bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultrafast conformation equilibration in triphenyl methane dyes analyzed by time resolved induced photoabsorption
Authors: Mokhtari A. , Fini L. , Chesnoy J.
Published: July 2002

Dual frequency pump–probe induced photoabsorption recovery is studied in two triphenylmethane dyes, malachite green and crystal violet, using femtosecond laser pulses. Comparison of the decays observed for different polarization conditions allows us to analyze the symmetry of electronic states. We observe that the simplest C2 or D3 symmetries cannot account for all our observations. In addition to the previously reported electronic relaxation decays, we observe two additional subpicosecond relaxation components. A first rising component is attributed to the conformational relaxation of torsional modes and a faster decay can be interpreted as originating from solvent limited vibrational relaxation. We attribute the solvent dependence of these relaxations to the nonrigid nature of the triphenylmethanes.
Photodissociation dynamics of methylnitrite (CH3O–NO) in the 300–400 nm range: An a
b i
n
i
t
i
o quantum mechanical study
Authors: Hennig Steffen , Engel Volker , Schinke Reinhard , Nonella Marco , Huber J. Robert
Published: July 2002

We report the results of a two-dimensional, quantal study of the photodissociation of CH3O–NO within the first continuum (S0→S1, 300–400 nm) taking into account only the O–N and the N=O separations. The S1 potential energy surface is taken from recent ab initio calculations. The calculated absorption spectrum consists of two band progressions of narrow resonance lines with widths of ∼0.3 and ∼5 meV, respectively. These resonances can be associated with excitation of the O–N bond (m=0,1) and excitation of the N=O chromophore (n*=0,1,2,...). The intensities of the m=1 band are negligibly small compared to those of the m=0 band. The decay mechanism in the two cases is different: The m=0 resonances decay primarily via vibrational predissociation, i.e., a nonadiabatic transition from n* to n*−1, and yield NO products with a preferential population of the (n*−1) level. The m=1 resonances decay mainly via tunneling through a potential barrier yielding preferentially NO products in state n*. Several of the theoretical results agree qualitatively (ratio of peak intensities) or even quantitatively (energy spacing between peaks) with the measurements. Most important, however, is the good agreement found for the vibrational NO distributions at several excitation wavelengths of the parent, which reveals that vibrational predissociation within the S1 state is the main decay mechanism.
Photofragment spectroscopy of CS2 at 193 nm: Direct resolution of singlet and triplet channels
Authors: Waller I. M. , Hepburn J. W.
Published: July 2002

The 193 nm dissociation of CS2 has been studied under supersonic molecular beam conditions, with the atomic S products detected by vacuum ultraviolet laser-induced fluorescence. These experiments measure the branching ratio between singlet and triplet channels to be S(3P)/S(1D) =2.8±0.3. Doppler spectroscopy on the S(3P2) and S(1D2) products was used to study the dynamics of each channel separately. The energy release in each channel was similar, with the CS fragments formed vibrationally inverted with about 50% of the available energy in CS rotation and vibration. The recoil velocity anisotropy in each channel was also measured, found to be the same for both, and was consistent with a 2 ps lifetime for the 1Σ+u(1B2) excited state of CS2.
Supersonic molecular jet spectroscopy of ethylbenzene, the ethyltoluenes, and the diethylbenzenes
Authors: Breen P. J. , Bernstein E. R. , Seeman Jeffrey I.
Published: July 2002

Time-of-flight mass spectra are presented for the S1←S0 origin region of jet-cooled 1,2-, 1,3-, and 1,4-diethylbenzene, and 2-, 3-, and 4-ethyltoluene. The spectra for the diethylbenzenes exhibit two origins which are assigned to syn and anti conformations of the ethyl groups relative to the aromatic ring. The existence of two origins in the 1,3-diethylbenzene spectrum, and only one in the 3-ethyltoluene spectrum, strongly implies that the stable orientation of the ethyl groups is with the β-carbon atom of the ethyl group projecting perpendicular to the plane of the aromatic ring. The size and shape of the potential barrier to rotation of the ring methyl group is obtained by treating the methyl group as a one-dimensional rigid rotor and fitting the calculated energy levels to observed features in the spectra. The results (for the para isomer B=5.20 cm−1, V6=31.00 cm−1; for the meta isomer, B=5.40 cm−1, V3=78.00 cm−1; and for the ortho isomer, B=5.50 cm−1, V3=89.00 cm−1) indicate that the methyl group experiences an increased barrier to rotation in the order para<meta<ortho isomer.
Rovibronic energy transfer from N2(a 1πg) to CO(A 1π) studied by laser REMPI spectroscopy
Authors: Sha G. , Proch D. , Kompa K. L.
Published: July 2002

The collision-induced electronic energy transfer N2(a 1πg,v′) +CO(X 1Σ+,v″=0) →N2(X 1Σ+g,v″) +CO(A 1πg,v′)+ΔE is studied in a gas cell. N2(a 1πg,v′, J′) is prepared by two-photon (2hν1) absorption from the ground state. CO(A 1πg,v′, J′) is probed by two-photon (hν1+hν2) ionization via CO(B 1Σ+) as the resonant intermediate state. Experiments show that the overall energy transfer cross sections exceed that of gas kinetic collisions by a factor of 3–4. The energy mismatch ΔE is the determining factor controlling the branching ratio from one N2(1πg,v′) donor to different vibrational levels of CO(A 1π,v′). For small values of ΔE, CO(A 1πg,v′, J′) shows a Boltzmann-like rotational level population. Its rotational temperature scales with ΔE. About 28% of the excess energy funnels into the rotation of CO(A 1π). An explanation for the observed rotational distribution of CO* and the energy transfer mechanism is given. The rate constants are analyzed in terms of the surprisal.
Optical line broadening of triplet excitons in dibromonaphthalene: Isotopic impurity and phonon scattering
Authors: Port H. , Nissler H. , Silbey R.
Published: July 2002

