analysis of bilayers composed of both DPPC and cholesterol shows that a two-state model does not adequately describe the thermodynamic behavior of lipid–cholesterol mixtures implying that intermediate states have to be introduced to account for the experimental data.
A time dependent wave packet approach to three-dimensional gas–surface scattering
Authors: Drolshagen Gerhard , Heller Eric J.
Published: February 2003

A time dependent wave packet approach to gas–surface scattering is presented. This semiclassical method is based on Gaussian wave functions whose average positions and momenta are those of classical trajectories. The initial states are represented as superpositions of Gaussian wave packets. These wave packets are propagated individually and after the collision, the scattering information is extracted by projecting them onto the final states in a coherent way, according to the initial expansion coefficients. The powerful new approach allows the treatment of surface imperfections as well as the inclusion of more degrees of freedom of the surface or the gas particle. The accuracy of the present semiclassical method is tested by a comparison with exact quantal calculations for He–LiF diffractive scattering and in most cases excellent agreement is found. In addition, the wave packet approach is used to calculate diffraction probabilities at high energies and to examine the interference structure of the scattered particles as a function of the size of the surface from which the scattering occurs.
On the optical properties of trans-polyacetylene
Authors: Eckhardt H.
Published: February 2003

The optical properties of undoped polyacetylene are discussed. Specular reflection spectra from freestanding cis- and trans-(CH)x films are compared to transmission spectra from thin films. It is shown that the ‘‘true’’ absorption spectrum of well ordered, long chain trans-polyacetylene has a well defined 0–0 transition at 12 000 cm−1 followed by vibrational structure. This suggest an excitonic assignment for the lowest energy optical transition. In thin films conjugation length dispersion is shown to be a major source of disorder dominating the optical properties. (AIP)
Study of O2 (1Δ) production in a glow discharge at large molar flow rates
Authors: Torchin L. , Jegou R. , Brunet H.
Published: February 2003

O2 (1Δ) and O (3P) produced in the oxygen glow discharge at large molar flow rates (3–12 mmol/s) have been measured. Up to 1.1% of initial molecular oxygen is converted into singlet 1Δ oxygen, and 3.3% into atomic oxygen. Incidentally, a new emission band at 5885 Å, not previously reported, has been observed and ascribed to AuO.
The microwave spectrum of the fluoromethyl radical, CH2F
Authors: Endo Yasuki , Yamada Chikashi , Saito Shuji , Hirota Eizi
Published: February 2003

The microwave spectrum of the fluoromethyl radical, CH2F, has been observed by using a source-frequency modulation spectrometer with a 1 m long free space absorption cell. The CH2F radical was generated directly in the cell by the reaction of methyl fluoride with 2450 MHz discharge products of CF4. The a-type R-branch transitions, N=1 ← 0, 2 ← 1, and 3 ← 2 for the ground vibrational state and N=2 ← 1 and 3 ← 2 for the first excited state of the out-of-plane bending mode (ν4), have been observed; all of them were found to be split into fine and hyperfine components, and individual lines were measured precisely. The hyperfine structure observed for the ground state is consistent with electronic symmetry of B1 and with odd parity with respect to the reflection on the ‘‘molecular plane.’’ A relative intensity measurement shows that the v4=1 state of even parity is located 300 (30) cm−1 above the ground state. These observations suggest that the CH2F molecule is essentially planar, although the presence of a small hump at the planar configuration cannot be excluded.
Mechanism for the decay of Ar(3P2) atoms in pure argon. Reconciliation of low pressure and high pressure values for the decay constant
Authors: Manzanares E. R. , Firestone R. F.
Published: February 2003

A new set of decay constant values for Ar(3P2) in pure argon at 298 K determined by means of single frequency cw laser probe spectrophotometry in the 100–700 Torr region is reported. An excimer forming mechanism initiated by a reversible termolecular step and carried forward by a sequence of reversible collisional relaxation stages with provision for radiative decay at each stage is proposed to account for the kinetic behavior of Paschen-1s atoms. It is demonstrated that this mechanism is quantitatively consistent with measured collisional decay constant values at pressures in the 0–22 and 100–700 Torr regions and that adherence of such values to form, k3P2=AP2+BP, is fortuitous and that the values of A and B have no mechanistic significance. It is similarly indicated that the nature of the probable decay mechanism precludes the possibility of recovering excited atom precursor decay constant values by analysis of excimer emission and absorption data without detailed numerical modeling.
The photodissociation of nitromethane at 193 nm
Authors: Butler L. J. , Krajnovich D. , Lee Y. T. , Ondrey G. S. , Bersohn R.
Published: February 2003

The dissociation of nitromethane following the excitation of the π* ← π transition at 193 nm has been investigated by two independent and complementary techniques, product emission spectroscopy and molecular beam photofragment translational energy spectroscopy. The primary process is shown to be cleavage of the C–N bond to yield CH3 and NO2 radicals. The translational energy distribution for this chemical process indicates that there are two distinct mechanisms by which CH3 and NO2 radicals are produced. The dominant mechanism releasing a relatively large fraction of the total available energy to translation probably gives NO2 radicals in a vibrationally excited 2B2 state. When dissociated, other nitroalkanes exhibit the same emission spectrum as CH3NO2, suggesting little transfer of energy from the excited NO2 group to the alkyl group during dissociation for the dominant mechanism. This conclusion is supported by the apparent loss of the slow NO2 product in the molecular beam studies to unimolecular dissociation to NO+O, which will occur for NO2 with 72 kcal/mol or more internal energy. Evidence is presented which suggests that the NO2 produced via the minor mechanism, which releases a smaller fraction of the available energy to translation, has a large cross section for absorbing an additional photon via a parallel transition and dissociating to NO+O.
Photofragmentation of nitromethane in a molecular beam at 193 nm
Authors: Blais Normand C.
Published: February 2003

