bluuebunny/crossref_metadata_2025 huggingface dataset:
Potential energy surfaces for the insertion of Re and Re+ into H2
Authors: Dai Dingguo , Balasubramanian K.
Published: July 2002

Potential energy surfaces of 12 electronic states of ReH2 and 13 electronic states of ReH+2 are computed. Complete active space multiconfiguration self-consistent field (CAS-MCSCF) followed by multireference configuration interaction calculations, which included up to 1.5 million configurations, were carried out on these electronic states of ReH2 and ReH+2. In addition, spin-orbit effects were included using the relativistic configuration interaction (RCI) method. Our computations reveal that the ground states of Re(6S) and Re+(7S) do not insert into H2 while the excited Re(6D) and Re+(5G) states insert into H2 spontaneously. The ground state of ReH2 is found to be a high-spin linear 6Σ+g state while the ground state of ReH+2 is found to be a bent 5B2 state. There are low-lying bent sextet and quartet minima for ReH2 while there are corresponding triplet and singlet bent minima for ReH+2. The 3B2 and 1B2 bending potential energy surfaces of ReH+2 exhibit double minima attributed to two possible Re(d) hybridizations. The spin-orbit effects were found to be non-negligible for both ReH2 and ReH+2.
Mixed-exponentially generated wave function method for ground, excited, ionized, and electron attached states of a molecule
Authors: Nakatsuji Hiroshi
Published: July 2002

Wave functions of excited, ionized, and electron attached states are produced by applying the excitator method to the mixed-exponentially generated (MEG) wave function for a ground state. This method is called excited-(EX-)MEG method and the computational algorithm is summarized. The MEG/EX-MEG method is a generalization of the SAC(symmetry adapted cluster)/SAC-CI method. Test applications are given for singlet, triplet, ionized, and anion states of hydrogen fluoride at different internuclear distances (R=Re, 2.1815 Re, and 3.2722 Re). Full-CI calculations are also done for all of these states in order to examine the accuracy of the MEG/EX-MEG method. It is good and reasonably constant among the various states with different energies, different multiplicities, different numbers of electrons, and different internuclear distances, though the sizes of the calculations are smaller than those of the conventional CI.
Trajectory calculations of ion–molecule momentum transfer rate constants
Authors: Liu Jing , Su Timothy
Published: July 2002

A classical trajectory method is used to calculate thermal energy momentum transfer rate constants for ions in polar and nonpolar gases. The calculations include the consideration of noncapture scattering. For ions in nonpolar gases, the anisotropy of the polarizability and the ion–quadrupole interaction are also included in the calculations. The results show that the noncapture scattering plays an important role in momentum transfer for ions in both polar and nonpolar neutral gases. For ions in anisotropic nonpolar gases, both the anisotropy of the polarizability and the ion–quadrupole potential are important contributors to the momentum transfer rate constants. The theoretical rate constants are in good agreement with a large collection of experimental results. The theoretical results for polar gases are parametrized to an analytical expression which serves as a useful tool for estimating thermal energy ion–polar molecule momentum transfer rate constants. The present theoretical model is more realistic and provides a more accurate estimation of ion–molecule momentum transfer rate constants than previous theoretical treatments.
Laser excitation spectroscopy of Ne2 n
pπ Πg Rydberg states observed in the afterglow of a corona discharge
Authors: Kane D. J. , Kim S. B. , Eden J. G. , Ginter M. L.
Published: July 2002

Several Rydberg series attributable to transitions originating from the a 3Σ+u metastable state of the neon dimer have been observed by laser excitation spectroscopy in the afterglow of a pulsed corona discharge. The n=4–9 members of an npπ Πg←a (0–0) series have been rotationally resolved and the upper states characterized. The probable multiplicity of the Πg states is discussed. An unusual loss of intensity in the Q branches for the higher n member transitions and a weak perturbation between the v=1 vibrational level of the 5fσ 3Σ+g state and the 6pπ Πg, v=0 level are also reported.
Reaction dynamics from liquid structure
Authors: Lee Lloyd L. , Li Y. S. , Wilson Kent R.
Published: July 2002

The possible connection between the equilibrium structure of a solution and the chemical reaction dynamics that occur in that solution has been discussed by Adelman and co-workers. In this work, we present a computational demonstration of this connection using molecular dynamics simulations and the generalized Langevin equation (GLE). A favorable example of a reaction loosely based on thermally activated Cl+Cl2→Cl2+Cl in argon solvent is used for this demonstration by (1) computing equilibrium solution structural information in terms of the Ar–Ar and Ar–Cl radial distribution functions, both from integral equations and from molecular dynamics; (2) deriving a memory function for Cl in argon solvent from the radial distribution functions and the Ar–Cl potential; and (3) using this memory function in a simple GLE to compute the dynamics of the reaction. Energy flow results both for climbing and descending the barrier are in gratifying agreement with the dynamics of the same reaction as computed by full deterministic molecular dynamics.
A force field analysis of the methyl radical X̃ 2A″2 state stretching potential using the local mode—coupled Morse oscillator model
Authors: Westre S. G. , Liu X. , Getty J. D. , Kelly P. B.
Published: July 2002