We present experimental data on the linewidths of the ground state to triplet exciton optical transition in 1,4-DBN at low temperatures. The linewidths are theoretically modeled by contributions due to 13C impurity scattering, defect scattering, deuterium impurity scattering, and one- and two-phonon scattering mechanisms. A consistent and complete description of the contribution of all these processes to the linewidths is achieved.
State-resolved inelastic cross sections from CN A 2Π v=8 to X 2Σ+ v=12: Quenching of the even–odd alternation in the final rotational state populations
Authors: Ali Ashraf , Jihua Guo , Dagdigian Paul J.
Published: July 2002

An optical–optical double resonance technique has been employed for the study of collisional transfer from the CN A 2Π v=8 manifold to X 2Σ+ v=12 with initial and final rotational state resolution. In contrast to our previous studies of vA=3 to vX=7 and vA=7 to vX=11 collisions, no significant even–odd oscillation in the final X state rotational populations was observed for any of the incident A state levels investigated. The absence of this alternation for vA=8 collisions is ascribed to the dependence on CN vibrational coordinate of the off-diagonal matrix element connecting the Σ and Π(A′) wave functions describing the CN(A,X)–Ar interaction. For all incident vA=8 levels studied, a preference for formation of X state e vs f levels of a given N was observed.
Accurate quantum mechanical reaction probabilities for the reaction O+H2→OH+H
Authors: Haug Kenneth , Schwenke David W. , Truhlar Donald G. , Zhang Yici , Zhang John Z. H. , Kouri Donald J.
Published: July 2002

We report converged quantum mechanical reaction probabilities for O+H2(v=0,1)→OH+H for zero total angular momentum as obtained by an ℒ2 expansion of the reactive amplitude density. These provide a benchmark for testing approximate dynamical theories, and this is illustrated by comparisons to centrifugal sudden distorted wave and least-action calculations and vibrationally adiabatic threshold energies.
Studies of the energy dependence of reactions of Ar+ and Ar+2 with CH4 and CS2
Authors: Shul R. J. , Passarella R. , Yang X. L. , Keesee R. G. , Castleman A. W.
Published: July 2002

The rate coefficients for the reactions of Ar+ and Ar+2 with CH4 and CS2 are obtained as a function of energy by using a selected ion flow tube with an electric drift field (SIFT-DRIFT). A diagnostic analysis of the system and technique has been completed and allows rate coefficients to be reported with a high level of confidence. Rate coefficients and product branching ratios are determined for the methane reactions from thermal (room temperature) to center-of-mass kinetic energies of about 0.15 eV and for the carbon disulfide reactions from thermal to about 0.6 eV. The overall rate coefficients for these reactions do not vary a great deal over the investigated energy range. The greatest dependence is found for the slowest reaction Ar+ with CS2, where a minimum in the rate coefficient is observed near 0.15 eV. The behavior of the branching ratios for S+ and CS+2 suggests curve crossings between a repulsive state and the A, B, and C electronic states of CS+2.
Light scattering in a dilute microemulsion. II. Radius dependence of interactions
Authors: Dozier William D. , Kim Mahn Won , Klein Rudolf
Published: July 2002

We have measured the static light-scattering intensity and mutual diffusion coefficient in dilute water-in-oil microemulsions made of sodium diethylhexylsulfosuccinate (AOT), water, and n-decane. From these measurements we determine the second virial coefficient and linear correction to the mutual diffusion coefficient. We discuss these results in the context of a model potential of a hard core plus attractive square well and determine the parameters of the square well from the data. We find the results consistent with prior small-angle neutron scattering results: assuming a constant range for the square well of about 3 Å, we see a linear dependence of the depth of the well upon the radius of the droplets. Then assuming a ‘‘chain interpenetration model’’ for the interaction, we calculate the interaction per overlapping surfactant to be ≈kBT/5.
Analysis of cloud chamber measurements of gas phase polymerization
Authors: Reiss H. , Rabeony H. , El-Shall M. S. , Bahta A.
Published: July 2002

This paper provides a summary of some of the principal mathematical and theoretical tools for the analysis of experimental data accumulated in the course of the study of true gas phase chain polymerization, using nucleation and growth (in cloud chambers) of liquid monomer drops for detection. Included in the discussion are (1) a theory for the nucleation of supersaturated monomer vapors by single polymer molecules formed from the same monomer, (2) the development of the appropriate stochastics for the growth and transport of the polymers in a variety of experiments involving different configurations of space and time, (3) the development of mathematical methods for determining, from the data, rate constants and critical (for nucleation) polymer sizes, and (4) the development of methods for the extraction of mechanistic information from the noise (fluctuating rate) in the reaction involving only a small number of product molecules. All of these ideas are used in the treatment of experiments reported in the following and subsequent papers.
The role of electron transfer stabilization in several gas phase ion–molecule reaction processes
Authors: Gislason Eric A. , Ferguson Eldon E.
Published: July 2002