We report the cross section for dissociation of nitromethane at 193 nm. It is (1.7±1.0)×10−17 cm2 measured relative to that for H 79Br. From this, we infer that the quantum yield for the process is nearly unity. Photofragments of the dissociation were observed at masses 15, 16, and 30 corresponding to CH3, O, and NO, but only a very small amount of fragments at mass 46 (NO2) was measured. Time-of-flight spectra were obtained for all fragments except for mass 46. Our data is explained by assuming that two sequential absorptions of photons occur. The first photon dissociates CH3NO2 to CH3 and NO2. The second photon is absorbed by the NO2 fragment to give NO+O. About 30% of the NO2 participate in the second absorption and the rate determining step is the primary absorption. The primary absorption fragments are characterized by having a most probable kinetic energy of 7% of the maximum available energy with an average of 19%. The remainder of this energy appears as internal energy of the fragments which can include electronic excitation of NO2 to the A 2B2 state or even to further dissociation of NO2 to NO+O.
Electron transfer in the O+2(X 2Πg, v=0–8) +O2(X 3Σg, v=0) system
Authors: DePristo Andrew E.
Published: February 2003

Total charge exchange cross sections for the O+2/O2 system were calculated for vibrational levels of the ion up to v=8 with ground state neutrals. Four different interaction potentials were utilized, each with different coupling strength between the two electronic arrangements, and the results compared to experimental data. Quantitative agreement was not obtained and qualitative agreement was determined to reflect mainly the use of accurate vibrational overlaps and energy gaps rather than the features of the interaction. The implication of these results for the general theoretical treatment of ion–molecule reactions is discussed briefly.
Photofragment spectroscopy and dynamics of the visible photodissociation of ozone
Authors: Moore David S. , Bomse David S. , Valentini James J.
Published: February 2003

We report collision-free coherent anti-Stokes Raman scattering (CARS) spectra of the O2 fragment formed in the photodissociation of ozone at 532 and 578 nm. From these spectra we have extracted the nascent electronic, vibrational, rotational, and translational energy distributions of the photofragments. The O2 fragment is produced only in the ground 3Σ−g electronic state, even though the 1Δg excited state is energetically allowed. High (J≊23 to 51) rotational states in the first five vibrational levels of O2(3Σ−g) are populated in the photodissociation. For both photolysis wavelengths most of the available energy, 65%–67%, appears in translation, while 15%–18% is in rotation, and 17%–18% is in vibration. These energy distributions are discussed in terms of simple models of the dissociation dynamics.
Electron transfer between magnetic ions in mixed valence binuclear systems
Authors: Girerd J.-J.
Published: February 2003

This paper describes how the Hubbard model in the atomic limit implemented by taking into account molecular vibrations can give a description of mixed valence binuclear systems with both metallic ions simultaneously magnetic. A FeIII (high spin) FeII (high spin) binuclear complex would constitute an example. In such systems electron transfer and electron exchange are expected. If the compound belongs to class II (Robin and Day classification) we find that the activation energy of the thermal electron transfer and the intensity of the intervalence band are spin dependent but that the energies of spin states are given by the Heisenberg Hamiltonian with a new expression for the exchange parameter. For class III binuclear complexes a quite different behavior is found. The energy of the intervalence band is spin dependent but energies of spin states are no longer given by the Heisenberg Hamiltonian.
The high-resolution photoelectron spectrum of N2O. The B̃(2Πi) state
Authors: Cvitaš T. , Klasinc L. , Kovač B. , McDiarmid Ruth
Published: February 2003

The complex photoelectron spectrum of the B̃ state of N2O+ has been remeasured at high resolution and a novel model proposed to interpret the improved data. In the model, a progression of variable strength Fermi resonances is postulated to occur between pairs of vibrational subbands that differ by ν3−2ν2−(2v−1)x33 rather than between members of the usual overtone progression. The model is seen to accurately describe the experimental data.
Properties of solid potassium cyanide: Is the physics of cyanide crystals really chemistry?
Authors: Klein Michael L. , McDonald Ian R.
Published: February 2003

Molecular dynamics simulations are reported for the orientationally disordered phase I of solid potassium cyanide. The calculations are based on an interionic potential incorporating a charge distribution for the cyanide ion which gives realistic values for the electrostatic moments up to and including the hexadecapole. Semiquantitative agreement with experiment is obtained for a wide range of static and dynamic properties of the crystal. Comparison with results obtained earlier for simpler electrostatic models shows that the high order moments are an essential ingredient of a satisfactory potential.
The influence of adsorbate interactions on kinetics and equilibrium for CO on Ru(001). I. Adsorption kinetics
Authors: Pfnür H. , Menzel D.
Published: February 2003