The local mode-coupled Morse oscillator model was utilized to determine the quadratic, cubic, and quartic force constants for the vibrational stretching potential energy functions of the CH3, CD3, CH2D, and CHD2 using stretching fundamentals and overtones derived from resonance Raman studies. The Morse harmonic frequency and anharmonic constant of the methyl radical indicate that bonding in the methyl radical and a variety of ethylenic molecules is primarily a function of the sp2 hybridization of the central atom and that the bonding is not extensively influenced by the methyl radical’s unpaired electron or the π bonding in the ethylenic molecules. The vibrational states of the methyl radical are best described by wave functions containing significant amounts of normal mode character. The stretching frequencies for the tritiated methyl radical isotopomers are calculated.
The role of matrix material and CCl4 (electron acceptor) on the ionization mechanisms of matrix-isolated naphthalene
Authors: Salama F. , Allamandola L. J.
Published: July 2002

The effects of matrix atoms (Ne vs Ar), photoionizing energy and CCl4 (electron acceptor) on the ionization mechanisms of naphthalene are discussed through their impact on the strength of the 300–900 nm spectrum of C10H+8. In Ne and Ar matrices containing naphthalene and CCl4, under 10.2 eV VUV irradiation, CCl+4 is predominantly formed (characterized by a broad, strong absorption at 408 nm in a neon matrix), severely limiting the naphthalene cation yield. As shown by Andrews et al. [J. Phys. Chem. 86, 2916 (1982)], with lower energy photon irradiation (5.6 eV), CCl4 is not ionized, but rather acts as a very efficient electron trap in argon, leading to a strong enhancement of the C10H+8 yield. However, naphthalene cations are not produced under the same conditions in a neon matrix. The difference between the ionization mechanism in the two matrices is explained in terms of spin–orbit coupling.
Rainbow effect in classical and quantum probability packets
Authors: Bosanac S. D.
Published: July 2002

A rainbow-like effect is described in the dynamics of the wave packets (probability packets). The effect manifests itself as a narrow and quite stable packet for which ordinary rules of the wave packet propagation do not apply.
CCl Rydberg states
Authors: Johnson Russell D.
Published: July 2002

CCl radical has been observed from 280 to 410 nm by resonance enhanced multiphoton ionization (REMPI) spectroscopy. Transitions from the ground state to Rydberg states are observed in the range of 40 000–70 000 cm−1, and to the previously known A 2Δ state at 36 000 cm−1. The states display short vibrational progressions with spacings of 1100 cm−1, which is similar to the ion vibrational frequency. Vibrational progressions are observed in a number of Rydberg states and the origins are assigned by isotope shifts and hot band analysis. From two of these Rydberg states an ionization potential is estimated to be 8.83±0.01 eV.
Zero electron kinetic energy spectroscopy of Au−6
Authors: Gantefor G. F. , Cox D. M. , Kaldor A.
Published: July 2002

Zero electron kinetic energy (ZEKE) spectroscopy and photodetachment cross section measurements have been carried out on Au−6. The transition frequencies of ZEKE peaks and resonance peaks are nearly identical, consistent with the proposal that the excited negative ion of Au6 has a geometry quite similar to that of neutral Au6, and supporting the picture of the resonances as weakly bound ‘‘surface states.’’ From these studies the 0–0 transition frequency between Au−6 and Au6 is measured to be 16 541±17 cm−1, and the vibrational frequencies for the neutral Au6 ground state, the Au−6 ground state, and the (Au−6)* anion excited electronic state are 107, 73, and 107 cm−1, respectively. Lastly, we observe evidence for a previously unreported resonance transition to the excited anion which we suggest is the 0–0 transition of Au−6 to (Au−6)*.
Reply to: The critical behavior of model electrolytes [J. Chem. Phys. 9
3, 8405 (1990)]
Authors: Kholodenko Arkady L.
Published: July 2002

A reply to the comment on critical behavior of model electrolytes is given.The claim that electrolyte model can be mapped onto the spherical model is examined. (AIP)
Chemical equilibrium in fluids from the gaseous to liquid states: Solvent density dependence of the dimerization equilibrium of 2-methyl-2-nitrosopropane in carbon dioxide, chlorotrifluoromethane, and trifluoromethane
Authors: Kimura Y. , Yoshimura Y.
Published: July 2002

We have studied the dimerization equilibrium of 2-methyl-2-nitrosopropane in carbon dioxide (CO2), chlorotrifluoromethane (CClF3), and trifluoromethane (CHF3) at 60 °C from the gaseous to liquid states. The dimerization equilibrium constants are not monotone functions of the solvent density in all solvent fluids. The density dependence of the equilibrium constant shows two inversions: The equilibrium constants in CO2 and CHF3 increase with increasing solvent density up to ρr≂0.3, where ρr is the reduced density by the critical density of solvent, whereas the equilibrium constant in CClF3 shows little dependence on the solvent density in this density region. From ρr≂0.3 to about 1.4, the equilibrium constants decrease with increasing solvent density. In the higher-density region, the equilibrium constants increase again. We measured the equilibrium constants in CClF3 and CHF3 at 35 °C, and obtain the density dependence of the equilibrium constant which is almost the same as that at 60 °C. The effects of the temperature and the solvent species on the equilibrium constant are larger in the low-density region than in the high-density region. The differences between the low- and high-density regions suggest that the energetic factor is dominant in the low-density region, and that the packing effect is dominant in the high-density region. The experimental results are qualitatively reproduced by a simple reaction model where two spherical molecules dimerize in a fluid composed of spherical molecules, although the reaction model overestimates the equilibrium constant.
Theoretical studies of the structure and thermochemistry of FO2 radical: Comparison of Mo/ller–Plesset perturbation, complete-active-space self-consistent-field, and quadratic configuration interaction methods
Authors: Francisco J. S. , Zhao Y. , Lester W. A. , Williams I. H.
Published: July 2002