A simple theoretical method is presented for estimating the bond energies of ion–molecule complexes such as O+2–M and NO+–M, where M is a neutral molecule. The theory has one adjustable parameter, H12, which is the electronic coupling between the state O+2–M (or NO+–M) and the charge– transfer state O2–M+ (or NO–M+). H12 has a fixed value for each ion. Good agreement is obtained with experimental bond energies where available. The theoretical bond energies are then compared with vibrational quenching rate constants and with three-body association rate constants measured for O+2–M and NO+–M systems. In each case there is a strong correlation, in agreement with earlier predictions. A similar comparison is made using the incremental bond energy which can be attributed to the H12 term. The correlation is even better, suggesting that the anisotropy in the ion–molecule interaction plays an important role in stabilizing the collision complexes.
Two-electron atoms near the one-dimensional limit
Authors: Doren D. J. , Herschbach D. R.
Published: July 2002

If the Hamiltonian of a two-electron atom is generalized in a natural way to arbitrary spatial dimension D, an especially simple case is found in the D=1 limit. While the ground state energy is singular at this point, a scaling argument reduces the problem to a limiting Hamiltonian with only two degrees of freedom in which the Coulombic potentials all reduce to δ functions. Since the singularity at D=1 dominates the energy at nearby dimensions, this limit forms the basis for an expansion in (D−1)/D which is reasonably accurate at D=3. By combining results from this expansion with the 1/D expansion about the D→∞ limit, estimates of the energy at D=3 are obtained with accuracy orders of magnitude better than that of either series alone. The simplicity of the D=1 and large-D limits and the accuracy of this method allow some qualitative insight into the physical features contributing to correlation effects in small atoms. Analysis of other singularities suggests that the 1/D series has zero radius of convergence for two-electron atoms. We conclude with a discussion of excited states and larger atoms and make some appealing connections with the orbital picture.
The reaction of Hg(6 3P1)+H2, D2, and HD: Product rotational and vibrational distributions; isotope effects
Authors: Bras N. , Jeannet J. C. , Perrin D.
Published: July 2002

The initial vibrational and rotational state distributions of HgH and HgD products of Hg(6 3P1)+H2, D2, and HD have been determined. No isotope effect was observed for the vibrationless molecules or for HgH(v=1) but this effect appears for vibrationally excited HgD. The relative yields of the various reactions are reported for each vibrational level. In the reaction with HD, HgD is preferentially produced: [HgD]/[HgH]≂6.7.
CH and CD overtone decay times in partially deuterated benzenes
Authors: Clarke David L. , Collins Michael A.
Published: July 2002

A recently developed classical model for the simulation of overtone dynamics in C6H6 and C6D6, is used to evaluate estimates for the decay times of CH and CD overtones in all deuterated benzenes for which experimental linewidths have been measured. The calculated and inferred decay times are compared for C6H6, C6H5D, p-C6H2D4, C6HD5, and C6D6. Reasonable quantitative agreement between theory and experiment is reported and marked qualitative trends are reproduced.
Infrared–microwave double resonance and diode laser spectroscopy of the ν2/ν4 bands of SnH4
Authors: Ohshima Yasuhiro , Matsumoto Yoshiyasu , Takami Michio , Yamamoto Satoshi , Kuchitsu Kozo
Published: July 2002

A pure rotational spectrum of SnH4 has been studied in the ν4 state by infared–microwave double resonance with a tunable diode laser. Twenty-seven double resonance signals have been observed for the 11 JJ–JJ type rotational transitions of the 5 Sn isotopic species. An infrared spectrum of the Coriolis-coupled ν2 and ν4 bands has been measured using a diode laser with Doppler-limited resolution. The ground-state scalar constants, B0, D0, and H0, have been determined by taking combination differences in the ‘‘forbidden’’ ν2 band, which borrows intensity through a strong Coriolis interaction. The observed microwave and infrared data have been analyzed simultaneously by including the Coriolis interaction explicitly. More than 20 molecular constants have been determined accurately for the ν2/ν4 dyad of seven major isotopic species.
Pulsed laser melting of silicon: A molecular dynamics study
Authors: Kluge Mark D. , Ray John R. , Rahman Aneesur
Published: July 2002

We present the results of a molecular dynamics study of the pulsed laser melting of crystalline silicon. The Stillinger–Weber potential is employed for the atomic interaction. A 23 ps laser pulse of 0.2 J/cm2 is assumed to deliver 9×1013 W/g to the crystal. The energy is delivered at a constant rate over the 23 ps time interval. For the first 14 ps the temperature increases in a linear manner as functions of time. At 14 ps the crystal reaches the limit of superheating and melts over the next 4 ps interval of time. We discuss various quantities as functions of time: temperature, density, energies, and structure factors. An upper limit of 1750 K is determined for melting temperature and a value of 932 J/g for the latent heat, compared to the experimental values 1683 K and 1800 J/g, respectively.
Determination of product electronic-state distributions in collision-induced dissociation experiments. II. Information available from the measurement of a single product velocity
Authors: Sizun Muriel , Parlant Gérard , Gislason Eric A.
Published: July 2002