After careful calibration, the work function change induced by CO adsorption on Ru(001) has been used to measure the sticking coefficient s as functions of sample temperature (between 100 and 400 K) and coverage, with the gas temperature always at about 295 K. The main results are: At θ=0, the sticking coefficient is independent of sample temperature between 100 and 400 K, at s0=0.7 for 0<θ<0.2. s drops proportional to (1–θ) for Ts≳200 K, and more gradually below 200 K; at 100 K, s drops slowly up to θ=0.5. Above θ=0.23 a sharp decrease of s sets in for Ts≳120 K; a second plateau becomes obvious around θ∼0.35 for Ts<360 K. Close to saturation, s drops to below 10−2. These results are discussed in terms of a precursor model with strong effects of the chemisorbed layer on the properties of the precursor. A critical evaluation of the formal precursor states involved suggests that classical precursor states exist only at appreciable coverage and are defined by the lateral and vertical interactions, while at low coverage the collisional dynamics are determined for sticking of molecules.
Fluorescence excitation spectrum of triptycene in a supersonic jet
Authors: Bersohn R. , Even Uzi , Jortner Joshua
Published: February 2003

The laser-induced fluorescence excitation spectrum of triptycene in pulsed supersonic expansions of He reveals the electronic origin of the S0→S1(1A′1→1E′) transition at 2750.85 Å (36 352 cm−1), followed by 19 vibrational excitations in the energy range 350 cm−1 above it. This vibrational structure corresponds to the excitations of four torsional modes of the phenyl rings. Mechanical models for these torsional modes indicate the existence of three doubly degenerate vibrations, exhibiting Jahn–Teller coupling with the 1E′ state, together with double excitations of a nondegenerate vibration. At excess vibrational energies of 350–1000 cm−1 above the S1 origin the laser-induced fluorescence spectrum is extremely weak and devoid of any structure, implying the onset of a new effective intramolecular decay channel, which may result in isomerization in the isolated molecule.
EXAFS study of Ni–Cl bonding in Ni(II) aqueous solutions at increasing Cl−/Ni2+ ratios
Authors: Licheri G. , Paschina G. , Piccaluga G. , Pinna G.
Published: February 2003

Three aqueous solutions of NiCl2 with a constant concentration in Ni2+ and increasing Cl−/Ni2+ ratios were examined by EXAFS spectroscopy. Fourier transforms of the experimental data give evidence of the increasing complex formation between Ni2+ and Cl− ions. A quantitative analysis of this phenomenon is also attempted, based on the interpretation of Fourier filtered EXAFS spectra.
Temperature dependence of the spin Hamiltonian parameters in KCl:Eu2+ crystals
Authors: Piña C. , Muñoz E. , Boldú J. L.
Published: February 2003

The temperature dependence of the spin Hamiltonian parameters of Eu2+ in orthorhombic sites of KCl crystals has been measured in the range 87–573 K using EPR techniques. In this work, we fit an analytical expression for the predominant parameter b02 as a function of temperature, b02=a0+a1 T+a2 coth θ/2T. With eight experimental points the equation for b02 was fitted and the following values were obtained a0=364.54×10−4 cm−1, a1=−0.03 ×10−4 cm−1 K−1, and a2=−5.72× 104 cm−1. The root of the mean squared deviation between calculated and reported values is σ=2.03×10−4 cm−1.
Frequency modulation coherent anti-Stokes Raman spectroscopy (FM-CARS): A novel sensitive nonlinear optical method
Authors: Lotem Haim
Published: February 2003

A frequency modulation coherent anti-Stokes Raman spectroscopy (FM-CARS) is proposed. Using phase-modulated Stokes input field, a frequency modulated CARS output is generated. Raman resonances are detected by a CARS heterodyne signal generated in a square-law photodetector. Theoretical estimations show that a high signal-to-noise ratio which is shot-noise limited and effective background suppression are expected with the proposed method independent of the lasers polarization. In typical solvents the sensitivity estimated is χ(3)R=4–8×10−18 esu, which corresponds to the detection of 1–2×10−4 M benzene in cyclohexane.
Three-body association reactions of NO+ and O+2 with N2
Authors: Speller C. V. , Fitaire M. , Pointu A. M.
Published: February 2003

The cluster ions NO+ (N2)n, n≤5, and O+2 (N2)n, n≤6, were observed in a N2–O2 gas mixture irradiated by α particles. The temperature dependence of equilibrium constants K for association reactions X+(N2)n−1+2N2⇄X+ (N2)n+N2, X=NO, O2, was determined using high-pressure mass spectrometry. With X=NO, the enthalpies and entropies of clustering (−ΔH°, kcal/mol;−ΔS°, cal/mol K) are found to be (4.4; 13.3) and (3.9; 12.6) for n=1 and 2, respectively. Estimation of K at 204 K yields 1.1×10−2 (±30%) Torr−1 for n=3 and ∼4×10−3 Torr−1 for n=4. For X=O2, the (−ΔH°;−ΔS°) values, for n=1, 2, 3 are (5.20; 15.8), (4.3; 13.8), and (3.46; 12.1), respectively. The value of K (n=4) is estimated to be ∼ 4×10−2 Torr−1 at 184 K, and ∼7×10−3 Torr−1 at 204 K; for n=5, K is ∼1×10−2 Torr−1 at 184 K. The results for n=1 are compared with previously reported measurements.
Conversion of bound states to resonances with changing internuclear distance in molecular anions
Authors: Lauderdale Jack G. , McCurdy C. William , Hazi Andrew U.
Published: February 2003