The structure of FO2 has been calculated for the X 2A″ ground state using Mo/ller–Plesset (MP) perturbation, complete-active-space self-consistent-field (CASSCF), and quadratic configuration interaction (QCI) ab initio molecular orbital methods. Basis sets with polarization and diffuse functions were used. Compared with the experimental structure, bond lengths obtained with MP perturbation methods are found to be consistently too short. CASSCF calculations yield a structure which varies considerably with the size of the active space and basis set used. Calculations using the single-configuration-based QCI in the single and double-space with perturbative inclusion of triple substitutions, denoted by QCISD (T), yield structures very close to the experimental structure of FO2. The thermochemistry of FO2 radical has been calculated using the MP, QCI, and gaussian-1 (G1) methods. The QCI method using isodesmic and isogyric schemes has predicted the heat of formation for FO2 at 0 K to be 8.9±3 kcal mol−1.
A test of generalized van der Waals theory for confined fluids of rigid particles
Authors: Nordholm Sture , Penfold Robert
Published: July 2002

The predictions of the generalized van der Waals (GvdW) theory for density profiles of hard rods and spheres confined between hard walls are considered as a function of chemical potential. The usual iterative solution of the stationarity condition of the functional becomes impractical at high μ and is replaced by a local steepest descent minimum search which is able to generate solutions with no apparent limit on μ. Further improvement can be achieved using the global method of simulated annealing to perform the combinatorial optimization and by consideration of the free-energy variation with the location of a test particle. The accuracy of the GvdW theory in one dimension is tested by comparison with exact results.
The interaction of atomic oxygen with thin copper films
Authors: Gibson B. C. , Williams J. R. , Fromhold A. T. , Bozack M. J. , Neely W. C. , Whitaker Ann F.
Published: July 2002

A source of thermal, ground-state atomic oxygen has been used to expose thin copper films at a flux of 1.4×1017 atoms/cm2 s for times up to 50 min for each of five temperatures between 140 and 200 °C. Rutherford backscattering spectroscopy was used to characterize the oxide formed during exposure. The observations are consistent with the oxide phase Cu2O. The time dependence and the temperature dependence of the oxide layer thickness can be described using oxide film growth theory based on rate limitation by diffusion. Within the time and temperature ranges of this study, the growth of the oxide layers is well described by the equation L(T,t)=3.6×108 exp(−1.1/2kBT)t12, where L, T, and t are measured in angstroms, degrees Kelvin, and minutes, respectively. The deduced activation energy is 1.10±0.15 eV, with the attendant oxidation rate being greater than that for the corresponding reaction in molecular oxygen.
Prediction of fluid phase equilibrium of ternary mixtures in the critical region and the modified Leung–Griffiths theory
Authors: Lynch John J. , Rainwater James C. , Van Poolen Lambert J. , Smith Duane H.
Published: July 2002

The modified Leung–Griffiths theory of vapor–liquid equilibrium (VLE) is generalized to the case of three components. The principle of ‘‘corresponding states’’ is reconsidered along with certain functions of ‘‘field variables’’ within the model. The mathematical form of the coexistence boundary in terms of the field variables remains practically unchanged and conforms to modern scaling theory. The new model essentially predicts ternary fluid mixture phase boundaries in the critical region from previous vapor–liquid equilibrium data correlations of the three binary fluid mixture limits. Predicted saturation isotherms of the ethane+n-butane+n-pentane and ethane+n-butane+n-heptane mixtures are compared with experimental ternary VLE data in the literature.
A low energy quasiclassical trajectory study of N++H2. Potential energy surface effects
Authors: Wilhelmsson Ulla , Nyman Gunnar
Published: July 2002

A low energy quasiclassical trajectory study has been performed on three potential energy surfaces, obtained by modifying the recent ground state ab initio potential energy hypersurface by Wilhelmsson et al. for the N+(3P)+H2(1Σ+g)→NH+ (2Π)+H(2S) reaction. The importance of explicit consideration of the asymptotic ion–induced dipole and ion–quadrupole interactions in the entrance channel is demonstrated. Reaction cross sections as a function of collision energy (approximately in the range 0.06–0.4 eV in c.m.) are presented.
Carbon cluster ion (C+n, 11≤n≤23) reactivity investigated via reaction with naphthalene: Kinetic energy dependence and evidence for long-lived complexes
Authors: Zimmerman Jeffrey A. , Creasy William R.
Published: July 2002

Reactions of carbon cluster ions (C+n, 11≤n≤23) with naphthalene have been investigated in a Fourier transform ion cyclotron resonance mass spectrometer (FTICR-MS). Reactive channels include the formation of adduct ions stabilized by radiative association, three-body collisions and loss of a hydrogen atom. Radiative association competes favorably with the dissociation of the ion/neutral complex and estimates of the average complex lifetime derived via kinetic modeling range from 28 to 59 (±50%) ms. The reactions have been found to be extremely dependent on the kinetic energy of the cluster ion. This dependency has been exploited to determine the average kinetic energy of the desorbed ions by comparison of reaction product distributions obtained as a function of kinetic energies. Calculated values were on the order of the trapping potential to twice that value (2–4 eV).
Static and contact electron transfer within photoinduced geminate ion pairs
Authors: Burshtein A. I. , Zharikov A. A. , Shokhirev N. V.
Published: July 2002