Recently, Gislason and Guyon (paper I) have presented a method to distinguish the two (nearly) degenerate product electronic states in a collison-induced dissociation (CID) process such as H+2 +He →H+ +H +He. This paper uses their method to systematically examine the relationship between the product electronic state and the final velocity vector of any one of the three products. The results show that if, for example, the velocity of H+ is determined, there is a range of fast H+ velocities where the products must be in the excited electronic state, a range of slow H+ velocities where the products must be in the ground electronic state, and an intermediate range of H+ velocities where the final electronic state is not uniquely determined. Analogous results are derived for the measurement of either the H or He final velocity. An example of this behavior is shown from our trajectory–surface-hopping study of CID in the H+2 +He system. In addition, several previous CID experiments are reconsidered here in light of the present considerations.
Numerical tests of a pseudoreactive algorithm for lattice chains
Authors: Madden William G.
Published: July 2002

A method for performing computer simulations of lattice chains (Ref. 1) which includes both reptational motion and the exchange of mass among chains is tested against the well-established reptational algorithm of Wall and Mandel (Ref. 2) at a variety of densities. The new method is found to produce results of much higher precision than the pure Wall/Mandel technique for simulations of similar length. Estimates of uncertainty suggest that, for some properties at high density, equivalent precision can be achieved using the new algorithm from simulations two orders of magnitude shorter than would be required with the more conventional technique. The two algorithms produce results which agree to within 1%, the estimated uncertainty achieved using the Wall/Mandel procedure. Application to bulk polymer under confinement, shows that the new method permits the detail in the spatial dependence of the properties of chains to be resolved much more completely.
Photodissociation dynamics of ethylsilane: A
b i
n
i
t
i
o and RRKM study
Authors: Francisco J. S. , Schlegel H. B.
Published: July 2002

The photodissociation dynamics of ethylsilane have been investigated by ab initio molecular orbital methods. Reactants, transition state structures, and products were fully optimized at HF/3-21G and HF/6-31G* levels of theory; relative energies have been calculated at MP4SDQ/6-31G*; and zero point energies and vibrational frequencies at HF/3-21G. The calculated geometries, barrier heights, and vibrational frequencies for primary dissociation channels of ethylsilane were used to compute a unimolecular dissociation rate constant k(E) by RRKM theory. The barrier height of H2 elimination (three center) is predicted to be 66.6 kcal/mol and is in good agreement with recent experimental estimates. These findings are discussed in the light of previous and recent experimental results on ehtylsilane.
Lyman-α, Lyman-α coincidence detection in the photodissociation of doubly excited molecular hydrogen into two H(2p) atoms
Authors: Arai S. , Kamosaki T. , Ukai M. , Shinsaka K. , Hatano Y. , Ito Y. , Koizumi H. , Yagishita A. , Ito K. , Tanaka K.
Published: July 2002

Photodissociation processes of the doubly excited states of H2 into H(2p)+H(2p) have been studied using a coincidence detection of two Lyman-α photons. Coincidence spectra have been measured in the energy region of 29.0–36.0 eV. The intensity of the observed coincidence peak corresponding to two Lyman-α photons increases with increasing energy from its threshold which is about 29 eV. The main precursor of the two H(2p) atoms is assigned to the doubly excited Q2 1Πu state.
Proton trapping and defect energetics in ice from FT-IR monitoring of photoinduced isotopic exchange of isolated D2O
Authors: Wooldridge Paul J. , Devlin J. Paul
Published: July 2002

The photoexcitation of 2-naphthol as a trace impurity in ice results in the injection of excess protons into the ice network. These protons are immobile at temperatures <100 K but warming to ∼120 K generates a near steady-state concentration of mobile protons which decays slowly. This behavior confirms the existence of shallow proton traps in ice which, following Kunst and Warman, are presumed to be intrinsic and, most probably, Bjerrum L defects. The quantity of mobile protons at a given temperature, in a pseudoequilibrium with immobile protons bound to the L defects, is controlled by the temperature coefficients of (a) the pseudoequilibrium constant and (b) the L-defect concentration. Since both the L and D defects are immobile below ∼130 K, the L-defect concentration can be taken to be temperature independent. Consequently, the temperature dependence of the rate at which D2O molecules isolated in H2O cubic ice are converted to (HOD)2 units by mobile protons is a direct measure of the binding energy between the excess protons and the L defects. This binding energy has been estimated at 10.0 kcal/mol. At the completion of each kinetic experiment at T<126 K, the predominant deuterated species is (HOD)2. Such samples are ideal for observation of the ice L-defect activity which is thermally activated by warming to above 130 K. By following the rate of conversion of (HOD)2 to isolated HOD for the range 134 to 150 K, the activation energy for the L-defect formation and mobility has been determined to be 12.2 kcal for cubic ice. This is close to the value of 12.0 kcal previously determined for cubic ice from isotopic exchange rates, but is less than the accepted value for hexagonal ice of 13.1 kcal/mol. Further, the enthalpy change for ice self-ionization has been estimated as 16.8 kcal from a combination of the activation energies for proton transport (9.5 kcal) and L-defect formation (7.8 kcal) with the L-defect–proton binding energy of 10.0 kcal.
Neutralized ion-beam studies of the rare gas hydrides: Observation of unique metastability for NeH
Authors: Selgren Susan F. , Hipp David E. , Gellene Gregory I.
Published: July 2002