The complex self-consistent field (CSCF) method has been used to compute the complex potential-energy curves of the lowest 2Σ+u autodetaching resonance state of F−2 and the 2Πg resonance state of N−2. The calculated width of the 2Πg resonance of N−2 as a function of internuclear distance is in excellent agreement with the results of previous calculations by other methods. The SCF energies of the anion and the neutral molecule in these examples cross at an internuclear distance different from that at which the SCF energy of the anion becomes complex. We find this to be a general feature of the SCF description of shape resonance states. Correlation effects appear to be of critical importance in determining the behavior of the resonance states in the crossing region.
Temperature dependence of near-resonant vibrational energy transfer between CO2 and deuterated methanes
Authors: Carlos Manzanares I , Carrazza Jose , Carrazza Jose
Published: February 2003

Vibrational deactivation of CO2(00°1) by CD4, CHD3, CH2D2, and CH3D has been studied using the laser fluorescence method. Rates are determined as a function of temperature in the range 220–300 K. The rate constants increase as the temperature decreases for all the systems studied. The deactivation of CO2(00°1) correlates linearly with the number of stretching modes between the 2109 and 2263 cm−1 region of CHnD4−n(n=0–4) molecules which, in turn, correlates with the number of deuterium atoms in the collision partner. The linear correlation occurs at all the temperatures studied. This result is interpreted as a near-resonant vibrational-rotational energy transfer process in which two vibrational quantum numbers change as the energy is shared between the collision partners. The rotational distribution of population of each CHnD4−n molecule is calculated and compared with the rotational distribution of the initially excited CO2 molecule. Sharma–Brau probabilities are calculated and compared with experimental probabilities. These calculated probabilities include average over the distribution of velocities, impact parameter, and rotational distributions. Experimental and calculated results indicate the effect of attractive forces in the energy transfer process and the importance of the rotational transitions decreasing the ΔE released to translational motion of the collision pair.
Nitric oxide vibrational excitation from the N(4S)+O2 reaction
Authors: Herm Ronald R. , Sullivan Brian J. , Whitson Milo E.
Published: February 2003

Measurements of the vibrational distribution of NO produced in a room temperature flowtube study of N(4S) + O2→NO(0≤v≤7) + O are reported. Ultraviolet laser induced fluorescence detection of NO(v) in levels v=0–7 was employed to study NO production under conditions where O2 vibrational quenching was insignificant. The results indicate that 42% of the NO molecules are produced in infrared-active states, 38% of them in levels ≥2. This is considerably more NO vibrational excitation than had been inferred from infrared chemiluminescence studies. Moreover, production of infrared active states of NO by the N(4S)+O2 reaction may be even more efficient than these numbers indicate owing to possible vibrational relaxation of the nascent NO(v) distribution by nitrogen atoms in the flowtube.
Nascent NO vibrational distribution from 2485 Å NO2 photodissociation
Authors: Slanger T. G. , Bischel W. K. , Dyer M. J.
Published: February 2003

The initial NO vibrational level distribution has been determined for NO2 photodissociation at 2485 Å. Excitation spectra of the NO vibrational levels were measured by using both the NO A 2Σ+←X 2Π and B 2Π←X 2Π transitions, the latter being somewhat stronger due to saturation effects. It was determined that the NO population was strongly inverted, with most of the nascent NO being in v=6–8; the thermodynamic limit is v=8. Injection locking of the KrF laser output permitted study of the 2491 Å NO2 band, and it was evident that the increased absorption in this region gave greatly enhanced signal levels in the excitation spectra, at those wavelengths where NO2 and NO absorption lines coincide. It was demonstrated that in the 2640–2850 Å wavelength region, NO2 can be detected by use of a single dye laser, simultaneously dissociating NO2 and electronically exciting the resultant vibrationally hot NO. Deactivation of NO(v=8) by NO2 was found to proceed with a rate coefficient of 1.1×10−11 cm3 molecule−1 s−1, whereas the coefficient for quenching by N2 and He was ≤2×10−15 cm3 molecule−1 s−1. The peculiar NO rotational distributions noted by Zacharias et al. in their study of NO2 dissociation at 3371 Å were also observed in the present work.
Reply to ‘‘Comment on ‘Force in SCF theories’ ’’
Authors: Nakatsuji H. , Kanda K. , Hada M. , Yonezawa T.
Published: February 2003

Pulay’s1 comments on the author’s accurate Hellmann-Feynman force method2−4 to calculating wave functions are answered. The comments concern reducing the computer time with an energy gradient method. (AIP)
FTIR observation of the N2---HF complex in solid argon
Authors: Andrews Lester , Kelsall Benuel J. , Arlinghaus Robert T.
Published: February 2003

Sharp absorptions at 3881.5 and 262 cm−1 in argon matrices containing HF are shown to depend upon N2. Substitution with DF indicates assignments to the νs and νl modes of the linear N2---HF complex.
A new uniform semiclassical wave function for single surface and multisurface scattering
Authors: Herman Michael F.
Published: February 2003