The quantum yield and a time of ion pair separation were calculated at any viscosity of solution and arbitrary distance between them at initial moment when geminate charge recombination starts. The results are compared with those obtained in a contact approximation and in a frame of ‘‘exponential model.’’ The way is proposed how to use experimental data on back electron transfer to answer the questions where the ions were born and how long an exchange interaction is between them.
Fourier transform intracavity laser absorption spectra of the 6ν1 band of CHD3
Authors: Domingo C. , del Olmo A. , Escribano R. , Bermejo D. , Orza J. M.
Published: July 2002

Absorption spectra in the 16 100–16 320 cm−1 region have been recorded for gaseous CHD3 at room temperature with 0.12 cm−1 resolution using a Fourier transform intracavity laser absorption technique. Two bands belonging to the N=6 polyad, 61 (6ν1) and 62 (5ν1+2ν5), were identified and a rotational analysis for 61 including transitions up to J=13, verified by means of ground-state combination differences, is reported. Comparison with rotational contants obtained from photoacoustic laser spectra at 77 K is discussed.
Methyl group dynamics in α-crystallized toluene as studied by deuteron spin–lattice relaxation
Authors: van der Putten D. , Diezemann G. , Fujara F. , Hartmann K. , Sillescu H.
Published: July 2002

Deuteron spin–lattice relaxation experiments on α-crystallized toluene-d3 are reported. It is shown that the two distinct methyl groups existing in this crystal can be identified by 2H–T1 experiments. The temperature dependence of the broadenings is found to be different, particularly in an intermediate temperature range, where transitions to the second excited librational state dominate the dynamics for one type of methyl group. For the second type, the crossover from a low temperature behavior to a ‘‘quasiclassical’’ regime takes place in a very narrow temperature range. The apparent activation energies are compared to the known transition energies of protonated toluene.
Reaction rates of ions with dipolar molecules in 2Π electronic states
Authors: Wickham A. G. , Stoecklin T. S. , Clary D. C.
Published: July 2002

Calculations of rate coefficients for the fast reaction of a structureless ion with a dipolar linear molecule in a 2Π electronic state are presented. The rotationally adiabatic capture theory is used together with a simple classical electrostatic potential. An analytical formula is derived for the rate coefficient in the low temperature limit. The reaction of H+3 with OH(X 2Π) is taken as an example. For temperatures less than 90 K, the rate coefficients differ from those calculated for molecules in 1Σ states. The most interesting new prediction is that the rate coefficient for the reaction of a 2Π polar molecule with an ion is proportional to T−1/2 for temperatures close to absolute zero.
Potential of mean force for an aqueous chloride ion pair: Simulation with a polarizable model
Authors: Karim Omar A.
Published: July 2002

The potential of mean force for an aqueous chloride ion pair at room temperature is calculated using a polarizable-polar model and molecular dynamics computer simulation. A shallow minimum in the potential of mean force is observed near an ion separation of 4.8 Å, with approximately two water molecules bridging the chloride ions at this separation. The barrier to further separation is less that 1 kcal/mol. This result does not support the strong association of chloride ion pairs.
The shape of alkaline earth dihalide molecules: The molecular geometry of strontium dibromide from electron diffraction
Authors: Hargittai Magdolna , Kolonits Mária , Knausz Dezsö , Hargittai István
Published: July 2002

Gaseous strontium dibromide has a quasilinear molecular structure according to a joint electron diffraction/vibrational mode analysis in agreement with recent ab initio studies. The thermal-average (rg) and equilibrium (re) bond lengths are 2.783±0.006 and 2.738±0.013 Å, respectively. The estimated vibrational frequencies ν1, ν2, and ν3 are, respectively, 165±21, 31±10, and 283±36 cm−1.
Polarization effects in infrared–infrared double resonance in methyl fluoride
Authors: Shin Uhyon , Schwendeman R. H.
Published: July 2002

A series of infrared–infrared double-resonance experiments are described in which a low-pressure gas sample interacts with pump and probe beams that have a variety of different polarizations. It is shown that orientation or alignment of the sample, previously demonstrated for electronic spectra with visible lasers, can be performed by irradiating a sample with a polarized infrared laser that is coincident with a vibration–rotation transition. By switching the polarization of the pump beam between horizontal and vertical polarization, linear dichroism is observed. By switching the polarization of the pump beam between right and left circular polarization, either circular dichroism or circular birefringence may be seen with the appropriate experimental arrangement. Also, as shown previously for electronic transitions, a combination of dichroism and birefringence experiments can be used to determine whether pump and probe transitions in the infrared region are P, Q, or R branch transitions. It is found that induced molecular alignment or orientation can be observed in four-level double resonance, indicating that alignment or orientation survives one or more collisionally induced rotational transitions. All of the three-level double-resonance effects may be predicted by means of the Jones calculus by using a Jones matrix for the pumped sample considered as an optical device. Equations for the elements of the Jones matrix have been derived for a sample pumped by either plane-polarized or circularly polarized radiation of arbitrary intensity.
Solvent-electronic state interactions measured from the glassy to the liquid state. I. Ultrafast transient and permanent hole burning in glycerol
Authors: Yu Jongwan , Berg Mark
Published: July 2002