The rare gas hydride radicals have been produced by charge exchange of the corresponding ion in a mass selected, high velocity beam with K, Na, Mg, Zn, or Hg target metals. NeH shows unique behavior, being produced in both dissociative and long-lived (>5.0 μs) metastable states. Arguments based on energetic considerations are presented that the observed metastability should be associated with the ground state of the NeH radical, requiring that it have a shallow well and dissociation barrier analogous to those previously determined for the ground states of the isoelectronic NH4, H3O, and H2F radicals. The existence of a structured radiative transition near 6.1 eV is predicted for the NeH radical. The other rare gas hydrides exhibit only dissociation of the radical with the kinetic energy released explainable in terms of production of known electronic states of the radicals by near resonant electron transfer. For HeH and ArH the first excited electronic state (A 2Σ+) is observed to efficiently predissociate into the repulsive ground state, whereas for KrH and XeH, lower limits of 4.5 and 3.9 eV, respectively, are determined for the energies of the corresponding excited states with respect to the separated ground state atoms.
Proton transfer reactions of H+(H2O)n=2–11 with methanol, ammonia, pyridine, acetonitrile, and acetone
Authors: Viggiano A. A. , Dale F. , Paulson John F.
Published: July 2002

We report here the first measurements of rate constants involving cluster ions with more than five ligands. We have measured rate constants, or lower limits to rate constants, for the reactions of H+(H2O)n=2–11 with NH3, CH3CN, CH3OH, CH3COCH3, and C5H5N (pyridine). The experimental techniques needed to study these ions and neutrals at low temperatures are described. The reactions all proceed rapidly by proton transfer with varying degrees of ligand transfer. At low temperatures the rate constants are larger than the collision rate constant based on an ion–dipole potential. Reasons for this are examined. Thermal dissociation appears to control the size distribution of the primary ion clusters and to affect the observed product distribution.
An analytic representation of the six-dimensional potential energy surface of hydrogen fluoride dimer
Authors: Hancock Gene C. , Truhlar Donald G. , Dykstra Clifford E.
Published: July 2002

We present a semiglobal potential energy surface for HF dimer that should be especially realistic in the vicinity of the two equivalent hydrogen-bonded-complex geometries and the transition state separating them. It is based on fits to 106 correlated calculations and 378 SCF calculations of the energy of the planar complex plus a separate fit to the empirical out-of-plane bending potential of Barton and Howard at a sequence of geometries along the minimum energy path for the degenerate rearrangement connecting the two minima.
Energy dependent relaxation and the theory of resonance Raman scattering
Authors: Li Pusheng , Champion P. M.
Published: July 2002

We present a ‘‘transform’’ theory for analyzing resonance Raman scattering under the condition of energy or state dependent dampibng. The energy dependence of the damping is taken to be linear in the Raman quantum but arbitrary in the remaining subspace of vibrations. The theory is tested using exact sum-over-states model calculations. The effects of truncation error are explicitly assessed. The method is applied to a Raman excitation profile of cytochrome P450, which is ‘‘red shifted’’ with respect to the resonant absorption band.
Matrix isolation study of electron impact on H2O. Infrared spectrum of OH− in solid argon
Authors: Suzer Sefik , Andrews Lester
Published: July 2002

A thermionic electron source capable of delivering 10–200 mA of electrons at low energies (30–200 eV) has been mated to a matrix isolation apparatus and employed to study electron impact on water. Three products were trapped in the argon matrix; OH radical (3452.7, 3478.1 cm−1), ArnH+ (903.7 cm−1), and a new species absorbing at 3548.9 cm−1 with a 3554.0 cm−1 shoulder. The latter is identified as OH− owing to diatomic D and 18O isotopic shifts and agreement with recent gas phase work. The 3548.9 cm−1 absorption is assigned to the R(0) vibration–rotation band of OH−, which is red shifted 42.6 cm−1 by the argon matrix. This shift is slightly more than the 38.4 cm−1 matrix shift for the R(0) line of HF. Product band intensities showed no dependence on electron energy above 30 eV but showed a nonlinear dependence on electron current, which suggests that OH is produced by primary electron impact on water and that OH− and ArH+ solvated by argon are produced by secondary processes.
Temperature dependence of the CIDEP spectrum of acetone
Authors: Tominaga Keisuke , Yamauchi Seigo , Hirota Noboru
Published: July 2002

The CIDEP spectrum of acetone has been studied by means of time-resolved EPR over a wide range of temperature. It was found that the spectrum varied remarkably depending on temperature indicating that different CIDEP mechanisms are involved at different temperatures. The CIDEP mechanisms operating in each temperature region are determined quantitatively mainly from the simulations of the spectra and their dependence on the diffusion constant of the solvent; from room temperature to −90 °C, the radical pair mechanism involving S–T0 mixing (ST0M)is always dominant, but the triplet mechanism and the radical pair mechanism involving S–T−1 mixing (ST−1M)become important above and below −45 °C, respectively. The relative importance of ST−1M as compared with ST0M determined below −60 °C is in good agreement with the prediction of the theory. We have also found two interesting phenomena that relatively strong E/A peaks appeared in the center portion of the complete E/A spectrum around −45 °C, and that the line shapes of the peaks were seriously distorted below −70 °C. We examined in detail these observations in terms of the contribution of the radical pair spectrum. The temperature dependence of a hyperfine coupling constant for the hydroxyl proton in the acetone ketyl radical is discussed in terms of spin polarization and hyperconjugation. CIDEP spectra of similar aliphatic carbonyls were investigated and the results support the generality of the conclusion obtained in acetone.
The torsional spectrum of CH3CH3
Authors: Moazzen-Ahmadi N. , Gush H. P. , Halpern M. , Jagannath H. , Leung A. , Ozier I.
Published: July 2002