A procedure for the evaluation of one-dimensional uniform semiclassical wave functions is explored. This form of the approximate wave function is tested for the case of an exponential potential and is compared to the exact quantum mechanical and WKB wave functions. It is found that the uniform approximation provides a very accurate representation of the exact wave function, even at the turning points of the classical motion, where the WKB approximation diverges. It is demonstrated how correction terms can be constructed, leading to a formal expansion for the exact wave function. The individual terms of this expansion each involve a finite number of reflection points with uniform semiclassical propagation between these reflections. The analysis is extended to multisurface (nonadiabatic) problems, removing the turning point singularities which are generally present in semiclassical formulations of nonadiabatic problems.
Product distributions in the reactions of excited noble-gas atoms with hydrogen-containing compounds
Authors: Balamuta John , Golde Michael F. , Ho Yueh-Se
Published: February 2003

Dissociation and ionization of small H-containing molecules following energy transfer from electronically excited (3P0,2) states of Ar, Kr, and Xe have been studied in discharge–flow systems, using the atomic resonance fluorescence and the saturation ion–current techniques. Chemi-ionization is observed in all reactions in which Penning ionization is exothermic, but molecular dissociation is the major channel in all cases. Elimination of H atoms is strongly favored over both loss of H2 and cleavage of central bonds of larger molecules, in contrast to the known vacuum–UV photochemistry of several of the molecules. When strongly exothermic, elimination of two H atoms in a single reactive event is a major and, in some cases, the dominant channel.
Influence of the rotational energy of the molecular perturber on the Rb(5P)–H2,D2 fine structure changing collisions
Authors: Cuvellier J. , Mestdagh J. M. , Ferray M. , de Pujo P.
Published: February 2003

A crossed beam apparatus has been used to study the Rb(5P1/2 → 5P3/2) fine-structure-changing (FSC) transitions induced by D2 and H2 molecules. The rotational temperature of the molecular perturber and the relative energy ER of the colliding atom–molecule pair were varied independently. The observed FSC cross section for H2 remains almost unchanged as the rotational temperature is varied. On the contrary, the rotational temperature of D2 results, in some cases, in a rather distinct variation of the FSC cross section; this is most pronounced for ER∼0.08 eV and Trot(D2)<200 K. Reference is made to the theoretical model of Hickman; it accounts well for the general trend in the cross section measured with H2. The results for D2–Rb collisions imply that the various rotational levels of D2 do not seem to contribute equally to the observed FSC cross sections. This work supports the assumption that for D2 molecules in the rotational level J=2 (and probably also J=3), the FSC cross section is larger than for D2 in its other rotational levels.
Comments on two treatments of symmetry rules in chemical reactions
Authors: Metiu H. , Schatz George C. , Ross John
Published: February 2003

In this paper we compare a recently presented approach to the determination of symmetry rules for chemical reactions based on adiabatic electronic states to earlier work in which a quasiadiabatic representation was used. The adiabatic approach is shown not to lead directly to Woodward–Hoffmann symmetry rules, and a number of the conclusions of the adiabatic work are shown to be incorrect and inconsistent. An indirect approach for deriving the Woodward–Hoffman rules in the adiabatic representation is presented which uses the connection between the adiabatic and quasiadiabatic representations to relate barrier height to the quasiadiabatic electronic matrix elements.
Theoretical studies of van der Waals dimer depletion mechanisms in free jet expansions: The Ar2+X (X=CO2, CO, N2) systems
Authors: Viswanathan R. , Raff L. M. , Thompson Donald L.
Published: February 2003

Collisional energy transfer, exchange, and complex formation mechanisms for Ar2 dimer depletion in free jet expansions have been investigated using quasiclassical trajectory methods on several different potential-energy surfaces. Computed Ar2 dissociation cross sections show that V → V energy transfer is an unimportant mechanistic pathway for Ar2 dissociation in collisions with CO2, N2, and CO. An R → V energy transfer pathway is found to be important at translational energies of 0.03 eV. However, there is very little difference among the results obtained for CO2, N2, and CO. At higher translational energies, around 0.10 eV, the importance of an R → V energy transfer mechanism in Ar2 dissociation decreases. The results are found to be insensitive to moderate variations in the pairwise LJ(12,6) potential parameters. Three-body potential terms are shown to be of negligible importance. Rate coefficients for collisional dissociation, exchange, and complex formation have been computed for (CO2, Ar2) and (N2, Ar2) systems under conditions that approximate those existing in the experiments reported by Yamashita et al. [J. Chem. Phys. 75, 5355 (1981)]. For the CO2 system, collisional dissociation is the major mechanistic pathway for Ar2 depletion. Exchange plays only a minor role. Complex formation does not occur. For the N2 system, collisional dissociation predominates for a rotational temperature equal to 298 K. At lower rotational temperatures, exchange becomes the major process. Complex formation does not occur. Yield ratios computed from a simple pseudo-first-order rate model are found to be in good accord with the experimental data for most systems. The exceptions are (C2H4, Ar2) when the mole fraction of C2H4 is 0.10 or greater, and the [C4H6, (CO2)2] system.
Dissociation curves for nine low lying states of Tl2 from REP CI calculations
Authors: Christiansen P. A.
Published: February 2003