Picosecond transient and permanent hole burning spectroscopies are used to study the interaction between glycerol and the electronic states of the nonpolar solute dimethyl-s-tetrazine. Variable temperature measurements from the low temperature glass into the fluid region have identified a phonon-modulated interaction in agreement with a previous hypothesis [J. Chem. Phys. 94, 5787 (1991)]. However, this mechanism alone cannot account for the magnitude of the subpicosecond line broadening seen at room temperature. A second relaxation mechanism is identified, which is connected to the structural coordinates involved in the glass transition and reaches subpicosecond relaxation times at room temperature.
Rotational relaxation measurements in ozone: Temperature and collision partner effects
Authors: Flannery C. C. , Steinfeld J. I.
Published: July 2002

Time resolved infrared double resonance experiments have been conducted on ozone mixtures in order to determine rotational relaxation rates. A pulsed CO2 laser pumps a ν3 ←ground state transition, while a diode laser is tuned to a hot band transition so that the relaxation process in v3=1 may be observed. Nitrogen, oxygen, rare gas, and self-relaxation rates have been measured, and the temperature dependence of these rates has been investigated over the 200–300 K range.
Structure and dynamics of Cl−(H2O)20 clusters: The effect of the polarizability and the charge of the ion
Authors: Perera Lalith , Berkowitz Max L.
Published: July 2002

The effect of the polarizability and the sign of the ionic charge were studied in Cl−(H2O)20 clusters using molecular dynamics computer simulation technique. From our simulations we concluded that the reduction in the ionic polarizability did not significantly change the structure and dynamics of the Cl−(H2O)20 cluster, but the inversion of the sign of the ionic charge produced a large effect. The energetic considerations helped us to understand why Cl− is located on the surface of the cluster. By being on the surface the anion permits the creation of the hydrogen bonded network between water molecules and that lowers the total energy of the cluster. Simulations with the inverted sign of the ionic charge correspond to that with a hypothetical ‘‘Cl+ ’’ ion which is similar in size and polarizability to a Cs+ ion. The dynamical structures and the quenched structures of Cl+(H2O)20 clusters are compared with the idealized structure of the Cs+(H2O)20 cluster proposed recently [A. Selinger and A. W. Castleman, Jr., J. Phys. Chem. 95, 8442 (1991)].
A theory of Raman scattering due to dipole active vibrational modes of molecules adsorbed on small metal particles
Authors: Xu Muliang , Dignam Michael J.
Published: July 2002

The theory of intense Raman scattering, generated by dipole active vibrational modes of molecules adsorbed on small metal particles, is reformulated here to make use of our recently developed approach to calculating the polarizability of small metal spheres and to include terms missed in our earlier development. The approach is based on the local-dielectric-response approximation and requires a knowledge of the conduction electron density profile at the metal/ambient interface. The modulation of the polarizability of a metal particle, due to the quadrupolar field generated by a phonon mode of the adlayer, is calculated using an integration procedure that incorporates both electron density and induced charge density profiles determined by Lang and Kohn for jellium. Scattering intensities for both Raman and dipole active modes of CO and CO2 on small silver particles are calculated and compared. The results show that the ratio of the scattering intensity for a Raman active molecular mode to that for a dipole active mode increases rapidly with the particle radius, being comparable for the two stretching modes of CO2 on Ag particles of radius 25–50 Å.
Chemi-ionization in K2/BrCN collisions
Authors: Durham M. A. , Marawar R. W. , Lindsay B. G. , Smith K. A. , Dunning F. B.
Published: July 2002

Ion production in thermal-energy collisions between K2 dimers and BrCN is examined using a novel time-of-flight technique incorporating position-sensitive detection. The reaction channels are identified and the energetics of reaction investigated.
Rotational temperature dependence of the branching ratio for the reaction of Kr+(2P3/2) ions with HD
Authors: Dotan I. , Viggiano A. A. , Morris Robert A.
Published: July 2002

Branching ratios for the reaction of Kr+(2P3/2) with HD, which produces KrH+ and KrD+, have been measured in a variable temperature-selected ion flow drift tube apparatus as a function of average center-of-mass kinetic energy 〈KEcm〉 at two temperatures, 93 and 300 K. At the lowest energy employed, 〈KEcm〉=0.012 eV, the KrD+ channel is favored, and its contribution decreases with increasing kinetic energy. The data are in agreement with previous measurements. The dependence of the branching ratio on the rotational temperature of HD is derived from the data. The rotational temperature dependence is small and appears to be in the opposite direction to the kinetic energy dependence, i.e., increasing rotational energy appears to decrease the fraction of KrH+ produced. The data are compared with models and with the rotational temperature dependence found for the reaction of O+ with HD. Rate constants for the reactions of Kr+(2P3/2) with H2 and HD were measured at 300 K and no drift field and found to be 2.8±0.7×10−10 cm3 s−1 and 4.0±1.0×10−10 cm3 s−1, respectively. The latter number supports the previous beam measurement and disagrees with a previous selected ion flow tube result.
Vibrational distributions of N2O+(Ã 2Σ+) produced by electron impact on jet-cooled N2O
Authors: Tokue Ikuo , Kobayashi Mikio , Ito Yoshio
Published: July 2002

Fluorescence spectra of the N2O+(Ã 2Σ+–X̃ 2Πi) system produced by electron impact on N2O have been studied in the impact energy range of 22–100 eV in order to determine vibrational-state distributions of the N2O+(Ã 2Σ+) state. Emission bands from the 00, 21K1, 22K0, 11, 12, and 31 levels have been assigned. The populations of the 11 and 12 levels corrected for effects of predissociation are similar to those obtained by the threshold photoelectron method. It is concluded that these results are affected by autoionization via the Rydberg states converging to the C̃ 2Σ+ ion state. The population of the 21K1 level is enhanced remarkably more than in photoionization data. This enhancement indicates that, in the electron impact, transitions with ΔK=±1 are allowed in the formation of Ã 2Σ+.
High resolution vacuum ultraviolet Stark measurement of the dipole moment of Ã 1A″ HCN
Authors: Jonas David M. , Solina Stephani Ann B. , Zhao Xinsheng , Field Robert W. , Kittrell Carter
Published: July 2002