The perturbation-allowed torsional spectrum of gaseous CH3CH3 has been measured between 225 and 340 cm−1 at a resolution of 0.015 cm−1 using a Fourier transform spectrometer. The absorption path length was 124 m; the gas temperature and pressure were 295 K and 107 Torr, respectively. The P, Q, and R branches have been observed for all four torsional sublevels in the bands v4=1←0 and 2←1. A torsion–rotation Hamiltonian containing seven parameters was used to analyze 204 frequencies assigned in the P and R branches of the two bands. Effective values were obtained for the reduced rotational constant AR, the two leading coefficients V3 and V6 in the Fourier expansion of the hindering potential, the B rotational constant, and three distortion parameters. Estimates for the three zeroth order torsional parameters free of higher order contributions are: AR =2.6711(50) cm−1, V3=1012.0(1.0) cm−1, and V6=10.71(0.60) cm−1. The torsional energies calculated from the current model are compared to those obtained by earlier experiments.
A lattice model of a supported monolayer of amphiphile molecules: Monte Carlo simulations
Authors: Harris Jonathan , Rice Stuart A.
Published: July 2002

We describe a lattice model of a supported monolayer of flexible amphiphile molecules which treats their full three-dimensional structure and includes nearest neighbor interactions between segments of the amphiphile chains, an attraction between the chain segments and the underlying substrate, and the inter- and intrachain excluded volume. The thermodynamic and conformational properties of this system are determined by a Monte Carlo simulation technique using a Metropolis sampling. At low surface coverage, the thermodynamic behavior predicted by the simulations is significantly different from that predicted by the mean-field theory of Wang and Rice; notably, the liquid–vapor critical point predicted by the mean-field theory is too high. In contrast, the mean-field theory prediction of the density profile along the normal to the surface is rather good. When the surface coverage is small, the conformations of the chains are sensibly independent of density. However, as the surface coverage increases the interchain interactions compress the chain parallel to the surface and force extension of the chain along the normal to the surface. Even at infinite dilution significant portions of the chains lie in the planes above the plane in which the head group lies.
The constraint on the integral kernels of density functional theories which results from insisting that there be a unique solution for the density function
Authors: Lovett Ronald
Published: July 2002

All predictive theories for the spatial variation of the density in an inhomogeneous system can be constructed by approximating exact, nonlinear integral equations which relate the density and pair correlation functions of the system. It is shown that the set of correct kernels in the exact integral equations for the density is on the boundary between the set of kernels for which the integral equations have no solution for the density and the set for which the integral equations have a multiplicity of solutions. Thus arbitrarily small deviations from the correct kernel can make these integral equations insoluble. A heuristic model equation is used to illustrate how the density functional problem can be so sensitive to the approximation made to the correlation function kernel and it is then shown explicitly that this behavior is realized in the relation between the density and the direct correlation function and in the lowest order BGYB equation. Functional equations are identified for the kernels in these equations which are satisified by the correct kernels, which guarantee a unique solution to the integral equations, and which provide a natural constraint on approximations which can be used in density functional theory. It is also shown that this sensitive behavior is a general property of density functional problems and that the methodology for constructing the constraints is equally general. A variety of applications of density functional theory are reviewed to illustrate practical consequences of this sensitivity.
Transition rates of a non-Markovian Brownian particle in a double well potential
Authors: Talkner Peter , Braun Hans-Benjamin
Published: July 2002

The transition rate of a non-Markovian Brownian particle in a double well potential is determined analytically by means of asymptotic methods and compared with both current theories and numerical simulations by Straub, Borkovec, and Berne [J. Chem. Phys. 83, 3172 (1985)]. We obtain good agreement with these simulations. The ranges of validity for the different current theories which we find do, however, not exhaust the complete parameter range. In particular, for large static friction we identify a region of bath correlation times in which the rate differs grossly from the value predicted by either Grote–Hynes theory or non-Markovian energy diffusion. Additionally, corrections to the Grote–Hynes rate are determined and an analytical expression for the non-Markovian energy diffusion rate is obtained.
Geometry and torsional motion of biphenyl in the ground and first excited singlet state
Authors: Im Hoong-Sun , Bernstein E. R.
Published: July 2002

The lowest excited singlet state of biphenyl (BP) and a number of its isotopically and chemically substituted analogs have been studied by supersonic jet laser spectroscopy. The symmetry species of this excited state in BP can be classified as B+2u in the G16 extended molecular symmetry group G16 (EM). The symmetry-allowed origin of the biphenyl -h10 S1←S0 electronic transition occurs at 35 268 cm−1. The frequency of the torsional motion in S1 is determined to be ∼65 cm−1. The potential parameters for this motion in S1 are V2=1195 cm−1, V4=−190 cm−1, and V6=18 cm−1. The torsional motion for the ground state (∼50 cm−1) can be described by V2=50 cm−1 and V4=−148 cm−1. The change in the dihedral angle between the two benzene rings in BP upon S0 to S1 excitation is determined to be ∼44° based on a Franck–Condon factor calculation. Several fundamentals of the molecular vibrations are assigned in the S1 state. The exciton interaction between the coupled benzene rings in biphenyl is suggested to be large (>103 cm−1).
Recombination of Br−2 photodissociated within mass selected ionic clusters
Authors: Alexander Michael L. , Levinger Nancy E. , Johnson M. A. , Ray Douglas , Lineberger W. C.
Published: July 2002