Dissociation curves for nine low lying states of Tl2 have been computed using relativistic effective potential and configuration interaction techniques. We can identify the molecular ground state as 0−u with a dissociation energy of 0.16 eV. 0+g and 1u states are about 0.1 eV higher in energy. The computed vibrational frequencies for these states are well below values from the Tl2 thermal emission spectrum. The discrepancy between our bond energy and the value (0.63 eV) given in recent mass spectrometric work is due, in part, to the use of inappropriate molecular parameters in the mass spectrometric analysis.
Use of dimer potentials to calculate the energy levels of alkali atoms in rare-gas matrices
Authors: Balling L. C. , Wright J. J.
Published: February 2003

A simple procedure is described for calculating the energy levels of an alkali atom perturbed by rare-gas neighbors in a solid rare-gas matrix, using alkali-rare-gas dimer potentials available in the literature. Straightforward analysis leads to an expression for the perturbation due to an arbitrary number of rare-gas neighbors in terms of alkali-rare-gas dimer potentials. The results of calculations for Na trapped in Ar are presented and compared with experiment and with earlier pseudopotential calculations.
Theoretical calculation of the absorption and magnetic circular dichroism spectrum of a Jahn–Teller distorted excited state: The 1E′ excited state of cyclopropane
Authors: Duch W. , Segal G. A.
Published: February 2003

This paper reports the ab intio calculation of the absorption and magnetic circular dichroism spectrum of a Jahn–Teller distorted system, the 1E′ (3s) excited state of cyclopropane, interacting with the doubly degenerate ring stretching mode. The vibronic problem is treated in the adiabatic approximation. Good agreement is found with the observed vibronic spectrum and a distinctive behavior is predicted for the MCD.
On the low-lying states of CuO
Authors: Bagus Paul S. , Nelin Constance J. , Bauschlicher Charles W.
Published: February 2003

Self-consistent field and correlated wave functions have been computed for the ground and for several low-lying states of CuO. The ground state is X 2Π and the lowest excited state, at ∼8000 cm—1 above X 2Π, is a previously unidentified 2Σ+ state. The separation of these states is compared to that for the similar states of KO and is analyzed in terms of integrals between orbitals of the separated free ions. We consider a classification of the states of the molecule based on states of Cu+ and O− which leads to a division into manifolds of states arising from Cu+ 3d10 and Cu+ 3d9 4s1. We predict that the states of the 3d9 4s1 manifold are 10 000–30 000 cm−1 above the ground state and assign the observed A 2Σ+ state at 16 500 cm−1 to this manifold.
Hindered rotational energy levels of a tetrahedron in a tetragonal crystalline field
Authors: Smith David
Published: February 2003

The hindered rotational energy levels of a tetrahedron in a tetragonal crystalline field have been computed. The symmetry group of the Hamiltonian is T̄×D2, where T̄ is the tetrahedral group of rotations about body-fixed axes and D2 represents the tetragonal group of rotations about space-fixed axes. The parameters in the potential function are determined for the tetragonal phases of NH4Br and NH4I assuming an electrostatic model.
Ultrasonic study of critical behavior in the binary liquid 3-methylpentane + nitroethane
Authors: Garland C. W. , Sanchez G.
Published: February 2003

The acoustic velocity and attenuation have been measured in a critical mixture of 3-methylpentane + nitroethane over the frequency range 1–17 MHz and the temperature range 0.004≤T−Tc≤13.5 K. No critical dispersion was observed in the velocity, which varied linearly with T. A substantial critical attenuation was well characterized as a function of the reduced frequency ω*≡ω/ωD over the range 10−1≤ω*≤106. The experimental data in this system are extensive enough to permit quantitative tests of the predictions of dynamic scaling theory, dynamic renormalization-group theory, and several versions of mode-coupling theory. Universality in the scaling function for the critical attenuation is demonstrated by comparison with comparable experimental results in two pure fluids.
Trigonal-13C hyperfine coupling in CF3CCl2
Authors: Walther Brian W. , Williams Ffrancon
Published: February 2003

An intense isotropic ESR spectrum of CF3CCl2 is generated by annealing γ-irradiated samples of CF2CCl3 above 145 K, the sudden spectral change being irreversible and probably corresponding to the loss of a chloride ion from the CF3CCl3− radical anion. Satellite lines detected at high gain are assigned to CF313CCl2 with a 13C hyperfine coupling of 67.3 G, showing that th radical possesses a more planar configuration than that of the CCl3 radical (13C hfc≂116 G). This result strongly supports a previous argument that increasing the electronegativity of the α substituent is much less important than the effect of π-conjugative destabilization in promoting the pyramidality of alkyl radicals. (AIP)
Radiationless transitions in linear polyenes
Authors: Dinur U. , Scharf B.
Published: February 2003

Electronic radiationless transitions in linear polyenes are discussed from a theoretical point of view. Only in-plane relaxation is considered. It is concluded that for a given energy gap, the nonradiative decay rate is a decreasing function of the chain length. Model calculations of the nonradiative lifetime as a function of the polyene chain length are presented. For those cases where comparison with the experiment is possible (8 to 12 membered polyenes) agreement with the calculations is reasonable. The decrease in the rate of radiationless transitions with the increase in the molecular system is shown to have implications for biological systems, such as rhodopsin, that undergo a fast photochemical change.
Electron diffraction studies of supersonic jets. V. Low temperature crystalline forms of SF6, SeF6, and TeF6
Authors: Valente Edward J. , Bartell Lawrence S.
Published: February 2003