We report here the first measurement of the electric dipole moment of the Ã 1A″ state of HCN. Fluorescence excitation spectra of the Ã←X̃ 210K10 band were recorded using narrow band (<0.1 cm−1) vacuum ultraviolet (VUV) produced by four-wave sum mixing in an improved strontium heat pipe. Accurate (±0.007 cm−1) term values for this band are reported. Surprisingly, we found that the asymmetry doubling of the upper state does not extrapolate to zero at J=0. We suggest that the most plausible explanation for this anomalous asymmetry doubling is a spin–orbit perturbation by a nearly degenerate level of the 1 3A′ state. The a component of the dipole moment has been determined by the Stark splitting of the Q(1) transition as a function of electric field in a novel Stark cell capable of sustaining high electric fields (41 kV/cm) at pressures of 50 mTorr. The observed value of the dipole moment μa=0.99(10) D is in agreement with simple molecular orbital expectations and a semiempirical study.
Picosecond stimulated emission pumping: Intramolecular vibrational relaxation in the excited electronic state of p-cyclohexylaniline
Authors: Smith P. G. , McDonald J. D.
Published: July 2002

Intramolecular vibrational energy redistribution (IVR) is studied by picosecond time resolved stimulated emission pumping (TRSEP). This technique is used to obtain decays for 3 of the 19 previously studied vibronic levels in p-cyclohexylaniline. These TRSEP decays allow one to quantitatively measure the ratio of fast to slow fluorescence intensities in order to obtain accurate values for the number of coupled states. This data inspired us to take a closer look at the time decays which eventually led to the understanding of why the medium energy levels exhibit temporal behavior with fast initial decays and small damped quantum beats when large undamped quantum beats are expected. This paper demonstrates that the rotational state interactions are the cause for this interesting behavior.
Isotopic and anharmonic perturbations to the dipole active vibrational modes of buckminsterfullerene
Authors: Weeks David E.
Published: July 2002

Force constants of C60 are obtained by a least squares fit to experimental infrared spectra. Vibrational normal modes are computed and used as a zero order basis to calculate the isotopic perturbation as one of the 60 carbon atoms is continuously varied from 12 to 15 amu. A perturbing mass of 13 amu splits the four, threefold degenerate, dipole active modes by amounts ranging from ∼0.5 to over 2 cm−1. Stereoscopic figures of the perturbed normal modes are displayed. The perturbed modes are used as initial conditions for numerically integrating Hamilton’s equations of motion. Using a realistic anharmonic potential, power spectra are generated for one quanta of excitation, and display anharmonic redshifts that range from 0.06 to 0.67 cm−1.
Poly(di-n-hexyl-silane) in solid solutions: Experimental and theoretical studies of electronic excitations of a disordered linear chain
Authors: Tilgner A. , Trommsdorff H. P. , Zeigler J. M. , Hochstrasser R. M.
Published: July 2002

The results of a study of the electronic structure and energy transfer dynamics of poly(di-n-hexyl-silane), representative of a large class of conjugated polymers, in glassy solution at low temperature are reported. Optical excitation at ca. 3500 Å yields a Frenkel exciton with possibly some small admixture of charge transfer states. The exciton is only weakly coupled to nuclear motions and small polaron effects are absent except for a slight expansion in the direction perpendicular to the chain axis on excitation. The excitation is localized by a continuous disorder distributed along the chain and apparently not by a lumped disorder in which segments of essentially perfectly ordered regions are separated by defects. The results are consistent with an exciton bandwidth of 4.4 eV. Energy transport between localized states can be understood in terms of a simple kinetic model which permits simulation of results from fluorescence and hole-burning experiments. A novel mechanism of hole burning occurring in one-dimensional disordered systems and a method to extract the range of energy transfer from fluorescence data is presented. A conformational change on the length scale of a few Si–Si bonds is predicted to occur during the phase transition.
Vibrational and electronic spectra of matrix-isolated para-dichlorobenzene radical cations
Authors: Szczepanski Jan , Personette William , Pellow Robert , Chandrasekhar T. M. , Roser Dennis , Cory Marshall , Zerner Michael , Vala Martin
Published: July 2002

A new pulsed-glow discharge technique has been used to generate the radical cations of para-dichlorobenzene (PDCB) and trap them in an argon matrix at 12 K. Three visible absorption bands with origins at 520.5, 490.6, and 322.4 nm are shown to be consistent with gas-phase photoelectron results and intermediate neglect of differential overlap/spectroscopy calculations. Resonance Raman and fluorescence spectra compare well with previous gas-phase electron impact excited emission and freon matrix (77 K) studies. The infrared spectrum of the PDCB cation has been measured for the first time and shows bands at 843, 986, 1110, and 1429 cm−1, all of which correlate well, in intensity, with the 520 nm electronic band. Ab initio self-consistent-field level calculations on the PDCB neutral and radical cation species reproduce the spectra well, after scaling is performed. It is shown that, upon ionization, the C–Cl bond force constants and the ring central C–C bond force constants increase, while the force constants for the ring C–C bonds adjacent to the C–Cl bond decrease. This accounts for the observed blue shift for certain bands and the red shift for certain others when PDCB is ionized.
Detailed rate coefficients and the enthalpy change of the equilibrium reaction OH+C2H4=MHOC2H4 over the temperature range 544–673 K
Authors: Diau Eric Wei-Guang , Lee Yuan-Pern
Published: July 2002