Photofragmentation studies of mass selected Br−2⋅(CO2)n, n≤24, cluster ions are presented. The heterogeneous cluster ions are synthesized in a supersonic expansion crossed with an electron beam. These ionic clusters are mass analyzed in a time-of-flight mass spectrometer prior to photodissociation of the Br−2 absorber in the cluster. Mass analysis of the ionic photofragments shows that in some cases the photodissociated Br−2 has recombined and in other cases a Br atom has escaped from the cluster. The branching ratio for recombination depends on both the size of the initial cluster ion and the photodissociation wavelength. The photofragmentation data are consistent with a recombination mechanism dominated by attractive forces in the smaller clusters and repulsive interactions in the larger clusters.
The effect of the nuclear motion on the electronic decay of core hole excited states in molecules
Authors: Murphy R. , Lyo In-Whan , Eberhardt W.
Published: July 2002

We have measured the vibrational fine structure in the electronic decay spectrum of the N 1s→1πg core electron excited state in molecular nitrogen. The Franck–Condon analysis of the vibrational substructure shows that within the 10−14 s lifetime of the core hole excited state the molecule adjusts to the excited state configuration, characterized by a change in bond length by 0.071 Å. Interference effects due to the short lifetime of the intermediate state play only a minor role.
Electron thermalization times for helium–krypton mixtures
Authors: Shizgal Bernie , Hatano Yoshihiko
Published: July 2002

The electron energy relaxation times for a gaseous helium/krypton mixture, calculated with a modification of a formalism reported in previous papers, are found to vary nonlinearly with the mole fractions. Recent experimental results for helium/krypton mixtures are reanalyzed and the previous disaccord of theory and experiment no longer exists.
Pulsed photoelectron spectroscopy of negative cluster ions: Isolation of three distinguishable forms of N2O−2
Authors: Posey Lynmarie A. , Johnson Mark A.
Published: July 2002

Three different ionic species with stoichiometry N2O−2 are generated by varying the neutral precursors in an electron beam ionized free jet expansion. In each case, the ion is isolated by mass spectrometry and then probed using pulsed photoelectron spectroscopy (PES) at 532 and 355 nm (2.330 and 3.495 eV, respectively). The neutral starting materials used in the three preparations are (I) O2 seeded 5% in N2, (II) pure N2O, and (III) NO seeded 10% in Ar. Based on their PES and photofragmentation properties, the three species appear to be best described as (I) O−2 ⋅N2, (II) either O−⋅N2O or more likely a chemically bound species, and (III) NO−⋅NO. It is likely that two of these species are trapped intermediates in the O−+N2O→NO−+NO reaction, suggesting a double minimum potential energy surface. The formation mechanisms of these ions in our source are discussed in the context of previous preparation schemes.
Electron energy loss spectroscopy of ammonia on Ru(001)
Authors: Parmeter J. E. , Wang Y. , Mullins C. B. , Weinberg W. H.
Published: July 2002

The adsorption of ammonia on Ru(001) has been studied using high-resolution electron energy loss spectroscopy. Multilayer, second-layer, and monolayer ammonia have been characterized vibrationally. These three states desorb near 115, 130 and between approximately 150 and 350 K, respectively. The symmetric deformation mode of chemisorbed ammonia shifts down in frequency continuously with increasing coverage from approximately 1160 cm−1 in the low-coverage limit to approximately 1070 cm−1 at (monolayer) saturation. The frequency of this mode in coordination compounds of ammonia is sensitive to the charge on the metal atom (increasing with increasing positive charge), and the frequency shift of this mode on the Ru(001) surface can be correlated with the work function decrease that this surface undergoes as the ammonia coverage increases. Off-specular EEL spectra allow the weak NH3 rocking mode and the frustrated translation of the ammonia perpendicular to the surface (i.e., the metal–nitrogen stretch) of chemisorbed ammonia to be resolved near 625 and 340 cm−1, respectively. These modes have not been identified in previous EELS studies of chemisorbed ammonia on hexagonally close-packed metal surfaces. Second-layer and multilayer ammonia yield EEL spectra similar to those observed on other metal surfaces. In agreement with previous results, the adsorption of ammonia on Ru(001) at 80 K, followed by annealing, leads only to reversible desorption.
Monodisperse particle production by vapor condensation in nozzles
Authors: Turner Jay R. , Kodas Toivo T. , Friedlander Sheldon K.
Published: July 2002

When a gas containing a condensable vapor expands from a reservoir through a subsonic or supersonic nozzle, the gas is cooled and the vapor can condense to form particles. A theoretical framework to describe such systems is given. Calculations based on the theory show that approximately uniform submicrometer particles can be produced at high mass rates by subsonic nozzle expansion. Four operating conditions (exit Mach number and the reservoir temperature, pressure, and saturation ratio) and the nozzle shape (area distribution and length) control the spread of the particle size distribution. Two modes of nozzle operation are predicted. In one case nucleation is confined to the nozzle while in the other case nucleation occurs both in the nozzle contraction and in a straight section following the nozzle. Nearly monodisperse particles can be produced for a wide range of operating conditions for both types of nozzle operation. Particle properties are less sensitive to operating conditions, however, when the conditions are chosen so that nucleation is confined to the nozzle.
Trajectory study of the formation and decay of silicon trimer complexes in monomer–dimer collisions
Authors: Gai Huadong , Thompson Donald L. , Raff Lionel M.
Published: July 2002