Condensation of SF6, SeF6, and TeF6 in nozzle flows with inert carrier gases produces microcrystals of these materials. All form the higher temperature body-centered cubic structure at higher partial pressures of hexafluoride. At lower partial pressures and colder nucleation conditions a lower symmetry form of each has been produced. Electron diffraction powder patterns are consistent with the space group Pnma to which metal hexafluorides of UF6 type belong. Low temperature phases of the present materials differ from those of the metal compounds, however, in being less dense than the cubic forms. Aspects of the gas dynamics affording a control over nucleated species are briefly discussed.
Relative quantum yield of I*(2P1/2) in the tunable laser UV photodissociation of i-C3F7I and n-C3F7I: Effect of temperature and exciplex emission
Authors: Smedley John E. , Leone Stephen R.
Published: February 2003

Wavelength-specific relative quantum yields of I* from pulsed laser photodissociation of i-C3F7I and n-C3F7I in the range 265–336 nm are determined by measuring the time-resolved infrared emission (λ=1.315 μm) from the atomic I(2P1/2→2P3/2) transition. The quantum yields of I* from i-C3F7I and n-C3F7I are constant and appear to be unity from 265–298 nm but decrease to 0.57 and 0.40, respectively, at 336 nm. The lower quantum yields suggest the existence of two distinct bands in this region of the absorption continua. To determine the quantum yields a correction is made for enhanced emission due to formation of exciplexes between I* and the parent (i- or n-C3F7I) species. The exciplex emission increases linearly with parent gas pressure, but decreases with increasing temperature. For both molecules the integrated absorption in the long wavelength region (≳290 nm) increases substantially with temperature, while at selected wavelengths the quantum yields are found to remain constant with temperature. The results are discussed in terms of the development of solar-pumped iodine lasers.
Collisional coupling and relaxation of N2(B3Πg) and N2 (W 3Δu) vibrational levels in Ar and Ne
Authors: Sadeghi N. , Setser D. W.
Published: February 2003

Laser excitation from N2(A 3Σ+u, v=0–6) in Ne and Ar carrier gas in a flowing afterglow apparatus has been used to prepare N2(B 3Πg) molecules in individual levels from v=3–10. The subsequent N2(B–A) fluorescence intensity was observed to study the coupling and relaxation kinetics of N2(B 3Πg, v=2–10). The decay curves from each level show double (or higher) exponential relaxation. The first component is assigned as primarily the rapid coupling of the N2(B 3Πg,v) and N2(W 3Δu, v or v+1) levels, however, the N2 (B′ 3Σ−u, v−4) levels also may be involved for N2(B, v≥5). The slowest decay component represents the subsequent relaxation of the coupled levels. The combination of the two steps is equivalent to apparent stepwise vibrational relaxation of N2 (B 3Πg, v), such relaxation occurs even at low Ar pressures because of the large rate constants and long radiative lifetimes of N2(B) and N2(W). The coupling pattern between N2(B) and N2(W) is analyzed, and rate constants are assigned to elementary steps in the relaxation sequence and the scheme is evaluated. From the analysis of the slow decay components in argon, estimates (20–90 μs) are obtained for the radiative lifetimes for the reservoir levels, which are assigned as N2 (W 3Δu, v) for N2(B, v≤4) and N2(W 3Δu, v or v+1) levels plus N2(B′3Σ−u, v−4) levels, for some cases, for N2(B, v≥5).
Laser flash photolysis of benzene. VIII. Formation of hot benzene from the S2 state and its collisional deactivation
Authors: Nakashima Nobuaki , Yoshihara Keitaro
Published: February 2003

Time-resolved absorption spectra of gaseous benzene have been observed in the time range from 0 to 4 μs with an ArF laser (193 nm) as an excitation source, which pumps the S2 state. The absorption strength in the transient spectrum observed immediately after excitation with a 10 ns pulse decreases monotonically from 210 to 290 nm. This spectrum is postulated to be due to B**(S0) (a hot benzene with internal energy 624 kJ/mol). The transient spectrum can be simulated as part of the S3(1E1u)←S0 transition at 3390 K on the basis of the Sulzer–Wieland model. The agreement between calculated and observed molar extinction coefficients of B**(S0) indicates that the yield of internal conversion from S2 to B**(S0) is close to unity. Deactivation processes have been detected from B**(S0) to a thermal equilibrium state in inert gas baths. The time profile of absorption intensity due to collisional deactivation is nonexponential and depends upon observing wavelength, being faster at longer wavelength. The deactivation processes are explained in terms of a simple energy transfer model, which assumes that the removal energy per collision depends on the internal energy of the hot molecule. Internal conversion from S2 to B**(S0) is suggested to be the initial step in the photochemical reactions of gaseous benzene. The absorption spectrum and collisional deactivation processes following excitation with a KrF laser (248 nm), which pumps the S1 state, have been analyzed in the same way.
On the origin of the dynamical differences between the Tang and Dalgarno–Henry–Roberts potentials for rigid rotor H2–H collisions
Authors: Margolies Seth , Garetz Bruce A. , Sathyamurthy N.
Published: February 2003