The reaction between OH and C2H4 in He has been studied over the pressure range 278–616 Torr and the temperature range 544–673 K by means of the laser-photolysis/laser-induced-fluorescence technique. Analysis of the temporal profile of [OH] yielded the equilibrium constant for the reaction OH+C2H4+M■HOC2H4+M. The temperature dependence of the equilibrium constant led to the enthalpy of reaction ΔH=−(30.3±0.8) kcal mol−1 and the entropy of reaction ΔS=−(30.9±1.0) cal K−1 mol−1 near 600 K, in agreement with previous predictions. Analysis of the detailed rate coefficients suggests that the H-atom abstraction reaction is relatively unimportant in the temperature range of our study; the rate coefficient determined previously for the H-atom abstraction near 673 K may have been overestimated. The temperature dependence of the rate coefficients for the forward, the reverse, and the adduct-loss reactions have been determined to be kf =(4±3)×10−13 exp[(1200±800)/T] cm3 molecule−1 s−1, kr =(6.2±1.0)×1011 exp[−(11 900±400)/T] s−1, and ka=(8.8+25−1.5)×109 exp[−(10 400+1600−400)/T]  s−1, respectively.
Nonadiabatic electronic spin transition in ligand–heme protein binding kinetics and the influence of the heme Fe molecular environment
Authors: Gerstman Bernard S. , Sungar Nilgun
Published: July 2002

Nonadiabatic spin transitions can significantly reduce rates in reactions involving transition metals. Zero field spin splittings can further modulate the reaction rate at temperatures with thermal energies of the same size as the zero field splittings, and the low temperature reaction rate can be completely shut off in ligand fields of relatively low symmetry, C2v or D2, for example, due to a vanishing of the electronic matrix element between the initial and final ground spin states. We show in this paper that if the site symmetry is lowered even further, the electronic matrix element coupling the ground spin states can become nonzero, allowing the reaction to occur as the temperature approaches zero. We show what factors in the metal’s ligand environment are important for the electronic coupling between the initial and final electronic states. This is especially relevant for transition metals in sites of low symmetry, such as are found in biomolecules. We focus specifically on heme proteins and show how factors in the immediate environment of the Fe, such as the proximal histidine, may lower the site symmetry sufficiently to allow a nonzero low temperature nonadiabatic electronic spin transition that may be necessary for ligand binding. We also use these ideas to make predictions that could resolve the question of whether the reversible binding of CO to heme proteins is a nonadiabatic electronic process.
Investigation of XNO and XON (where X=Cl or Br) and their protonated isomers
Authors: Meredith Cynthia , Quelch Geoffrey E. , Schaefer Henry F.
Published: July 2002

A b initio molecular electronic structure methods have been used to examine nitrosyl chloride (ClNO) and nitrosyl bromide (BrNO), their hypohalite isomers (i.e., ClON and BrON), and all possible singly-protonated forms of these species. Equilibrium geometries have been optimized and harmonic vibrational frequencies obtained for each parent structure and protonated isomer at the self-consistent-field (SCF) double zeta plus polarization (DZP) level of theory. Molecular structures also have been optimized with the DZP basis set, utilizing the single and double excitation configuration interaction method (CISD). Our study has located stable minima corresponding to the nitrogen hypohalites XON (where X=Cl or Br), neither of which has been observed experimentally. In addition, we found that, for all of the species under study, protonation at the nitrogen is energetically favored over protonation at the oxygen atom. Protonation at the halogen atom of either XNO structure, however, results in the formation of stable trans and cis isomers. Similar protonation of XON results in a rearrangement to the halogen-protonated XNO form. By contrast, fluorine protonation of FNO or FON yields only one structure. Among the protonated isomers obtained, the trans halogen-protonated species lie lowest energetically relative to their parent structures. It is also found that, in terms of total energies, all protonated forms of XNO lie lower than those of XON. Moreover, protonation is found to preferentially stabilize XNO relative to XON.
Free ion yields, electron thermalization distances, and ion mobilities in liquid cyclic hydrocarbons: Cyclohexane and t
r
a
n
s- and c
i
s-decalin
Authors: Gee Norman , Freeman Gordon R.
Published: July 2002

Free ion yields GEfi were measured as a function of electric field strength E in liquids of the cyclic hydrocarbons cyclohexane, trans-decalin, and cis-decalin at temperatures up to 514, 386, and 372 K, respectively. By comparing the measured GEfi to values calculated using the extended Onsager model, zero-field free ion yields G0fi and thermalization distances bGP were obtained. The values of G0fi in trans-decalin were used to correct previously reported values of the electron mobility obtained using estimates of G0fi. Values of the density-normalized thermalization distances bGPd were ∼9% higher than in the linear n-alkanes, that is, at a given reduced density d/dc, energy transfer from the thermalizing electron is less efficient than to the n-alkanes. Measured molecular cation mobilities μ+ were correlated with liquid viscosity and compared with neutral molecule diffusion coefficients. The mobility of the molecular cations was three to four times smaller than that of the corresponding neutral molecules. A free volume model was used to describe the temperature dependence of μ+. Ion transport in a liquid is made easier by increasing the free volume of the liquid, which occurs when it is heated under its vapor pressure. The amount of thermal energy required to activate the actual transport of the ions is considerably smaller than suggested by the Arrhenius model.
The effect of low frequency vibrations in CH4 on the rate constant for the reaction of O+2 (X 2Πg, v=0) with CH4
Authors: Viggiano A. A. , Morris Robert A. , Van Doren Jane M. , Paulson John F.
Published: July 2002