The dynamics of the formation and dissociation of Si3 complexes in atom–dimer collisions have been investigated using quasiclassical trajectories on a potential-energy surface that is a global fit to the available experimental data and ab initio CI and MP4 results. The fitted surface yields excellent results for the equilibrium geometries of both Si2 and Si3. The rms deviation of the energies from the MP4 calculations is 0.117 eV. The Si3 fundamental frequencies agree with the MP4 results to within 37 cm−1 or less. Formation cross sections for Si3 formation are reported as a function of initial relative translational energy. These cross sections are found to exhibit a near double exponential dependence upon relative translational energy. The thermal rate coefficients are on the order of 1015 cm3/mol s and show a negative temperature dependence at temperatures below 200 K. Microcanonical dissociation rate coefficients for Si3 complexes are reported for six different internal Si3 energies. These values follow an RRK energy dependence with s=2.67. The average Si3 lifetimes are found to lie in the range 4.71×10−13−5.02×10−11 s.
The nuclear displacement operator and formulation of the Born couplings of molecular Born–Oppenheimer wave functions
Authors: Zhang Yongfeng , Porter Richard N.
Published: July 2002

The nuclear displacement operator (NDO) for Born–Oppenheimer electronic wave functions (BOEWF) is introduced and some recurrence formulas are obtained. The formulas for Born couplings and higher derivatives of BOEWF with respect to nuclear coordinates are derived from very general considerations and relations among these quantities are given. The series form, exponential, and integral forms of the NDO are exhibited. Particularly, it is proven that for the two-state systems the NDO has a very simple form by which it is convenient to study two-state dynamical processes. It is shown that the NDO satisfies a differential equation which is analogous to that for the time-evolution operator in the presence of a time-dependent perturbation. The physical meanings of these two operators are compared. It is demonstrated that the NDO is uniquely determined by the vector Born coupling matrix, and that the nuclear motion may be analyzed in terms of a local non-Abelian gauge transformation.
The generation of 12C31P and 13C31P by reactive laser vaporization for rare gas matrix electron spin resonance studies: Comparison with a
b i
n
i
t
i
o theoretical calculations
Authors: Knight Lon B. , Petty J. T. , Cobranchi S. T. , Feller David , Davidson E. R.
Published: July 2002

The 12C31P and 13C31P diatomic radicals have been generated by the laser vaporization reaction between carbon and phosphorus which were pressed into a pellet to form the laser target. This method is applicable to a wide range of nonmetallic systems for generating new reactive intermediates which cannot be done with more conventional approaches. The radicals were isolated in neon and argon matrices at 4 K for detailed ESR investigations. The magnetic parameters (MHz) for neon were: g∥=2.0009(3); g⊥=1.9902(3); A∥(31P)=145.0(3); A⊥(31P)=−269.0(2); A∥(13C)=580.0(3); and A⊥(13C)=422.0(3). The argon A and g tensors were virtually indistinguishable from these neon results. Extensive ab initio theoretical calculations were conducted for CP which yielded nuclear hyperfine A values in close agreement with the experimental results. Valence orbital spin populations extracted from the calculated CI wave functions are compared with those determined directly from the ESR hyperfine parameters for 13C and 31P. The observed g shift for CP agreed with the gas phase spin–rotation constant and indicates the presence of a low lying 2πr state that has not been previously observed.
Use of K(n
d) Rydberg atoms to investigate low-energy electron attachment to C2Cl4
Authors: Marawar R. W. , Walter C. W. , Smith K. A. , Dunning F. B.
Published: July 2002

An investigation of electron attachment to tetrachloroethylene (C2Cl4) at subthermal energies through studies of K(nd)–C2Cl4 collisions (12≤n≤100) is reported. The data indicate that electron capture can result in the formation of Cl− ions, relatively long-lived C2Cl−4 ions, and C2Cl−4 ions that undergo rapid autodetachment. Cross sections for the production of Cl− and long-lived C2Cl−4 ions through electron capture are derived from the Rydberg atom data and are compared to results obtained using swarm and threshold photoelectron spectroscopy techniques.
Nonspectator electron processes in resonant Auger spectra of SiCl4
Authors: Carlson Thomas A. , Mullins David R. , Beall Charles E. , Yates Brian W. , Taylor James W. , Grimm Frederick A.
Published: July 2002

Angle-resolved photoelectron spectroscopy has been carried out on the valence shell of SiCl4 over the range of photon energies from 101.5 to 112.5 eV. Both intensities and angular distribution parameters β were monitored over the preionization resonances for silicon 2p. Little effect was seen for emission from the chlorine lone-pair orbitals 2t1 , 8t2 , and 2e, but striking effects were seen for the bonding 7t2 orbital. The resonant behavior was compared with photoabsorption data. The results are discussed in terms of direct participation of the resonantly excited electron and this process compared with spectator electron and shakeup processes.
Series expansions for clustering in continuum–percolation models with interactions
Authors: Sen Asok K. , Torquato S.
Published: July 2002

The low-density expansions of the concentration of monomers, dimers, trimers, and the mean cluster size are computed exactly, up through three-body cluster integrals, for a continuum–percolation model of spheres characterized by both exclusion-volume repulsions and short-range attractions. The compactness of trimers for our model is studied in the dilute limit. Interestingly, the [1,1] Padé approximant of the mean cluster size yields percolation thresholds which exhibit the proper qualitative dependence on the strengths of the repulsive and attractive interactions. The predicted thresholds agree qualitatively well with the trends reported in a Monte Carlo study by Bug et al.