We compare our quasiclassical trajectory results for the J=0 → 2 rotational excitation in rigid rotor H2–H collisions with the close-coupling results of Choi and Tang on two different potentials. We trace the dynamical differences between the two potentials to the differences in the ellipticity of the repulsive potential-energy contours. We also discuss the application of our results to rotational excitation on the most accurate Siegbahn–Liu–Truhlar–Horowitz surface for the collision.
Supplemental basis functions for the second transition row elements
Authors: Walch Stephen P. , Bauschlicher Charles W. , Nelin Constance J.
Published: February 2003

Self-consistent field calculations are reported for the second transition row elements. The configuration interaction optimization of the 4f functions was carried out using the IBM BIGGMOLI[5) and noname(6] programs. (AIP)
Tracer diffusion of counterions through a solution of polyelectrolyte
Authors: Zero Karl , Cyr Donna , Ware B. R.
Published: February 2003

Fluorescence photobleaching technique is used to determine. Tracer diffusion coefficient of an anionic dye through a solution of cAlionic polymer. (AIP)
A molecular dynamics study of the translational and rotational motions in an aqueous LiI solution
Authors: Szász Gy. I. , Heinzinger K.
Published: February 2003

The spectral densities of the hindered translations and librations for the ions and the water molecules in a 2.2 m LiI solution have been derived from an MD simulation with the ST2 model for water. The reorientational times of the dipole moment vector, the intramolecular proton–proton vector as well as the ion–oxygen and ion–hydrogen vectors are also reported. The separate calculation of various properties for the three water subsystems—hydration water of the cation, of the anion and bulk water—contributes significantly to the understanding of macroscopically measured quantities on a molecular level.
Orientation function of the electric birefringence and dichroism of rodlike polyelectrolytes on the basis of the saturating dipole mechanism
Authors: Yoshioka Koshiro
Published: February 2003

The orientation function of the electric birefringence and dichroism of rodlike polyelectrolytes is derived in a simple case where the induced dipole moment is proportional to the field strength at low fields and becomes constant above a critical field strength. This function is expressed in terms of the field strength E, the electrical polarizability α, and the critical field strength E0. The field strength dependence of the orientation function Φ is represented graphically in the form of Φ vs E* plots or Φ vs γ plots for various values of γ0, where E*≡E/E0, γ≡αE2/2kT, and γ0≡αE20/2kT. With decreasing γ0, the deviation from the classical orientation function for the induced dipole orientation becomes larger.
The vibrational spectra of solid II ethane and ethane-d6
Authors: Wisnosky M. G. , Eggers D. F. , Fredrickson L. R. , Decius J. C.
Published: February 2003

Mid-infrared spectra and Raman spectra are reported for solid II ethane, ethane-d6, and their mutual solid solutions, at temperatures near 20 K. The results are consistent with a recent x-ray crystal structure determined for this phase of ethane. Observed vibrations are assigned and compared with several different lattice-dynamical calculations based on various atom–atom potentials. External vibrations are reproduced moderately well by several potentials, one of which has electrical charges on the atoms. Splittings of internal vibrations are not fit well. Several puzzling discrepancies are noted in the Raman intensities for the solid.
A metastable solid phase of ethane
Authors: Wisnosky M. G. , Eggers D. F. , Fredrickson L. R. , Decius J. C.
Published: February 2003

A solid phase of ethane, different from the other solid phases that have been previously described, is reported. Infrared and Raman spectra show it to be ordered and crystalline. The numbers of observed components in the infrared and Raman spectra are consistent with space groups of the types C4, D4, C2h, and Ci. Consideration of molecular packing favors the C2h and Ci space groups. Further work is needed before the crystal symmetry and space group can be determined.
Renormalization group studies of tunneling systems in the condensed phase
Authors: Jongeward Gary A. , Wolynes Peter G.
Published: February 2003

In the path integral representation of the thermal density matrix, the sum over quantum paths is equivalent to the configurational partition function sum for a system composed of long polyatomic molecules. We give an iterative (renormalization group) approach which successively reduces the number of degrees of freedom in this isomorphic classical many-body problem to a more tractable few-body problem. We give as examples a two state tunneling system coupled to an adiabatic and nonadiabatic bath and an impurity with spin coupled to conduction band electrons.
Chemistry in noninteger dimensions between two and three. II. Fractal surfaces of adsorbents
Authors: Avnir David , Farin Dina , Pfeifer Peter
Published: February 2003

The concept of fractal dimension D of surfaces, advanced as natural measure of surface irregularity in part I of this series, is shown to apply to a remarkable variety of adsorbents: graphites, fume silica, faujasite, crushed glass, charcoals, and silica gel. The D values found for these examples vary from two to almost three (for smooth and very irregular surfaces, respectively), thus covering the whole possible range. They quantify the intuitive picture that surface inhomogeneities are minor, e.g., in graphites, but dominant, e.g., in charcoal. The analysis is based on adsorption data, with main focus on adsorbates of varying molecular cross section. They include N2, alkanes, polycyclic aromatics, a quaternary ammonium salt, and polymers. The straight-line plots so obtained confirm also a number of reported on-surface conformations of specific adsorbates. The converse method to get D from varying the size of adsorbent particles is exemplified for fume silica and crushed glass.