The rate constant for the reaction of O+2 with CH4 has been measured as a function of average center-of-mass kinetic energy, 〈KEcm〉, at several temperatures over the range 93–545 K. The rate constant decreases with increasing 〈KEcm〉 at low 〈KEcm〉 and increases at higher 〈KEcm〉; the position of the resulting minimum depends on temperature. We have observed small differences in the rate constants measured at the same total 〈KEcm〉 but at different temperatures in the low temperature regime (T≤300 K). These differences, i.e., decreasing rate constants with increasing temperature, are discussed in terms of the possibility of a rate constant which is dependent upon rotational energy. At higher temperatures, 430 and 545 K, we find that the rate constant is significantly larger than that measured at lower temperatures at a given 〈KEcm〉. This enhanced reactivity is attributed to vibrational excitation in the CH4. From these data, we derive a rate constant for vibrationally excited CH4 (in the low frequency modes) that is approximately a factor of 20 greater than the rate constant for the ground vibrational state. The data disagree with an early study of this type at low temperature. The disagreement is discussed, and arguments are presented which indicate that the earlier data are in error at low temperature with an applied drift field. The use of these data for deriving effective reaction temperatures in other nonthermal experiments is also discussed.
Effect of enhanced collision energy on product vibrational excitation for the proton transfer reaction: O−+HF→F−+OH(v=0,1)
Authors: Knutsen Karen , Bierbaum Veronica M. , Leone Stephen R.
Published: July 2002

Relative vibrational state populations are determined for the OH product in the reaction O−+HF→F−+OH(v=0,1) as a function of reactant center-of-mass collision energy in a flow-drift tube. The OH(v=0,1) vibrational states are measured using laser-induced fluorescence detection, and the relative vibrational signals are converted to absolute populations by normalizing the results at thermal energy to the absolute measurement by Hamilton et al. [Chem. Phys. Lett. 94, 4 (1983)]. At thermal energy, the OH(v=1) fractional population {OH(v=1)/[OH(v=0)+OH(v=1)]} is 0.18±0.01. At enhanced average collision energies of 9.6 and 15.4 kJ mol−1, the OH(v=1) population increases to 0.25±0.02 and 0.33±0.03, respectively. The results are considered in terms of the deep attractive well in the ion-molecule potential-energy surface which dominates the reaction dynamics.
Bound states in Coulomb three-body symmetric systems
Authors: Bishop David M. , Frolov Alexei M.
Published: July 2002

The results of highly accurate variational calculations of the D(L=2) states in some Coulomb three-body symmetric systems are presented: ∞He, ddμ, and ttμ. The isotopic shifts in the helium atom are calculated and the existence of the quasistationary 1D state in the Ps− system is briefly discussed.
Low field theory of polymer transient electric birefringence
Authors: Bowers John S. , Prud’homme Robert K.
Published: July 2002

Using the Rouse/Zimm model for polymers, we have derived the low field dynamics for a flexible polymer in transient electric birefringence (TEB). Three orientation mechanisms were included; a permanent dipole from fixed charges, a local induced dipole dependent on the local conformation, and a global induced dipole dependent on the global conformation. The rise, decay, and reverse pulse behavior have been determined for all three, independently and combined. The condensed ion theory of Manning has been used to describe the global orientation mechanism of DNA. We find good agreement for the steady state birefringence and the predictions from theory with no adjustable parameters.
Rovibrational distributions of CH(A 2Δ) produced in energy-transfer reactions from Ar(3P2), Kr(3P2), and Xe(3P2) atoms to C2H5 radical
Authors: Tsuji Masaharu , Kouno Hiroyuki , Ujita Hiroki , Nishimura Yukio
Published: July 2002

The CH(A 2Δ–X 2Π) emission system from v′=0 was observed in energy-transfer reactions from the metastable Ar(3P2), Kr(3P2), and Xe(3P2) atoms to C2H5 radical in the flowing afterglow. The nascent rotational distributions of CH(A:v′=0) were represented by an effective Boltzmann temperature of about 1000 K for all three reactions. The average fractions of total available energies channeled into rovibrational modes of CH(A) were less than 8.4%, suggesting that most of available energies were deposited as relative translational energy of products and/or rovibrational energy of CH4. The observed rovibrational distributions for the Ar(3P2) and Kr(3P2) reactions were colder than those predicted from a simple statistical theory. The best agreement between the observed and statistical rovibrational distributions was obtained in two-body dissociation [C2H*5→CH(A)+CH4] mechanism assuming that the energy transfer takes place with significant momentum transfer, leaving long-lived C2H*5 at ∼8.2 eV, in which energy randomization is complete before dissociation. For the Xe(3P2) reaction, the observed rovibrational distributions were in good agreement with the statistical ones obtained in two-body and three-body dissociation [XeC2H*5→CH(A)+CH4+Xe] models without taking account of momentum transfer. It was therefore concluded that CH(A) is formed through near resonant energy transfer, leaving long-lived C2H*5 and/or XeC2H*5 intermediates.